Fig. 61. Pitches of roofs.—¹⁄₂, ¹⁄₃, ¹⁄₄, ¹⁄₈.
Since roofs are of various pitches, they require rafters of various lengths and bevels. Farmers and many carpenters have much difficulty in getting the length and bevels of both rafters and braces. Most carpenters’ squares have so-called brace rules stamped upon their tongues.[3] These give the length of the brace for the shorter and more common runs,[4] but they do not give the angles of the ends of the brace. Then, too, the length is given in inches and hundredths of inches, and carpenters’ squares are not divided into hundredths, so this complicated brace-rule is as useful as a steam whistle on an ox-cart.
[3] The short end of the square.
[4] The perpendicular and horizontal distances covered by the brace.
The methods by which the length and bevels of any member of a frame which departs from any other member at an angle are so easily understood that the wonder is that all are not familiar with them. For a simple illustration, let it be supposed that rafters for a building 18 feet broad, with one-third pitch, are to be laid out ([Fig. 62]). The rafter, R, takes the form of a brace. The run is 9 feet horizontally or half the width of the building, and 6 feet perpendicularly. If the square be laid upon the stick designed for the rafter, as 6 is to 9, one side of the square will give the shorter and the other the longer angle or bevel ([Fig. 63]). If the square is laid on 12 times at 9 and 6 inches, it will give the length of the rafter, for 12 times 9 is 108, half the width of the building, and 12 times 6 is 72, the height of the peak above the plates. If the square is laid on 18 × 12 inches, the proportion is preserved, and hence the angles; the square would only have to be laid on six times.
Fig. 62. Laying out a roof.
Fig. 63. Laying out a rafter.
Fig. 64. Laying out a timber.
