Adam Jesberger | B.A.E. / M.A.E. | The Pennsylvania State University
Introduction
Wood structures are all around us in a variety of forms ranging from dimensioned lumber and engineered lumber framed residences to heavy timber structures. In order to maintain these structures, engineers and builders must understand how to identify defects that are present during construction as well as deterioration that occurs over a period of time. Proper techniques must be used to remediate the defect at hand and to prevent the further deterioration of the structure. Defects in wood can be classified as splits, checks, warps, rotting, insect infestation, or decay due to other biological organism. Understanding what defect is present and discovering the source of the defect is crucial in determining a potential solution to prevent further damage to the structure. Identifying a number of possible resolutions is important to determine a feasible remediation of the problem. This installment will focus on the deterioration of wood and its recommended remediation methods.
Understanding Wood
Wood has been used for thousands of years by cultures all throughout the world as a structural material. Even in its living form wood acts as the structural member supporting the leaves of the tree. When assessing wood in structural applications, it is important to understand it in its natural habitat, forests.
Wood gains its strength from two major components, cellulose and lignin. Cellulose is the compound on which a majority of the strength is formed. Lignin acts as the glue that holds the cellulose together to create the required strength. When either component breaks down or is destroyed, significant, if not all, strength loss occurs. Each wood type contains different proportions of cellulose and lignin. Hardwoods tend to be comprised of less lignin when compared to softwoods (Morris).
Each layer of a tree has a specific task that aids the tree throughout its life time. Only two of those layers are used in structural applications. Natural protection measures are eliminated with the other layers and need to be replaced in order to prevent wood from developing defects. The most outer layer of the tree is the outer bark. Outer bark acts as the first level of protection for the tree. Many organisms that attack wood are stopped by the outer bark. The next layer in is the inner bark, also known as the phloem. Phloem is responsible for transporting carbohydrates and amino acids that are used by a living tree for growth. Phloem is sometimes used as a storage device for them as well. The cambium layer follows, consisting cells that create phloem and the sapwood cells for the layers on each side of it. The growth of a tree occurs in the cambium layer. Sapwood is next, and is the first layer used in structural applications. Sapwood is responsible for conducting water from the roots to the leaves through the use of capillary action. This natural water flow is why sapwood absorbs water most quickly along the grain. There are many ways in which water can travel perpendicular to the grain for distribution. These ways contain large amounts of highly organic matter that is easily degradable. For this reason, sapwood is very vulnerable to organic defects. The innermost layer of a tree is the heartwood. Heartwood is sapwood that is no longer in use. Its sole purpose is to act as a structural core for the tree.
When sapwood is converted to heartwood, natural toxins are infused into it to aid in decay prevention. However, these toxins are easily dissolved by water, and leave cut wood while it is still the mill yard. Every tree species contains different levels of heartwood and sapwood, and the heartwood durability varies as well. Figure 1 is an excerpt from a table listing the wood species, predominant layer type (sapwood versus heartwood), and the relative heartwood durability (Morris). Please see Understanding Biodeterioration of Wood inStructures by P. Morris for the full table (pg. 4).
Figure 1 - Heartwood Durability Table Excerpt (Credit: P. Morris, Understanding Biodeterioration of Wood in Structures)
Over the course of its lifetime, wood can be exposed to a variety of humidity levels. As a result, the moisture content varies as well. Controlling the moisture content of wood is necessary in preventing a large number of defects. Moisture content can be calculated by taking the weight of the water and dividing it by the oven dried weight of the wood, then expressing it as a percentage (Morris). Below is Table 1 summarizing common moisture contents wood experiences throughout its lifetime.
Wood Condition
Moisture Content
Living Sapwood
150% to 200%
Dead heartwood
30% to 40%
Kiln dried wood
16% average / 19% max
Exterior wood equilibrated with air
12% to 18%
Wood in heated buildings
4% to 10%
Table 1 - Common wood conditions and
their corresponding moisture contents.
Derived from data from (Morris)
Types of Structures
Residential
Residential structures are predominately constructed using dimension lumber. Member sized range from 2” by 4” to 2” by 12”. Plywood and Oriented strand board are often used as floor and siding elements. In more recent years, the residential industry has adopted the use of more engineered products, replacing members that were previously dimension lumber. Defects associated with residential structures range from physical to thermal to organic. The organic defects that are most common to residential structures are those that require air.
Heavy Timber
Heavy timber structures have similar defects to those found in residential structures. In order to be classified as heavy timber, members must have minimum sizes that are determined by their function.
Waterfront
Waterfront structures were not extensively considered in this installment. Information on waterfront structures and their defects can be found in the following sources:
Controlling Decay in Waterfront Structures Evaluation, Prevention, and Remedial Treatments by T. Highley and T. Scheffer
Practical Repair of Timber Structures by J. Gerwick, T. Trenkwalder, and J. Kearney
Types of Wood Defects
Physical Defects
There are a large number of defects than can be classified as physical. Some of them are as follows: checks, splits, warping, and weathering. All influence the structural behavior in one way or another. Checks typically occur from differential shrinkage of the wood during the drying process. They are cracks in the wood that do no propagate entire through it. Checks usually do not have large impact on structural integrity. However, if there is a large number, or the checks are large, strength loss can occur. Checks that do propagate all the way through wood are known as splits. Splits do have a significant impact on the structural integrity of the wood. Splits can also be caused by member overload or an impact load. Other side effects of differential shrinkage include bowing, warping, twisting, and cupping (Dunham 2013).
Wood that is exposed to nature can experience weathering. Weathering does not typically have a significant impact on the long term strength of the wood, but it can affect the impact strength of the wood. It can be a sign of structural degradation below the surface. Weathering is caused by repeated wetting and drying cycles, ultraviolet light exposure, or wearing of the surface due to wind-blown debris (Dunham 2013).
More information on physical defects can be found in the following sources:
Inspection and Diagnosis Systems for Wood Flooring by A. Delgado, J. de Brito, and J.D. Silvestre
Inspecting Wood Frame Structures by P. Seirup
Thermal Degradation
Over long periods of time, wood can experience degradation due to different temperature exposures. Each species responds differently depending on the environmental conditions where the wood is placed.
More information on thermal degradation can be found in the following source:
Thermal Degradation by S. Levan
Organic Defects
At the end of a trees life, the organic components, cellulose and lignin, need to be broken down in order for a forest to progress through its natural cycles. Many of the causes of organic defects discussed in this post are responsible for this breakdown. Because this decay process is natural, using wood in structural applications that are expected to lasts for decades defies the very nature of wood (Morris).
In order for any organism to infect wood, four critical elements must be present: temperature, moisture, oxygen, and a food source. For many organisms that affect wood, the proper temperatures are very close to those associated with human comfort. Moisture can be added to wood through a number of ways. Some common causes of moisture in wood are faulty ventilation around the wood, exposure of the wood to humid environments, and wood contact with the ground. Oxygen from air is vital for the organisms to live. For this reason, many of the organisms discussed here only affect above water structures. Lastly the food source is often the wood itself. However, sometimes alternative foods sources must be present and the wood is used as a shelter for the organisms (Morris).
Fungi
As with all organisms, fungi require the four critical elements. Specifically to fungi, moisture in the wood needs to be in the form of free water. This is why it is very uncommon to find fungal infestations of completely submerged structures (Dunham 2013). Fungi spread through spores that are produced by fruiting bodies. Those spores are often spread by wind, water, or insects. In some form or another, the spores find their way to wood that is in an environment that meets the four critical elements. The most common time for spore production occurs in later summer and fall. Once wood has been infected with spores, mycelium start to grow, and function as the backbone of the infestation. Mycelia are tubular cells that grow like the branches on a tree to deliver essential items to the fungi.
An infection of a fungus occurs through three major stages incipient decay, intermediate decay, and advanced decay. Each stage adds upon the other, and results in less structural integrity of the wood. Incipient decay is the first stage. Signs of it include discoloration, minimal section loss, and punky surface. Next, in the intermediate decay stage, small voids appear on the surface as the fungus moves into the wood. Finally in advanced decay, the small voids grow larger and the fungus has infected most of the cross section. In this stage, the actual depth of the decay varies depending on the fungus, wood species, and environmental conditions (Anthony 2007).
Throughout the stages of decay, wood losses strength through two major avenues, section loss and mechanical property loss. In the early stages of fungal decay, wood section loss is not substantial. However, the mechanical properties can see a significant decrease. Relatively speaking, a 10% loss in section is often associated with a 50% loss in the mechanical properties of the wood. Impact strength of wood is the first property to degrade. The two most common strengths that follow are compression perpendicular to grain and compression parallel to grain. Because mechanical property loss can occur prior to and visible decay, some physical defects can be signs of fungal decay (Dunham 2013). Some fungi result in no strength loss, but can lead to other fungal infections. The microbial bio deterioration process is a competition between all types of fungi to infect the wood. Different conditions favor one type over another, but as conditions change an existing infection may allow for a new one to easily take over it (Morris).
Molds and Staining Fungi Molds and staining fungi do not cause significant section loss of wood. Normally only the impact strength is affected for even the most advanced stages of decay (Morris). Most require relatively high levels of humidity in order to raise the moisture content of the wood to acceptable levels. The minimum moisture content required for molds and staining fungi is 20%. Sapwood is the more susceptible to mold and staining fungi infections.
Molds have colorless or pale mycelium and often look like a “cottony” growth on the surface of the wood (See Figure 2). The growth on the wood can appear in a variety of colors depending on the species of the mold (Shupe et. al. 2008).
Staining fungi come in a variety of forms, each with its own degradation effects. In general, staining fungi has brown or black mycelium (Morris) (See Figure 3). Staining fungi do not die when the wood dries out, they merely go dormant. Dormant staining fungi can live for long periods of time, and will resume activity once the moisture level threshold is met. The major type of staining fungi is blue stain. It only occurs in sapwood, and given by its name turns the surface of the wood blue. The most economical impact of blue stain is the downgrading of the wood grade.
Figure 2 - Mold Growth on Lumber in Storage (Photo Credit: Building America Solution Center, US Department of Energy)
Figure 3 - Staining Fungi (Photo Credit: Dr. Thomas Boothby, P.E.)
Soft Rot Soft rot is most common in hardwoods, specifically oaks and maples. It is capable of breaking down the cellulose with the wood which leads to significant strength loss. Conditions that are permanently moist are the best environments for soft rot to grow. Wood that is buried in the ground or in constant contact with the ground is most susceptible to soft rot (Morris). In most other situations, other fungi are more aggressive and kill off soft rot before it can affect the wood. The attack of the wood occurs from the surface to the center of the wood. If caught early enough, the center of the wood may still be in good condition. Soft rot is typically a dull brown or a blue-gray color (Shupe et. al. 2008).
Brown Rot (Dry Rot) Brown rot, also known as dry rot, is one of the most common forms of decay found in structures. It tends to attack softwoods, which are the predominantly used wood type in residential struc tures. Similar to soft rot, brown rot reduces wood strength by breaking down cellulose. It also modifies the lignin in the wood, leaving it a brown appearance (See Figure 4). This is an easy way to distinguish brown rot from soft rot. Brown rot starts by bre aking down the long strains of cellulose into shorter lengths. Cracks perpendicular to the grain develop as a result. As time progresses the wood starts to shrink and develop cracks parallel to the grain. The combination of cracking results in a cubic crack in the surface (see Figure 5). Because brown rot attacks the cellulose first, significant strength loss can occur rapidly. Under ideal conditions, the compression strength, parallel and perpendicular to the grain, can undergo a 15% to 60% loss in under a week (Morris).
Figure 5 - Incipient Brown Rot Decay (Photo Credit: Controlling Decay in Waterfront Structures, USDA)
White Rot (Wet Rot) White rot, also known as wet rot, is just as common as brown rot, but occurs typically in hardwood. Because hardwood is less commonly used in structures, it is less common to find white rot. White rot degrades both the lignin and cellulose. The cellulose breakdown that occurs from white rot is a slower process compared to brown rot, thereby allowing for a slow loss in strength. After advanced decay starts to take hold, the wood surface appears stringy in texture (See Figure 6). The wood will fell punky and will have a bleached color (Morris).
Both brown and white rot occur in similar environmental conditions, but vary only because of the wood present. Through a variety of means, spores, produced by large fruiting structures, make their way to a piece of wood in an environment that satisfies all four critical elements. Once the spores land on the wood, a minimum of 29% moisture content is required for both brown and white rot in order for the fungus to start the infection. If wo od were left in an environment with 96% relative humidity at around 70F, the resulting moisture content would be around 30%. After the spores have infected the wood, mycelium and mycelia cords grow to help the fungus from drying. Mycelium and mycelia cords can spread to non-infected wood that has moisture content greater than 20%, which helps the fungus infect other areas that spores could not have infected. The ideal moisture content for fungus is between 40% and 80%. Beyond 120% it is very difficult for the fungus to spread due to a lack of free water, and it becomes dormant. On the other side of the threshold, wood that dried below 15% also causes the fungus to become dormant. IN either case the fungus is not dead. If moisture levels move back to optimum levels, the fungus will resume the infection. In the dormant state, many fungi can live up to 9 years (Morris). Figure 7 is a excerpt from a table that lists the moisture content of the wood and the corresponding colonization type, spore production, and strength loss associated with it. For the complete table see Understanding Biodeterioration of Wood inStructures by P. Morris (pg. 23)
Figure 7 - Fungi Growth Table (Credit: P. Morris, Understanding Biodeterioration of Wood in Structures)
Bacteria
Bacteria require a very high moisture content level. Wood that is completely submerged in water is typically the only situation where the required levels can be achieved. Bacteria that infect wood typically live on the non-structural components. This results a higher permeability, which can lead to other infections or infestations of other organisms. Some bacteria do break down the cellulose, but this process is extremely slow and can multiple decades to get to sufficient strength loss. Most bacteria are resistant to preservative treated woods and require other prevention methods.
Insects
Different areas in the world have different insects that infect wood. In many of those areas the natural species have resistive measure to the natural insects. As the trade between countries grows, wood species and insects are travelling to areas where the natural resistance does not exist. The four critical elements: temperature, moisture, oxygen, and a food source, are required by insects. While temperature and oxygen requirements are very similar to fungi, a minimum moisture content of 10% is needed (Anthony 2007). Since wood is typically kiln dried to around 16%, the wood in structures is very susceptible to insect infestations.
Termites Termites are one of the few insects that use wood as a food source for its entire lifetime (Anthony 2007). Termites are social insects that include a king, a queen, and workers (Morris). There are three major classifications of termites: damp wood termites, dry wood termites, and subterranean termites. Damp wood termites typically infect damp decayed wood. This has a very small economic impact on a structure since the structure was most likely already insufficient from the decay process. Dry wood termites infest sound dry wood. Most colonies are started from a mating pair of termites that fly into an area and start reproducing. Dry wood termites are usually found with blistering wood, six-sided pellets around the entry holes, and sounds of moving termites in the wood. Many times the extent of the damage cannot be observed on the surface. Finally, subterranean termites are the insect most people associate with termites. They require a connection to soil where all of their travel occurs. In order to create the connection, the termites build sheltering tubes out of earth, wood fragments, or bodily excretions (Morris). One of the most invasive subterranean termites is known as Formosan Termites.
Formosan Termites Formosan termites were introduced into the United States (US) from Asia during the late 1950’s through ports in the south (Gregorie 2012). Since their introduction, they have become the most destructive termite in the US, specifically New Orleans area. They annually cause roughly 500 million dollars in damage to the Greater New Orleans area, and roughly 2 billion dollars in damage nationwide. Significant research is being conducted to better understand these termites, but preliminary figures estimate the termites could spread as far north as Washington and Massachusetts (Marx 2000). These termites are extremely aggressive and defensive, extremely mobile, and have been known to eat through everything from PVC pipe and mortar to thin metal and electrical lines. Colonies are estimated to contain several million termites, which is significantly larger than native termites which typically have less than several thousand. They typically swarm in the spring and early summer. Although they are classified as subterranean termites, they are capable of flying and sometimes establish colonies independent from the ground. This is only possible if there is sufficient moisture present. Individual termites are known to devour southern wood twice as fast as the native species (Gregorie 2012).
Figure 8 - Carpenter Ant Damage (Photo Credit: Controlling Decay in Waterfront Structures, USDA)
Carpenter Ants Carpenter ants are easily identified by their large, completely black bodies. Similar to termites, colonies include a king, a queen, and workers (Shupe et. al. 2008). Carpenter ants do not use wood a food source, but excavate it for use as a shelter. Common ant food sources are vegetation and sugary household products (Morris). They tend to excavate softer wood, and prefer wood that has been softened by decay. Many infestations are found from hearing noise of excavation and movement and finding large clean holes in the wood (Anthony 2007) (See Figure 8).
Figure 9 - Powder Post Beetle Damage (Photo Credit: Controlling Decay in Waterfront Structures, USDA)
Powder Post Beetles Powder post beetles are one of the more common wood-boring beetles. They are commonly found with wood frass and small holes in the wood (~1/16”) (Anthony 2007) (See Figure 9). Adult powder post beetles do not consume the wood, they excavate it. This m akes them immune to many preservatives, which are meant to act as stomach poison for many insects. Powder post lava do consume the wood. They feed on the cellulose of the wood in order to grow. After they have sufficiently grown they exit through the surface, leaving behind the 1/16” holes. An individual generation of powder post beetles does not typically incur significant structural damage. If left go for a number of years, multiple generations of larva can and have reduced the structural integrity of many structures (Morris).
Other common insects Among the common insects that have specifically been discussed above, a number of other insects are known to infest wood structures. Each insect exhibits similar behavior to the powder post beetle, because the primary consumer of wood is the larva. Below is a list of other known insects that infest wood:
Lyctid Beetle
Anobiid Beetle
Bostrichid Beetle
Long-horned Beetle
Old House Borer
Carpenter Bees
(Shupe et. al. 2008)
Prevention Methods
Physical Defects
Physical defect prevention methods were not covered extensively in this installment. Please see the Physical Defects section under Types of Wood Defects for sources that contain information on physical defects and their corresponding prevention and remediation.
Organic Defects
Fungi, Bacteria, and Insects all require some level of the four critical elements: temperature, moisture, oxygen, and a food source. The easiest way to prevent organic defects in wood is to eliminate one of the four elements. In most cases three of the four are given in residential structures. The temperature is a given at a level comfortable for humans, ideal for organic growth. A food source is also given in the form of the wood itself, all fungi rely on wood as the food source. Oxygen is always present as well, since there are very little underwater applications in residential. The only controllable element in residential structures is the moisture (Shupe et. al. 2008). In many cases moisture can be controlled using proper construction techniques, materials, and details. Waterproofing details are critical in preventing moisture into indoor area. In other cases moisture cannot be avoided (Anthony 2012). In such cases, preservatives should be utilized to prevent decay (Shupe et. al. 2008).
Preservatives In many cases preservative treatments are warranted where moisture levels are not controllable. There are two major types of preservatives oil-type and waterborne. The primary difference is in the solution base used to transport the preservative chemicals into the wood. Oil-type preservatives are used in applications of high moisture, because oil is a natural replant to water. However, the surface of the treated wood keeps the oil finish and makes it difficult to apply other finishes. Waterborne preservatives are commonly used when the wood will be placed in interior applications. The wood has a dried finish surface making it very easy to apply alternative finished. Each preservative type has specific sub classes that are designed for specific applications. When selecting a preservative to be used in structural applications, it is important to understand all of the long term environments the wood may experience (Shupe et. al. 2008).
Termite Prevention The Formosan termite infestations are alarming a large number of residences. This section applies to all termites, but is specifically geared towards preventing Formosan termites. There are key steps that can be taken during the design and construction process that significantly help in preventing termites. First is removing any potential food sources. This includes construction waste, old tree stumps, and pretty much any wood item buried in the ground. By eliminating exterior food sources, termites are less like to come near the house. Soil treatment barriers can also be used in preventing termites. Chemicals can be added to the soil as well as physical barriers such as graded gravel and termite resistant steel mesh. While both items are better at preventing native termites, Formosan termites can fly and are not usually stopped by ground barriers. The most effective way to prevent termites is in the details. Minimizing small openings and properly detailing all foundations makes it very difficult for termites to find an entry point. Finally, preservative treated wood can be used at a minimal cost to the project. Treated wood should be utilized as the last line of defense for termites (Marx 2000)
Remediation Methods
Physical Defects
Physical defect remediation methods were not covered extensively in this installment. Please see the Physical Defects section under Types of Wood Defects for sources that contain information on physical defects and their corresponding prevention and remediation.
Organic Defects
All organic defects have similar remediation methods. Defects caused by organisms can only be repaired once the organism is removed. In order to remove the organism, pesticides and/or fungicides may be warranted. After the organism is removed it is important to eliminate one of the four critical elements. As stated before, moisture is the element that is easiest to control. Therefore, the first step is to eliminate any sources of the moisture. This can be achieved through allowing ventilation into the space or by fixing construction details that were not properly installed. Next the wood should be dried rapidly to prevent any further spread. All of the infected area should be removed. This is essential in preventing a relapse in the organism infecting the wood. The remaining wood should be treated with preservatives to prevent future problems with organisms. If enough of the wood has been affected support members or complete replacement may be required.
Some insects are not as dependent on moisture as the fungi and can be eliminated using more simple methods. Insects, whose primary use wood as a food source for their young, can be eliminated by preventing the insects from laying future eggs. This can be achieved by applying paint or a varnish to the wood. If the infestation is too large, pesticides will be necessary. Similar to the case above, if enough wood has been excavated by the insect, additional members or a complete replacement may be necessary.
Key Words: Wood Defects, Wood Remediation, Mold and Mildew, Staining Fungi, Dry Rot, Brown Rot, Wet Rot, White Rot, Wood decay, Termites, Carpenter Ants, Powder Post Beetles, Wood Preservatives
Annotated Bibliography
Anthony R., (2012) “Deterioration of Wood Structures – Basic Forensic Considerations”, Forensic Engineering 2012. November 2012, pp. 907-917
This article discusses common methods for determining decay rates. It also overviews common factors that affect decay rates and gives two case studies involving decaying wood structures.
Anthony, R. D., (2007), “Practice Points - Basics of Wood Inspection: Considerations for Historic Preservation”,APT Bulletin, The Journal of Preservation Technology, Vol. 38, No. 2-3.
This article discusses the need for wood inspections focusing on deterioration of the wood over time. It addresses evaluating material and properties of the wood and the tools required for inspections.
Delgado A., de Brito J. , and Silvestre J.D., (2013), “Inspection and Diagnosis System for Wood Flooring”, Journal of Performance of Constructed Facilites, ASCE, Vol 27, No. 5, pp. 564-574.
This article is aimed specifically towards the identification and remediation of wood floor systems. It relates the floor defect to probable causes. The article includes a case study detailing the evaluation of the defect and probable methods for repair.
Dunham, L. (2013), “Decayed Wood Structures”, STRUCTURE magazine, October 2013 Edition, pp. 21-24.
This article discusses probable causes of wood decay due to water and resulting material capacities. It focuses on identifying the duration of the decay process to aid engineers in providing a viable solution.
Gerwick, J., Trenkwalder, T., and Kearney, J., (2010),"Practical Repair of Timber Structures", Ports 2010. April 2010, pp. 999-1008
The authors discuss methods they have used in the repair of waterfront timber structures. They include other repair methods used throughout the industry today.
Gregorie, J., (2012), "Structural Evaluation and Repair of Formosan Termite Damage in Wood Structures: Lessons Learned and Two Case Studies", Forensic Engineering 2012. November 2012, pp. 1247-1256
This article explore overviews the current Formosan termite infestations occurring in New Orleans. Gregorie presents two case studies that involve Formosan termites on a historic structural and a utility shed.
Highley, T. and Scheffer T, (1989), “Controlling Decay in Waterfront Structures Evaluation, Prevention, and Remedial Treatments”, FPL-RP-494, September 1989, Forest Products Laboratory Research Paper.
This research paper includes information about the underlying causes of deterioration, identifying decay, building procedures that lead to decay, and procedures for preserving the decayed wood. The report is aimed at inspectors and maintenance crews of wood structures constantly exposed to water.
This article discusses common causes of wood thermal degradation. It also discusses the process under which each component of the wood undergoes.
Morris, P.I., “Understanding Biodeterioration of Wood in Structures”, Forintek Canada Corporation.
This document provides information about decay of wood structures by biological organism. It lays out the process that leads up to wood decay, and gives suggestions for repair and prevention.
Ratay, R.T. and Owen C.R. (2010), Forensic Structural Engineering Handbook, 2nd Ed. McGraw Hill, New York, N.Y.
The book contains a chapter on timber structures, chapter 14. The chapter details engineering properties, causes of failures, and repair types of timber. Eleven case studies are included with the chapter detailing a variety of nonperformance and failures involving timber structures.
Seirup, P., (1993), “Inspecting Wood Frame Structures”, ASHI Technical Journal, Spring 1993, pp. 15-26.
This article is directed toward home inspectors. It focuses on informing the inspectors about observations made during an inspection and what information can be obtained from those observations.
Shupe T., Lebow S., and Ring D., (2008), “Causes and Control of Wood Decay, Degradation and Stain”, Louisiana State University Agricultrual Center, Pub. 2703.
This report explores the decay of wood over time focusing on insect infested wood and stain causation. It presents multiple solutions to the insect infestations and stains that could be used to slow down or stop the decay.
Singh, J. and White, N., (1997), “Timber Decay in Buildings: Pathology and Control”, Journal of Performance of Constructed Facilities, ASCE, Vol 11, No. 1, pp. 3-12.
In this journal article, Singh and White discuss the interaction of structure with the other building components. They draw attention to the prevention of decay through control of the other building components.
An Overview
Table of Contents
Introduction
Wood structures are all around us in a variety of forms ranging from dimensioned lumber and engineered lumber framed residences to heavy timber structures. In order to maintain these structures, engineers and builders must understand how to identify defects that are present during construction as well as deterioration that occurs over a period of time. Proper techniques must be used to remediate the defect at hand and to prevent the further deterioration of the structure. Defects in wood can be classified as splits, checks, warps, rotting, insect infestation, or decay due to other biological organism. Understanding what defect is present and discovering the source of the defect is crucial in determining a potential solution to prevent further damage to the structure. Identifying a number of possible resolutions is important to determine a feasible remediation of the problem. This installment will focus on the deterioration of wood and its recommended remediation methods.Understanding Wood
Wood has been used for thousands of years by cultures all throughout the world as a structural material. Even in its living form wood acts as the structural member supporting the leaves of the tree. When assessing wood in structural applications, it is important to understand it in its natural habitat, forests.Wood gains its strength from two major components, cellulose and lignin. Cellulose is the compound on which a majority of the strength is formed. Lignin acts as the glue that holds the cellulose together to create the required strength. When either component breaks down or is destroyed, significant, if not all, strength loss occurs. Each wood type contains different proportions of cellulose and lignin. Hardwoods tend to be comprised of less lignin when compared to softwoods (Morris).
Each layer of a tree has a specific task that aids the tree throughout its life time. Only two of those layers are used in structural applications. Natural protection measures are eliminated with the other layers and need to be replaced in order to prevent wood from developing defects. The most outer layer of the tree is the outer bark. Outer bark acts as the first level of protection for the tree. Many organisms that attack wood are stopped by the outer bark. The next layer in is the inner bark, also known as the phloem. Phloem is responsible for transporting carbohydrates and amino acids that are used by a living tree for growth. Phloem is sometimes used as a storage device for them as well. The cambium layer follows, consisting cells that create phloem and the sapwood cells for the layers on each side of it. The growth of a tree occurs in the cambium layer. Sapwood is next, and is the first layer used in structural applications. Sapwood is responsible for conducting water from the roots to the leaves through the use of capillary action. This natural water flow is why sapwood absorbs water most quickly along the grain. There are many ways in which water can travel perpendicular to the grain for distribution. These ways contain large amounts of highly organic matter that is easily degradable. For this reason, sapwood is very vulnerable to organic defects. The innermost layer of a tree is the heartwood. Heartwood is sapwood that is no longer in use. Its sole purpose is to act as a structural core for the tree.
When sapwood is converted to heartwood, natural toxins are infused into it to aid in decay prevention. However, these toxins are easily dissolved by water, and leave cut wood while it is still the mill yard. Every tree species contains different levels of heartwood and sapwood, and the heartwood durability varies as well. Figure 1 is an excerpt from a table listing the wood species, predominant layer type (sapwood versus heartwood), and the relative heartwood durability (Morris). Please see Understanding Biodeterioration of Wood in Structures by P. Morris for the full table (pg. 4).
Over the course of its lifetime, wood can be exposed to a variety of humidity levels. As a result, the moisture content varies as well. Controlling the moisture content of wood is necessary in preventing a large number of defects. Moisture content can be calculated by taking the weight of the water and dividing it by the oven dried weight of the wood, then expressing it as a percentage (Morris). Below is Table 1 summarizing common moisture contents wood experiences throughout its lifetime.
Types of Structures
Residential
Residential structures are predominately constructed using dimension lumber. Member sized range from 2” by 4” to 2” by 12”. Plywood and Oriented strand board are often used as floor and siding elements. In more recent years, the residential industry has adopted the use of more engineered products, replacing members that were previously dimension lumber. Defects associated with residential structures range from physical to thermal to organic. The organic defects that are most common to residential structures are those that require air.Heavy Timber
Heavy timber structures have similar defects to those found in residential structures. In order to be classified as heavy timber, members must have minimum sizes that are determined by their function.Waterfront
Waterfront structures were not extensively considered in this installment. Information on waterfront structures and their defects can be found in the following sources:Types of Wood Defects
Physical Defects
There are a large number of defects than can be classified as physical. Some of them are as follows: checks, splits, warping, and weathering. All influence the structural behavior in one way or another. Checks typically occur from differential shrinkage of the wood during the drying process. They are cracks in the wood that do no propagate entire through it. Checks usually do not have large impact on structural integrity. However, if there is a large number, or the checks are large, strength loss can occur. Checks that do propagate all the way through wood are known as splits. Splits do have a significant impact on the structural integrity of the wood. Splits can also be caused by member overload or an impact load. Other side effects of differential shrinkage include bowing, warping, twisting, and cupping (Dunham 2013).Wood that is exposed to nature can experience weathering. Weathering does not typically have a significant impact on the long term strength of the wood, but it can affect the impact strength of the wood. It can be a sign of structural degradation below the surface. Weathering is caused by repeated wetting and drying cycles, ultraviolet light exposure, or wearing of the surface due to wind-blown debris (Dunham 2013).
More information on physical defects can be found in the following sources:
Thermal Degradation
Over long periods of time, wood can experience degradation due to different temperature exposures. Each species responds differently depending on the environmental conditions where the wood is placed.More information on thermal degradation can be found in the following source:
Organic Defects
At the end of a trees life, the organic components, cellulose and lignin, need to be broken down in order for a forest to progress through its natural cycles. Many of the causes of organic defects discussed in this post are responsible for this breakdown. Because this decay process is natural, using wood in structural applications that are expected to lasts for decades defies the very nature of wood (Morris).In order for any organism to infect wood, four critical elements must be present: temperature, moisture, oxygen, and a food source. For many organisms that affect wood, the proper temperatures are very close to those associated with human comfort. Moisture can be added to wood through a number of ways. Some common causes of moisture in wood are faulty ventilation around the wood, exposure of the wood to humid environments, and wood contact with the ground. Oxygen from air is vital for the organisms to live. For this reason, many of the organisms discussed here only affect above water structures. Lastly the food source is often the wood itself. However, sometimes alternative foods sources must be present and the wood is used as a shelter for the organisms (Morris).
Fungi
As with all organisms, fungi require the four critical elements. Specifically to fungi, moisture in the wood needs to be in the form of free water. This is why it is very uncommon to find fungal infestations of completely submerged structures (Dunham 2013). Fungi spread through spores that are produced by fruiting bodies. Those spores are often spread by wind, water, or insects. In some form or another, the spores find their way to wood that is in an environment that meets the four critical elements. The most common time for spore production occurs in later summer and fall. Once wood has been infected with spores, mycelium start to grow, and function as the backbone of the infestation. Mycelia are tubular cells that grow like the branches on a tree to deliver essential items to the fungi.An infection of a fungus occurs through three major stages incipient decay, intermediate decay, and advanced decay. Each stage adds upon the other, and results in less structural integrity of the wood. Incipient decay is the first stage. Signs of it include discoloration, minimal section loss, and punky surface. Next, in the intermediate decay stage, small voids appear on the surface as the fungus moves into the wood. Finally in advanced decay, the small voids grow larger and the fungus has infected most of the cross section. In this stage, the actual depth of the decay varies depending on the fungus, wood species, and environmental conditions (Anthony 2007).
Throughout the stages of decay, wood losses strength through two major avenues, section loss and mechanical property loss. In the early stages of fungal decay, wood section loss is not substantial. However, the mechanical properties can see a significant decrease. Relatively speaking, a 10% loss in section is often associated with a 50% loss in the mechanical properties of the wood. Impact strength of wood is the first property to degrade. The two most common strengths that follow are compression perpendicular to grain and compression parallel to grain. Because mechanical property loss can occur prior to and visible decay, some physical defects can be signs of fungal decay (Dunham 2013). Some fungi result in no strength loss, but can lead to other fungal infections. The microbial bio deterioration process is a competition between all types of fungi to infect the wood. Different conditions favor one type over another, but as conditions change an existing infection may allow for a new one to easily take over it (Morris).
Molds and Staining Fungi
Molds and staining fungi do not cause significant section loss of wood. Normally only the impact strength is affected for even the most advanced stages of decay (Morris). Most require relatively high levels of humidity in order to raise the moisture content of the wood to acceptable levels. The minimum moisture content required for molds and staining fungi is 20%. Sapwood is the more susceptible to mold and staining fungi infections.
Molds have colorless or pale mycelium and often look like a “cottony” growth on the surface of the wood (See Figure 2). The growth on the wood can appear in a variety of colors depending on the species of the mold (Shupe et. al. 2008).
Staining fungi come in a variety of forms, each with its own degradation effects. In general, staining fungi has brown or black mycelium (Morris) (See Figure 3). Staining fungi do not die when the wood dries out, they merely go dormant. Dormant staining fungi can live for long periods of time, and will resume activity once the moisture level threshold is met. The major type of staining fungi is blue stain. It only occurs in sapwood, and given by its name turns the surface of the wood blue. The most economical impact of blue stain is the downgrading of the wood grade.
Soft Rot
Soft rot is most common in hardwoods, specifically oaks and maples. It is capable of breaking down the cellulose with the wood which leads to significant strength loss. Conditions that are permanently moist are the best environments for soft rot to grow. Wood that is buried in the ground or in constant contact with the ground is most susceptible to soft rot (Morris). In most other situations, other fungi are more aggressive and kill off soft rot before it can affect the wood. The attack of the wood occurs from the surface to the center of the wood. If caught early enough, the center of the wood may still be in good condition. Soft rot is typically a dull brown or a blue-gray color (Shupe et. al. 2008).
Brown Rot (Dry Rot)
Brown rot, also known as dry rot, is one of the most common forms of decay found in structures. It tends to attack softwoods, which are the predominantly used wood type in residential struc
tures. Similar to soft rot, brown rot reduces wood strength by breaking down cellulose. It also modifies the lignin in the wood, leaving it a brown appearance (See Figure 4). This is an easy way to distinguish brown rot from soft rot. Brown rot starts by bre
aking down the long strains of cellulose into shorter lengths. Cracks perpendicular to the grain develop as a result. As time progresses the wood starts to shrink and develop cracks parallel to the grain. The combination of cracking results in a cubic crack in the surface (see Figure 5). Because brown rot attacks the cellulose first, significant strength loss can occur rapidly. Under ideal conditions, the compression strength, parallel and perpendicular to the grain, can undergo a 15% to 60% loss in under a week (Morris).
White Rot (Wet Rot)
White rot, also known as wet rot, is just as common as brown rot, but occurs typically in hardwood. Because hardwood is less commonly used in structures, it is less common to find white rot. White rot degrades both the lignin and cellulose. The cellulose breakdown that occurs from white rot is a slower process compared to brown rot, thereby allowing for a slow loss in strength. After advanced decay starts to take hold, the wood surface appears stringy in texture (See Figure 6). The wood will fell punky and will have a bleached color (Morris).
Both brown and white rot occur in similar environmental conditions, but vary only because of the wood present. Through a variety of means, spores, produced by large fruiting structures, make their way to a piece of wood in an environment that satisfies all four critical elements. Once the spores land on the wood, a minimum of 29% moisture content is required for both brown and white rot in order for the fungus to start the infection. If wo
od were left in an environment with 96% relative humidity at around 70F, the resulting moisture content would be around 30%. After the spores have infected the wood, mycelium and mycelia cords grow to help the fungus from drying. Mycelium and mycelia cords can spread to non-infected wood that has moisture content greater than 20%, which helps the fungus infect other areas that spores could not have infected. The ideal moisture content for fungus is between 40% and 80%. Beyond 120% it is very difficult for the fungus to spread due to a lack of free water, and it becomes dormant. On the other side of the threshold, wood that dried below 15% also causes the fungus to become dormant. IN either case the fungus is not dead. If moisture levels move back to optimum levels, the fungus will resume the infection. In the dormant state, many fungi can live up to 9 years (Morris).
Figure 7 is a excerpt from a table that lists the moisture content of the wood and the corresponding colonization type, spore production, and strength loss associated with it. For the complete table see Understanding Biodeterioration of Wood inStructures by P. Morris (pg. 23)
Bacteria
Bacteria require a very high moisture content level. Wood that is completely submerged in water is typically the only situation where the required levels can be achieved. Bacteria that infect wood typically live on the non-structural components. This results a higher permeability, which can lead to other infections or infestations of other organisms. Some bacteria do break down the cellulose, but this process is extremely slow and can multiple decades to get to sufficient strength loss. Most bacteria are resistant to preservative treated woods and require other prevention methods.Insects
Different areas in the world have different insects that infect wood. In many of those areas the natural species have resistive measure to the natural insects. As the trade between countries grows, wood species and insects are travelling to areas where the natural resistance does not exist. The four critical elements: temperature, moisture, oxygen, and a food source, are required by insects. While temperature and oxygen requirements are very similar to fungi, a minimum moisture content of 10% is needed (Anthony 2007). Since wood is typically kiln dried to around 16%, the wood in structures is very susceptible to insect infestations.Termites
Termites are one of the few insects that use wood as a food source for its entire lifetime (Anthony 2007). Termites are social insects that include a king, a queen, and workers (Morris). There are three major classifications of termites: damp wood termites, dry wood termites, and subterranean termites. Damp wood termites typically infect damp decayed wood. This has a very small economic impact on a structure since the structure was most likely already insufficient from the decay process. Dry wood termites infest sound dry wood. Most colonies are started from a mating pair of termites that fly into an area and start reproducing. Dry wood termites are usually found with blistering wood, six-sided pellets around the entry holes, and sounds of moving termites in the wood. Many times the extent of the damage cannot be observed on the surface. Finally, subterranean termites are the insect most people associate with termites. They require a connection to soil where all of their travel occurs. In order to create the connection, the termites build sheltering tubes out of earth, wood fragments, or bodily excretions (Morris). One of the most invasive subterranean termites is known as Formosan Termites.
Formosan Termites
Formosan termites were introduced into the United States (US) from Asia during the late 1950’s through ports in the south (Gregorie 2012). Since their introduction, they have become the most destructive termite in the US, specifically New Orleans area. They annually cause roughly 500 million dollars in damage to the Greater New Orleans area, and roughly 2 billion dollars in damage nationwide. Significant research is being conducted to better understand these termites, but preliminary figures estimate the termites could spread as far north as Washington and Massachusetts (Marx 2000). These termites are extremely aggressive and defensive, extremely mobile, and have been known to eat through everything from PVC pipe and mortar to thin metal and electrical lines. Colonies are estimated to contain several million termites, which is significantly larger than native termites which typically have less than several thousand. They typically swarm in the spring and early summer. Although they are classified as subterranean termites, they are capable of flying and sometimes establish colonies independent from the ground. This is only possible if there is sufficient moisture present. Individual termites are known to devour southern wood twice as fast as the native species (Gregorie 2012).
Carpenter Ants
Carpenter ants are easily identified by their large, completely black bodies. Similar to termites, colonies include a king, a queen, and workers (Shupe et. al. 2008). Carpenter ants do not use wood a food source, but excavate it for use as a shelter. Common ant food sources are vegetation and sugary household products (Morris). They tend to excavate softer wood, and prefer wood that has been softened by decay. Many infestations are found from hearing noise of excavation and movement and finding large clean holes in the wood (Anthony 2007) (See Figure 8).
Powder Post Beetles
Powder post beetles are one of the more common wood-boring beetles. They are commonly found with wood frass and small holes in the
wood (~1/16”) (Anthony 2007) (See Figure 9). Adult powder post beetles do not consume the wood, they excavate it. This m
akes them immune to many preservatives, which are meant to act as stomach poison for many insects. Powder post lava do consume the wood. They feed on the cellulose of the wood in order to grow. After they have sufficiently grown they exit through the surface, leaving behind the 1/16” holes. An individual generation of powder post beetles does not typically incur significant structural damage. If left go for a number of years, multiple generations of larva can and have reduced the structural integrity of many structures (Morris).
Other common insects
Among the common insects that have specifically been discussed above, a number of other insects are known to infest wood structures. Each insect exhibits similar behavior to the powder post beetle, because the primary consumer of wood is the larva. Below is a list of other known insects that infest wood:
- Lyctid Beetle
- Anobiid Beetle
- Bostrichid Beetle
- Long-horned Beetle
- Old House Borer
- Carpenter Bees
(Shupe et. al. 2008)Prevention Methods
Physical Defects
Physical defect prevention methods were not covered extensively in this installment. Please see the Physical Defects section under Types of Wood Defects for sources that contain information on physical defects and their corresponding prevention and remediation.Organic Defects
Fungi, Bacteria, and Insects all require some level of the four critical elements: temperature, moisture, oxygen, and a food source. The easiest way to prevent organic defects in wood is to eliminate one of the four elements. In most cases three of the four are given in residential structures. The temperature is a given at a level comfortable for humans, ideal for organic growth. A food source is also given in the form of the wood itself, all fungi rely on wood as the food source. Oxygen is always present as well, since there are very little underwater applications in residential. The only controllable element in residential structures is the moisture (Shupe et. al. 2008). In many cases moisture can be controlled using proper construction techniques, materials, and details. Waterproofing details are critical in preventing moisture into indoor area. In other cases moisture cannot be avoided (Anthony 2012). In such cases, preservatives should be utilized to prevent decay (Shupe et. al. 2008).Preservatives
In many cases preservative treatments are warranted where moisture levels are not controllable. There are two major types of preservatives oil-type and waterborne. The primary difference is in the solution base used to transport the preservative chemicals into the wood. Oil-type preservatives are used in applications of high moisture, because oil is a natural replant to water. However, the surface of the treated wood keeps the oil finish and makes it difficult to apply other finishes. Waterborne preservatives are commonly used when the wood will be placed in interior applications. The wood has a dried finish surface making it very easy to apply alternative finished. Each preservative type has specific sub classes that are designed for specific applications. When selecting a preservative to be used in structural applications, it is important to understand all of the long term environments the wood may experience (Shupe et. al. 2008).
Termite Prevention
The Formosan termite infestations are alarming a large number of residences. This section applies to all termites, but is specifically geared towards preventing Formosan termites. There are key steps that can be taken during the design and construction process that significantly help in preventing termites. First is removing any potential food sources. This includes construction waste, old tree stumps, and pretty much any wood item buried in the ground. By eliminating exterior food sources, termites are less like to come near the house. Soil treatment barriers can also be used in preventing termites. Chemicals can be added to the soil as well as physical barriers such as graded gravel and termite resistant steel mesh. While both items are better at preventing native termites, Formosan termites can fly and are not usually stopped by ground barriers. The most effective way to prevent termites is in the details. Minimizing small openings and properly detailing all foundations makes it very difficult for termites to find an entry point. Finally, preservative treated wood can be used at a minimal cost to the project. Treated wood should be utilized as the last line of defense for termites (Marx 2000)
Remediation Methods
Physical Defects
Physical defect remediation methods were not covered extensively in this installment. Please see the Physical Defects section under Types of Wood Defects for sources that contain information on physical defects and their corresponding prevention and remediation.Organic Defects
All organic defects have similar remediation methods. Defects caused by organisms can only be repaired once the organism is removed. In order to remove the organism, pesticides and/or fungicides may be warranted. After the organism is removed it is important to eliminate one of the four critical elements. As stated before, moisture is the element that is easiest to control. Therefore, the first step is to eliminate any sources of the moisture. This can be achieved through allowing ventilation into the space or by fixing construction details that were not properly installed. Next the wood should be dried rapidly to prevent any further spread. All of the infected area should be removed. This is essential in preventing a relapse in the organism infecting the wood. The remaining wood should be treated with preservatives to prevent future problems with organisms. If enough of the wood has been affected support members or complete replacement may be required.Some insects are not as dependent on moisture as the fungi and can be eliminated using more simple methods. Insects, whose primary use wood as a food source for their young, can be eliminated by preventing the insects from laying future eggs. This can be achieved by applying paint or a varnish to the wood. If the infestation is too large, pesticides will be necessary. Similar to the case above, if enough wood has been excavated by the insect, additional members or a complete replacement may be necessary.
Key Words:
Wood Defects, Wood Remediation, Mold and Mildew, Staining Fungi, Dry Rot, Brown Rot, Wet Rot, White Rot, Wood decay, Termites, Carpenter Ants, Powder Post Beetles, Wood Preservatives
Annotated Bibliography
Anthony R., (2012) “Deterioration of Wood Structures – Basic Forensic Considerations”, Forensic Engineering 2012. November 2012, pp. 907-917
Anthony, R. D., (2007), “Practice Points - Basics of Wood Inspection: Considerations for Historic Preservation”,APT Bulletin, The Journal of Preservation Technology, Vol. 38, No. 2-3.
Delgado A., de Brito J. , and Silvestre J.D., (2013), “Inspection and Diagnosis System for Wood Flooring”, Journal of Performance of Constructed Facilites, ASCE, Vol 27, No. 5, pp. 564-574.
Dunham, L. (2013), “Decayed Wood Structures”, STRUCTURE magazine, October 2013 Edition, pp. 21-24.
Gerwick, J., Trenkwalder, T., and Kearney, J., (2010),"Practical Repair of Timber Structures", Ports 2010. April 2010, pp. 999-1008
Gregorie, J., (2012), "Structural Evaluation and Repair of Formosan Termite Damage in Wood Structures: Lessons Learned and Two Case Studies", Forensic Engineering 2012. November 2012, pp. 1247-1256
Highley, T. and Scheffer T, (1989), “Controlling Decay in Waterfront Structures Evaluation, Prevention, and Remedial Treatments”, FPL-RP-494, September 1989, Forest Products Laboratory Research Paper.
Levan S., (1989), “Thermal Degradation”, Concise Encyclopedia of Wood & Wood-Based Materials, 1st Edition, Pergamon Press, Elmsford, N.Y.
Morris, P.I., “Understanding Biodeterioration of Wood in Structures”, Forintek Canada Corporation.
Ratay, R.T. and Owen C.R. (2010), Forensic Structural Engineering Handbook, 2nd Ed. McGraw Hill, New York, N.Y.
Seirup, P., (1993), “Inspecting Wood Frame Structures”, ASHI Technical Journal, Spring 1993, pp. 15-26.
Shupe T., Lebow S., and Ring D., (2008), “Causes and Control of Wood Decay, Degradation and Stain”, Louisiana State University Agricultrual Center, Pub. 2703.
Singh, J. and White, N., (1997), “Timber Decay in Buildings: Pathology and Control”, Journal of Performance of Constructed Facilities, ASCE, Vol 11, No. 1, pp. 3-12.