This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The ends of rafters are usually cut to fit accurately against one another and against the plates on which they rest. The cutting of these bevels is not at all difficult when the relation of the rafter to the roof surfaces is seen and the steel square is used to show this relation.
Method of Cutting Bevel. Fig. 192 shows the bevels that are used on the common rafters in a simple gable roof such as is illustrated in Fig. 163. In Fig. 191, B is the plate and E is a point midway between the two plates, and the distance D E is the run of the common rafter C. P is the point where the line drawn through D parallel to the edges of the rafters is directly above the point E. The distance D P is usually taken as the length of the rafter.
The length is taken from these points because the distance D E represents the exact run and E P represents the rise for this run.
The first step to take in laying out the rafter is to locate the point D on the uncut piece as shown in Fig. 193. The point is chosen so that it is far enough from the end to form the eaves F D, and the distance T D is usually taken 2 inches on a 2 X 4 inch and 3 inches or more on larger size rafters. It is well to remember that the measurement is to be taken from the top in order that the roof surface may always be even and smooth throughout. Now the edge of the blade of the square must coincide with D, but the position which the square will take depends entirely on the pitch.
In all cases 12 inches is used on the blade of the square and the figures on the tongue depend on the rise per foot of run. If the rise of the rafter is 12 inches to the foot, 12 inches should be used on the tongue also. If the rise is 10 inches, use 10 inches on the tongue. In the cut the rise is 8 inches to the foot, the run is 12 feet, the rise is 8 feet, and 12 and 8 are the figures used.
Fig. 192. Diagram Showing Bevel Used on Common Rafters.
Fig. 193. Method of Laying Out Rafter with Steel Square.
Usually the carpenter uses the edge M N to obtain this bevel but the line F P may also be used. The line D 0 is the heel or plate cut, as shown in Figs. 192 and 193. The rafter is sawed along the lines F D and D 0. Now the next step is to find the length D P. This may easily be determined by any one of four methods. The easiest of these is to turn to the rafter table on the square. Opposite 12 - 8 - 1/3 and under the 12 (indicating feet run) the length is given as 14 feet 5 inches. By extracting the square root of the sums of the square of rise and run the same result is obtained. The third way is to measure the distance in inches and twelfths from the 12 on the blade of the square to the 8. Each inch represents one foot of length and each twelfth represents 1 inch. The distance is 14 feet 5 inches. The fourth method is to use the method illustrated in Fig. 194. First locate the line D P and then beginning at D move the square along the edge of the rafter as many times as there are feet and fractions of feet in the run; thus the point P is determined.
Fig. 194. Method of Cutting the Bevel at the Top of the Rafter.
A little study of the figures will suffice to reveal to anyone the reason for this method of procedure. Every time the square is moved into a new position it has advanced 12 inches or 1 foot along the run of the rafter, since the distance D E is 12 inches and is measured horizontally. After the square has been moved twelve times it has advanced 12 feet along the run of the rafter or the distance required. This gives the position of the top bevel. It should be noticed that for a run of 12 feet the square must be moved along twelve times; for a run of 8 feet, eight times; and so on. The run of the rafter may be easily obtained by subtracting one-half the thickness of the ridge board from one-half of the total span of the roof from outside to outside of wall plates.
Fig. 195. Fitting a Rafter against Ridge Board.
Fig. 196. Cutting Rafter for Concealed Gutter.
Fig. 197. Another Method of Cutting Rafters for Concealed Gutter.
Fig. 194 shows the rafter in the position which it would occupy in a building, the plate and a part of the wall studding being indicated. When the rafter is cut along the line N S, Fig. 193, it is ready to be put on the building. In case, however, that a ridge board is used to hold the rafter in place, as shown by R in Fig. 192, the rafter is cut parallel to N S but shorter, as shown in Fig. 195, one-half of the thickness of the ridge board being cut away. The cut at D 0, Fig. 192, is horizontal, and the bevels at NS and VK, Figs. 193, 194, and 195, are plumb cuts.
Fig. 198. Bevels for Valley and Hip Rafters.
In case a concealed gutter is used and the rafter is set directly over the wall, the line DP coincides with the line M N, Fig. 193, and the rafter has only the horizontal cut at the bottom or a horizontal and vertical cut, as shown in Figs. 196 and 197.