A good general rule is to think first whether or not the problem can be done in one's head without the assistance of the square.
The table is made, as its name, Board-Measure (B. M.) implies, for measuring boards, which are commonly 1" thick. For materials more than 1" thick, multiply the B. M. of one surface by the number of inches thick the piece measures.
The rafter-table is found on the back of the body of the square, Fig. 197. Auxiliary to it are the twelfth inch graduations, on the outside edges, which may represent either feet or inches.
[Illustration: Fig. 198. The "Run" and "Rise" of a Rafter.]
By the "run" of the rafter is meant the horizontal distance when it is set in place from the end of its foot to a plumb line from the ridge end, i. e., one half the length of the building, Fig. 198. By the "rise" of the rafter is meant the perpendicular distance from the ridge end to the level of the foot of the rafter. 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. Try-Square. Fig. 201. Miter-Square. Fig. 202. Sliding-T Bevel.]
The _lumberman's board-rule_, Fig. 199. To measure wood by it, note the length of the board in feet at the end of the measure. The dot nearest the width (measured in inches) gives the B. M. for lumber 1" thick.