Wood used in roof framing is not a homogeneous material.
Various defects are present depending on the grade and method of cutting: plain sawn, quarter sawn and rift sawn.
Plain sawn (also known as flat sawn) is the type of cut most commonly found in lumber and the least expensive way to turn logs into lumber. The rings comprise 30 degrees or less of the board face.
In quarter sawn wood, the rings in the wood intersect at a 60 to 90-degree angle, creating an intersecting design in the wood.
Rift sawn wood is the most expensive type of wood, is milled perpendicular to the rings and has a unique linear design, but is extremely stable.
There are a number of defects such as knots, checks, pitch pockets and shakes that can be found in wood, which can affect its resilience:
Figure 1 shows a roof rafter failure brought on by a large knot that encompassed about half of the cross section of the board. The knot extended from the center all the way to the lower edge or tension side of the joist. The knot had not adhered well to the surrounding wood and a crack developed along the circumference of the knot. Since the crack entered the tension side of the joist, it propagated from the lower edge, resulting in severe reduction in strength. This left half of the cross section of the board supporting the load.
|1. Knots
A knot in a piece of framing lumber is a circular discontinuity caused by a tree branch. Small knots usually are not a problem since they adhere well to the wood board. Large knots cause a problem with strength in that they often become detached, leaving a large hole in the board and a significant reduction in strength.
Figure 2 shows a roof rafter with a large knot at the upper or compression end of a roof rafter. The knot has reduced the bearing strength of the board and deflection is noted at the location of the knot.
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