|As a tree grows, it responds to its environment in many ways. It inevitably develops characteristics that reduce the strength of lumber cut from it. The worst of these defects are: knots, cross grain, checks and splits, and reaction wood.
Knots are the most obvious lumber characteristic. A knot is a section of a branch that has been enveloped by the tree as it grew. If the branch was an early and low one that died and was encased along with its dead bark, the resulting knot is said to be loose or encased. A branch that was alive when the tree was cut produced intergrown or tight knots in the lumber, with uninterrupted contact between the wood cells and branch bark.
A knot’s shape is a function of how the plane of the lumber face intersects the branch. A face normal to the axis of the branch results in a round knot. If the face is nearly parallel to the branch axis, the knot is called a spike knot. Any orientation between normal and parallel makes oval knots.
Knots reduce lumber’s strength in several ways. A knot is an interruption in the fibers that would otherwise be aligned along the strong axis of the piece. The fibers that are left also deviate from the axis to pass around the knot. Finally, significant stress concentration exist near the knot.
Cross grain is a generic term describing wood fibers that are not aligned with the member’s major axis. Severe cross grain occurs when trees that have grown spirally or with pronounced taper are cut for lumber. Some cross grain is the inevitable result of cutting prismatic, rectangular pieces from a tapered mass of concentric cylinders. Cross grain drastically reduces the tensile strength of the lumber and can precipitate abrupt and early bending failures.
The most direct measure of cross grain is the slope of grain. Slope of grain is the angle, expressed as a ratio, between the wood fibers and the longitudinal axis of the lumber. A large slope (1 in 6) of grain is second only to knots in reducing the grade of a piece of lumber. Grain is a misleading term. The growth ring tracings on the lumber face can be parallel to the face, while the fibers are not. Often, a small pattern of checks will be the only indication of misaligned fibers.
Checks, Shakes, and Splits
Checks and splits are fractures in the wood that open as the lumber dries. Checks align with the longitudinal axis of the piece and are normal to the growth rings. Splits run through checks and are usually found at the ends of timber. The worst of the splits are often trimmed off after the lumber is dried as a final step in cutting lumber. Improper kiln-drying can accentuate splitting at the ends.
Shakes are thin voids within the tree that seem to occur naturally as the tree is growing. Shakes also align with the axis of the tree, but they lie in plane with the growth rings. It is as if the tree had delaminated slightly at a transition between spring and summer wood.
Timber’s strength relies upon the connection between adjacent wood cells. Though checks, shakes, and splits may close, the cells never reconnect across the fracture. By physically separating pieces of lumber, checks, shakes, and splits are detrimental in many ways. Just what and how much effect these separations have depends on the location and severity of the defects, and the end use of the timber.
Reaction wood is created by the tree as a response to its own changing dead load. As branches get longer and heavier, or if the tree is tilted for some reason, the wood is subjected to bending stresses. The tree responds by growing thicker-walled, denser cells at these locations. Most softwoods grow compression wood on the underside of the branches.
Most hardwoods, however grow tension wood on branch topsides. The denser reaction wood shrinks more as it dries than does the surrounding normal wood, resulting in dramatic and abrupt warps in any lumber that contains reaction wood.