How Does Rain Affect Wood?
Moisture Content
Many problems can arise for woodworkers or homeowners when wood shrinks or swells due to moisture. In nature, before a tree is cut for use, its cell structure is saturated and the moisture content is very high. When a tree is cut, those cells dry out and the wood shrinks until it reaches a balance with the humidity of its environment. In other words, the atmospheric humidity determines the final dimension of the wood. R. Bruce Hoadley explains in his book, "Understanding Wood: A Craftsman's Guide to Wood Technology," that the moisture content (MC) of wood is measured as the ratio of the weight of water in a given piece of wood to the weight of the wood when it is completely dry, which is usually accomplished by oven drying (see Resources below). However, wood can also reabsorb moisture after it has dried if exposed to elements such as rain, mist and sleet.
Cracking, Warping and Molding
When a piece of wood dries on its own, the water and natural moisture such as sap evaporates from the cell cavities and fibers of the wood structure. Every species of tree is slightly different in how much moisture it can hold and how quickly it will dry. The orientation of the wood cells determines how wood will dry out ,and radial cracking, warping, cupping or twisting can occur as the wood shrinks and uneven drying occurs. Over time, wood can swell due to temperature changes and humidity, or if exposed to a lot of rain. A perfectly straight preshrunk board can reabsorb moisture into its cell structure and expand tangentially in a radial manner, or longitudinally. Wood pieces such as a drawer in a cabinet or dresser will swell slightly in the humidity of summer and shrink slightly in the cold, dry air of winter. Wood furniture, decks and homes that are built of wood and are exposed to the elements will not maintain their original look and can collect mold and rot if not properly pressure treated with a sealant.
Wood Drying Techniques and Treatments
The best season for a tree to be cut and dried varies widely depending on the species. In order to control the drying process and accomplish equilibrium moisture content (EMC), there are several methods that have been developed. Air-drying and kiln-drying are the main methods. Green wood is when all fibers are fully saturated, and if exposed to natural outdoor conditions, the wood will lose its water content through air-drying. Kiln-drying is used for most structural lumber to quickly reduce the weight of the wood for shipping, but wood can still reabsorb moisture later on. Preservative treatments can be applied to wood to prevent fungi or insects and to seal out moisture to prevent dimensional change and rot. These chemical treatments penetrate the pores of the wood, and this wood is generally labeled as "pressure treated" because it is treated in a pressure chamber for optimal penetration. There are also non-pressure treatments that can be applied topically by brush (see Resources below).
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