Fig. 51. Finding Rafter Length by Scaling
Second Method: In determining rafter length, an equally common practice is to lay the framing square as is shown in [Fig. 45-a]. While in this position the seat cut is scribed, cf. Section 18, and also a short sharp line scribed along the other member of the square at the top edge of the rafter. The square is moved along, using the same numbers, and another advance mark scribed. This operation is repeated just as many times as there are feet in the run of the common rafter. With a span of 24' the operation would be repeated 12 times.
Should the run not happen to be in even feet, the square would be placed as many times as there were full feet in the run. In addition it would be advanced that fractional part which the fraction of the run was of 12". For example, in a run of 12' 7", with a roof of ¼ pitch, the square would be advanced 12 times using the number 12 on the tongue and 6 on the blade. In addition to this the square would be advanced using 7/12 of 12" or 7" on the tongue and 7/12 of 6" or 3½" on the blade. As these numbers do not allow enough of the square to rest on the rafter to give a full line, as soon as the advance limit of rafter length is indicated the square may be moved up, using the set of numbers first used, that is 12" and 6". On common rafters, this last operation is simplified by noting that the fractional run, divided by 12, times 12, always equals itself. The final position of the square, therefore, may be obtained by simply sliding the member, used in laying out the last full foot line which parallels the seat cut, an additional distance equal to the fractional foot of total run, [Fig. 44.] The tail length is obtained similarly, [Fig. 44.]
Fig. 52. Laying out Rafter
18. Laying off Common Rafter Seat Cut and End Cut.—First Method: Having determined the rafter length as directed in [Sec. 17,] first method, (1) lay off this length along the upper edge beginning at the plumb cut. The whole number of feet is more safely "taken off" by means of a pole marked in feet, and of good length. The rule or square may be used to transmit fractional parts of a foot. (2) Place square as at "b," [Fig. 52], standing as in [Fig. 45-b], and scribe a plumb line as indicated at 1-2, [Fig. 52.] (3) From the point 1, [Fig. 52], measure along the line marked 1-2 a distance equal to one-half that of 1-2. The distance 1-3 may be increased or decreased somewhat when an extreme pitch makes it advisable. As a rule this should be 2½" to 3". (4) Place the square as at c, [Fig. 52], with the edge of the tongue resting on 3 and scribe a line for the seat cut, as 3-4. These last marks give the bird's mouth joint which is to fit over the plate.
Fig. 53. Independent Rafter Tail
Fig. 54. Length of Ridge Piece