Wood used in roof framing is not a homogeneous ma- terial. 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 af-
fect its resilience.
Knot
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.
Check
A check is a crack in a board brought
on by shrinkage. Shrinkage can occur
naturally, but typically it is a result of the
drying process. Wood moisture content
must be reduced to approximately 15
percent for maximum strength and lon-
gevity. Kiln drying often results in
surface checks in thick pieces of
wood. Surface cracks can coalesce
and form larger, more damaging
cracks.
Pitch pocket
This is a cavity in a piece of wood
caused by insects or some impact
to the tree. Sap fills the void and
results in a resin-filled pocket that
may appear solid, but can severely
reduce the strength of a board.
Shake
A shake is a crack that forms along
the growth rings from bacteriological growth. This is typically a
result of high moisture content in
the wood.
The following photos illustrate
how these defects can impact
wood’s ability to handle the stress
from roofs. When inspecting a
roof failure, it is important to consider the contributing role of any
wood defects.
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.
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.
Figure 3 shows a pitch pocket at the
Lumber Defects and
Roof Frame Failures