How Does Wood Deteriorate?
While it is a strong material, in the right conditions, wood can experience a lot of deterioration. Damage can be from biological factors, such as insects and decay, or physical factors, such as sunlight and wind.
One major source of biological damage to wood is insects. There are a variety of insects that use wood for food, shelter, or movement. Termites are often associated with wood damage and cause severe damage from tunneling. Depending on the species, termites may live in the tunnels they create or in a nest nearby. For instance, Dampwood termites live in the wood while Subterranean termites live in the soil. Formosan termites, one of the most destructive species of termites, can live underground in the soil or above ground in nests.
While termites eat the wood they tunnel through, carpenter ants do not, although they still cause damage to wood. Carpenter ants are often attracted to a home by food waste, water sources, or overgrown vegetation. Once inside a home, they nest in wood, especially if it is wet and rotting. Carpenter ants can often be discovered by the sawdust piles that they leave behind during the nesting process.
Certain species of beetles can also cause damage to wood in your home. Beetles feed on the carbohydrates in the wood, in addition to using it as a place to lay their eggs. Depending on the type, the beetles infest the interior of the wood or stay near the surface. As you might guess, the wood boring beetle tunnels into the interior of the wood while bark beetles stay near the surface of the wood. Signs of a beetle infestation include piles of sawdust beside the wood and small holes in the wood. The amount of damage that beetles cause will vary depending on if they are reinfesting beetles or non-reinfesting beetles. Reinfesting beetles, will bore into the wood more than once, even after it is dry. Conversely, non-reinfesting beetles will only infest wood one time. This means they pose less of a threat to the wood.
Carpenter bees are also a potential source of wood damage since they bore tunnels in wood to create a place to lay their eggs. Although the tunnels created by carpenter bees, known as galleries, are relatively large in size, they are not an immediate threat to the wood structure. However, if the same areas are used over and over, the structural integrity can be significantly weakened.
Besides insects, the other major biological factor that can affect wood is fungi. The fungi that breaks down wood can be divided into 3 categories- decay, mold, and stain. All three types of fungi need some sort of moisture to be present and use the wood for nutrients.
Decay, also known as rot, is the most destructive of the fungi, as it can actually break down the wood. Depending on the specific type, decay may decompose the entire cell wall (white rot) or just the the cellulose and hemicelluloses in the cell wall (brown rot). No matter what type it is, decay can seriously impact the strength capacity of the wood.
Another type of fungi is mold which will grow on the surface of wood. Mold will appear as fuzzy, discoloration on the wood, when there are high moisture levels at the right temperature. Since mold only feeds on nutrients that are in the storage cells of wood and not the wood itself, it doesn’t have a significant impact on the strength. This makes mold relatively harmless to wood compared to decay. The issue with mold is the millions of spores it creates that travel throughout the air. These spores can be extremely detrimental to your living environment.
Similar to mold, stain fungi creates a discoloration in wood yet doesn’t cause significant damage to the structure. The difference is that stain fungi isn’t just on the surface and also fills openings in cellular structure. Because of this, stain fungi can’t be removed once it appears.The good news is that stain fungi only occurs in sapwood, which contains the needed sugars and starches that it feeds on.
In addition to mold and insects, physical factors can contribute to the deterioration of wood, as well. A notable source of physical damage to wood is sunlight, which can eventually lead to long term damage. When ultraviolet rays hit exposed sections of the wood, it can actually break down the wood fibers. Of course, when wood is outdoors, it’s not just sunlight that’s affecting it but also wind and water. The combination of sunlight, wind, and water eroding wood fibers is known as weathering. Typically, weathering is a slow erosion that only occurs on the surface of the wood. This means that weathering doesn't usually cause structural damage to your home. While weathering is a gradual process, it is faster with soft woods compared to hard woods.
Other physical factors that affect wood include checking and settling. Both of these are caused by the shrinkage that wood goes through during the drying process. As we mentioned in this article, wood contains two types of water- free water that circulates in the hollow parts of wood cells, and bound water that is attached to the wood cells. Since bound water is attached and thus harder to remove, only free water is removed when you begin drying wood. Removing free water won’t alter the shape of the wood. Once all of the free water is removed, wood reaches its Fiber Saturation Point (FSP). Any further drying of the wood will result in the removal of bound water. This means the structure of the wood cells will change, leading to a shrinkage of the wood. Typically, for every 3% of moisture reduction, the wood will lose 1% of it’s volume. Most of this shrinkage occurs across the the grain rather than in length.
One of the problems that wood shrinkage can lead to is settling. For instance, in a log home, where the logs are placed lengthwise, you may end up with a significant difference in height after settling. A 10 foot wall can settle more than 2 inches as it adapts to the moisture content of the environment.
Another issue that wood shrinkage may cause is checking. During the drying process, wood experiences uneven stress leading to wide cracks on the surface, known as checks. Unfortunately, checks in wood are unavoidable and are simply a natural part of the drying process.
Whether its wood boring beetles or weathering, there are many factors that negatively affect the strength of wood. This means it is crucial to ensure the wood you use is properly dried and properly maintained. Doing so, could help prevent costly damages.
Taylor, Adam M., Stephen L. Quarles, and Karen M. Vail. Wood Protection for Log Home Owners. The University of Tennessee. University of Tennessee Institute of Agriculture, n.d. Web. 20 May 2017.
"Technical Committee of the Log Homes Council, Building Systems Councils National Association of Home Builders. Preservation and Maintenance of Log Structures. N.p.: Technical Committee of the Log Homes Council, Building Systems Councils National Association of Home Builders, 2003. Log Homes Council, 2003. Web. 20 May 2017.