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Wood decay is a frequent problem in observed during Forensic inspections. On the microscopic level, the structure of wood is composed of hollow tubules running parallel to each other. The fibers are made up of cellulose & lignin. The point where the wood fibers can no longer absorb water is termed the Fiber Saturation Point, or FSP.

The FSP occurs around 20-30% moisture content in dimensional lumber but differs in manufactured wood products. When saturation passes the FSP, free water enables colonization by wood decay fungi, as the water is required for their metabolism.

Microscopic tubule structure of wood

Wood termite damage

Hydrogen bonding water distribution through a tree

A single water molecule, H2O, is made up of one Hydrogen and two Oxygen atoms. The molecule itself exhibits a electrical propensity and is termed a polar molecule, kind of like the north and south pole having different electrical charges. Water molecules stick together by way of this polarization, during which the Hydrogen molecule of Atom A is electrically connected to molecule B, like a magnet clinging to the fridge. This polar attraction is termed Hydrogen bonding.

Another example of the electrical bonding of water is surface tension. We all have seen a leaf floating in a lake. The downward weight of the leaf is less than the electrical forces holding the water together, so the leaf floats. In nature, Hydrogen bonding allows water to travel through the internal tubules and be distributed from root to branch.

Wood staining and decay commence when specific fungi colonize and inhabit the wood member, whether it be a fallen tree in a forest or a 2x4 inside the walls in your home. Multiple factors are required to occur in conjunction for decay to start and progress. Decay spores and free water must coexist in a suitable substrate that has a habitable temperature and pH. Once colonization occurs, it requires persistent moisture exposure to continue. At any time, when moisture exposure ceases, so does the progression of decay. Please visit our page on water mitigation and fungal remediation for further information.

Under the right conditions, the fungi initiate a metabolic breakdown of the wood fibers, effectively consuming the wood. This is a natural process that serves to return nutrients, recycled into the soil. It is undesirable in the built environment and a cause for concern as prolonged decay results in weakening of structural members.

Fungi colonizing a fallen tree

Various lumber cuts from a single tree

Moisture absorption of wood samples

Dimensional lumber for housing consists of many varying shapes cut from trees. The shape of each individual tree is analyzed and compared to demand for different wood profiles. The tree is then cut with an efficient layout to produce a variety of shapes. Larger members are sawn from the tree's heartwood, which is dense and dry. The outer sapwood layer is more porous and moist. Differences in lumber occur even in boards cut from the same tree and even more-so with boards from different stock or regions.

In a recent study we performed, we subjected wood samples to extended moisture exposure. Various sizes of solid wood lumber and common manufactured wood products absorbed water differently. Manufactured wood products absorbed water quicker initially and plywood absorbed more water by percent of sample weight. Moisture absorption patterns in different wood products influenced the staining produced during the study.

Wood veins run the long way up the piece of lumber or molding, with varying grain characteristics depending if the piece was cut from heartwood or sapwood. Due to the longitudinally oriented grain, solid wood tends to stain the long-way up a piece of wood. Moisture follows the small tubules which give wood its unique grain pattern, as the forest tree absorbs water from its root and distributes it to branches. I frequently observe this during inspections.

Longitudinal wood staining and decay

Plywood layers

Plywood long-way staining

Plywood is a manufactured wood product made of thin 1/8 inch layers bonded at right angles by synthetic resin adhesives. Plywood may stain longitudinally following the grain of a single layer. The alternating layers of wood interrupt the water from traveling too far within the piece, so staining may be localized, and not travel too far from the source.

Oriented Strand Board, or OSB, is another common manufactured wood product comprised of many small wood flakes. Cross-aligned interwoven strands are formed by multiple layered mats of wood flakes. This dense mesh of wood flakes is bonded under high heat and pressure. Due to the discontinuous, chopped-up wood grain, OSB stains are very localized in the presence of an isolated low-volume leak. The spread of water is interrupted by discontinuous grain.

OSB board wood flakes

OSB localized staining

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