The parts of a gable roof
the span. When you know the pitch of a roof plus the true span (measure the span from outside wall to outside wall with a long tape) or the run, you can determine the length of the common rafters. To do this, try using a book of rafter tables (see Sources on p. 198) or a pocket calculator.
A 24-ft.-wide building has a span of 24 ft. To find the rafter length of a 4-in – 12 pitch roof for this building, open your rafter-table book to the 4-in-12 page and look under the common-rafter table
at 24 ft. to see that the rafter length is 12 ft. 73/д in. If the span is 24 ft. 8 in., look under 8 in the inches column and add on an extra 41A in. That is the total length of the common rafter. It’s really that simple, so don’t make it difficult for yourself. After a few minutes with a rafter book, you can figure the length of almost any rafter for any pitch and any span. Subtract from this figure half the thickness of the ridge board (3A in. for a 2x ridge) and leave enough extra wood to cover the length of the tails in the overhang.