By the pitch is meant the ratio of the rise to twice the run, i.e., to the total width of the building.

In a 1/2 pitch, the rise equals the run, or 1/2 the width of the building; in a 1/3 pitch the rise is 1/3 the width of the building; in a 3/4 pitch the rise is 3/4 the width of the building.
[Illustration: Fig.199.

Lumberman's Board Rule.] To find the length of a rafter by the use of the table, first find the required pitch, at the left end of the table.

Opposite this and under the graduation on the edge representing the run in feet, will be found the length of the rafter; e.g., a rafter having a run of 12' with a 1/4 pitch, is 13' 5" long, one with a run of 11' and a 1/3 pitch, is 13' 2-8/12", one with a run of 7' and a 5/8 pitch, is 11' 2-6/12" long, etc.
When the run is in inches, the readings are for 1/12 of the run in feet: e.g., a rafter with a run of 12" and a 1/4 pitch is 13-5/12", one with a run of 11" and a 1/3 pitch, is 13-3/12".

Where the run is in both feet and inches, find the feet and the inches separately; and add together; e.g., a rafter with a run of 11' 6", and a 1/2 pitch, is 15' 6-8/12" + 8-6/12" = 16' 3-2/12".
[Illustration: Fig.200.