- Q.How do you figure rafter
lengths for a gable roof when one wall is 10 feet high, and the
other is 8 feet high (illustration A)? The span of the building
is 24 feet. We want the roof pitch to be the same on both
A.Will Holladay responds: Using a sample roof
pitch of 6/12, the first step is to calculate the rafter run
for the 2-foot difference in rafter heights. This is done by
dividing the difference in inches (24) by the rise ratio of
6/12 (.5), to arrive at a 48-inch run (24 ÷ .5 =
Next, add 48 inches to the 24-foot span to create a phantom
8-foot-high wall outside the 10-foot wall, as shown in
illustration B. Now, figure the rafter lengths for the 8-foot
wall using the 28-foot span.
The rafters on the 10-foot-high side can be most easily
figured by "installing" a 10-foot-high phantom wall 48 inches
in from the real 8-foot-high wall (illustration C), and
calculating the rafter lengths for a 20-foot span.
Will Holladay, a longtime framer, is the author of A
Roof Cutters Secrets to Framing the Custom House (W&H
Publishers, P.O. Box 3618, Santa Barbara, CA 93130).