There are different kinds of rafters, viz: Common, Jack, Cripple, Hip and Valley (see Fig. 13). A common rafter extends from the plate to the ridge. A Jack rafter extends from the plate to intersection of the hip. A cripple rafter extends from the valley to hip or ridge. A hip rafter extends from the corner of the plate to the ridge diagonally. A valley rafter extends from the intersection of two roofs at right angle to the ridge and runs parallel with the hip rafter. (See fig. 13.)
There are three lines on the square with which all cuts are obtained. The base, rise and hypotenuse (the angular line from base to rise), (see fig. 14). It is important to understand their relative uses in framing. Study the rules and demonstrations herein given. The base represents the horizontal line and is reckoned in feet. Every horizontal line represents one base foot in framing. Twelve inches is the base of the square for common, jack and cripple rafters, brace, truss, bracket, bridging, house and barn framing, and nearly all framing at right angles; the 12" base does not change on the square for obtaining the pitch of roof, angle of brace and various cuts, as the rise. (See fig. 15.)
The base of a hip or valley rafter is 17". The diagonal, or hypotenuse of a 12" square is nearly 17". Every foot on the plate represents one base foot of 17" for a hip and valley rafter (see fig. 16). This base does not change in obtaining the different pitches of roof (see fig. 17). Then a 17" base with the same rise used in the 12" base will meet at the same point on the hips after. (See fig. 18.)
The Rise is the perpendicular line, and is used on the square at right angle to the base. Every inch or fractional inch rise to one base foot gives a different pitch of roof, angle or cut. (See Figs. 15 and 17.)
The Hypotenuse is the angular line from the base to the rise and is the pitch of roof and length of rafter in one base foot which is obtained by raising the Rise to the desired pitch or angle (see fig. 15). It is used with the base instead of the rise for the angular or face cut for Jack, Cripple, Hip and Valley rafters that meet the ridge, Hip and Valley rafters, which will be explained.
The base of an Octagon is 13", used with the same Rise of the common rafter, to meet the same point. The difference of the radius and the distance to the corner of the octagon is 1" to every radius foot. (See fig. 19.)
We will use repetitions of many of the rules here given. If the Mechanic or Student does not understand them in one form, they may comprehend them in another.