Monday, September 8, 2008

Natural Features of a Timber

Timber is a natural material, and as such, is affected by its surrounding environment – its climate, area vegetation, and wildlife. For instance:

As timbers go through the drying process, they shrink or swell until they reach equilibrium with the constantly changing level of moisture in the air of its environment.

Water is stored in wood in two forms: 1) as free water in the vessels and/or cells, used to move nutrients within the tree; 2) as cell (or bound) water, which is an integral part of the cell walls (about 30% of the water). As the tree seasons, or dries, it loses its free water first. Then, as the water in the bound cells near the surface is exposed it evaporates and the wood shrinks at a faster rate than the cells deep inside the wood. Because of the cell structure, wood shrinks primarily in width and thickness and very little in length. This uneven loss of moisture may result in twisting and cracks called “checks”. These checks typically do not extend through the timber. Opinions vary as to “checking” adding character to the timbers. Either way, if you are considering green timbers for a timber frame or post and beam home, you can expect to have some checking occur.

Splits are large checks, or cracks, that penetrate through the entire thickness of the timber, most often occurring near the ends. Splits can occur from poor handling or by drying stresses (the cut end-grain of the timber releases moisture most rapidly). Because they reduce the strength of the timber, split timbers are unsuitable for most timber frame applications.

Knots: As a branch starts to grow from the trunk of a tree, cells in the trunk bend around the branch, leading to a knot in the timber that is cut through the young branch. Knots vary in quantity according to wood species and timber size. Typically they are not a structural concern; however, knots can affect the timber’s strength if they are found in or near joinery because the wood in the center of a knot may have lower strength and knots located near the edges of the timber may cause variations in the grain near the corners, resulting in a significant reduction of strength.

Some wood species (like hemlock) are subject to ring shake, which is the separation of the rings in a timber that occurs during the drying process. Hemlock, like other species, may be ordered “shake free” if desired, for an additional charge.

Storms producing high winds can bend trees enough to crack the layer beneath the bark, making an opening between the grains of wood. When this happens, pitch or sap can flow into the crack and then heal, creating a pitch pocket. Pitch pockets may be considered a defect in the wood and can compromise the structural integrity of a timber.

Bluestain is a blue-toned discoloration in softwoods, especially pines, occurring only in the outer part of the tree, the sapwood, closest to the bark. Many confuse this with mold, which can grow on the wood’s surface after it is processed; however, bluestain is caused by a type of harmless fungus. Bluestain does not affect the timber’s integrity, but where appearance is important bluestained wood may not be desirable.

Mold is one of the most common types of fungus on the earth. When stored in a humid setting, timbers, especially pine, are susceptible to mold. Stains or streaks of mold range from blue-black to blue-gray and can even be brownish or purple. Although the mold does not cause timber decay, the discoloration causes aesthetic concerns as well as health and air quality concerns directly related to the mold. Mold is generally treatable; however, you want to catch it in the early stages.

Most of these natural features are acceptable in timber frame applications and give a home a unique personality. Your timber frame architect will be able to explain these terms in relation to your timber frame design and outline your options in wood type, species, and grade.

Bonin Architects & Associates, PLLC

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