Wood, a natural, organic material, is inherently subject to change when exposed to the environment over time. These alterations are driven by a combination of physical forces, chemical reactions, and biological activity. Understanding these processes is fundamental to predicting wood’s lifespan and maintaining its structural and aesthetic properties. Ultimately, these changes lead to the material’s decay and return to the natural cycle.
Weathering and Aesthetic Changes
Weathering is the most visible change to wood exposed outdoors. This surface degradation is primarily initiated by the ultraviolet (UV) portion of sunlight. UV radiation causes photo-degradation, breaking down lignin, the complex polymer that acts as the rigid glue holding the wood fibers together.
As the lignin near the surface degrades, rain washes it away, leaving behind the more resistant, lighter-colored cellulose fibers. This gradual erosion results in the common silver-gray appearance, often called a patina. This process is superficial, typically penetrating only a small fraction of a millimeter, meaning structural integrity remains largely unaffected by this aesthetic change alone.
Cycles of wetting and drying also contribute to surface breakdown. The fluctuating moisture content creates internal stresses, which manifest as small cracks on the surface called checking. Surface checking increases the wood’s roughness and allows water to penetrate slightly deeper, accelerating the overall weathering effect.
Biological Decay Mechanisms
Structural failure in wood is most often caused by biological decay, primarily through the actions of fungi and insects. Fungal decay, commonly known as rot, requires four conditions:
- A food source (the wood itself)
- Oxygen
- A favorable temperature
- Sufficient moisture
Decay typically begins when the wood’s moisture content consistently exceeds 20%.
Wood-rotting fungi are classified into three main types based on their method of attack. Brown rot fungi consume cellulose and hemicellulose, leaving behind dark, brittle lignin residue that causes the wood to crumble into a cubical pattern. White rot fungi use enzymes to break down all three major wood components—cellulose, hemicellulose, and lignin—resulting in a bleached, stringy, or spongy material.
Soft rot fungi are less aggressive and typically affect wood exposed to high moisture for long periods, such as fence posts in soil. These fungi bore microscopic cavities into the cell walls, causing the surface to soften and develop a cross-hatched pattern.
While fungi cause the most damage, wood-boring insects like termites and powderpost beetles also contribute to structural compromise. Termites feed on the cellulose and lignin within the wood structure, relying on symbiotic microorganisms to digest these polymers. Powderpost beetles cause damage during their larval stage, as the grubs tunnel through the wood, reducing the interior to a fine, flour-like powder before emerging as adults.
How Moisture Causes Dimensional Changes
Wood is a hygroscopic material, meaning it readily absorbs or releases water vapor from the surrounding air. This constant exchange causes the wood to physically change dimensions when the internal moisture content falls below the fiber saturation point (FSP). The FSP is the point where cell walls are fully saturated with bound water, generally around 30% moisture content.
Below the FSP, the removal of bound water causes shrinkage, while the addition of water causes swelling. Wood is an anisotropic material, meaning its dimensional change is unequal across different axes. It shrinks and swells most significantly tangential to the growth rings, about half as much radially, and only minimally along its length.
This uneven movement creates internal stresses, leading to defects like warping, splitting, or checking. These dimensional changes result from the wood seeking equilibrium moisture content with its fluctuating environment. The resulting instability can compromise wood joints and fastening systems over time.
Protecting Wood for Longevity
Mitigating the effects of weathering, decay, and dimensional change involves chemical treatments, surface finishes, and smart design. Chemical preservation is the most effective method for resisting biological decay, particularly in ground-contact applications. Pressure treatment forces biocides, such as copper-based compounds, deep into the wood’s cellular structure, poisoning the food source for fungi and insects.
Surface finishes, including stains and sealants, primarily regulate moisture and block UV light. Stains often contain pigments that absorb UV radiation, shielding the underlying lignin from photo-degradation and slowing the graying process. Sealants create a water-repellent barrier, preventing liquid water absorption that would push the wood’s moisture content above the decay threshold.
Proper architectural design and maintenance are equally important for longevity. Ensuring good ventilation allows wood to dry quickly after rain, keeping the moisture content low and preventing fungal colonization. Avoiding direct contact between wood and the ground or concrete prevents the wicking of moisture. Routine inspection and reapplication of surface finishes can restore the UV and moisture barrier, extending the service life of the wood.