This section is from the book "The Carpenters' Guide", by Harvey Miller. Also available from Amazon: The carpenters' guide.

There are as many base feet in a common rafter as there are feet in half the width of the gable, or half the width of the building in a hip roof. (See fig. 21.)

The length of a common rafter in one base foot is equal to the hypotenuse of the base and rise and can be obtained by measuring from the base to the given rise. (See fig. 29.) This is true with 12" base and any number of inches rise. (See fig. 15.)

Then lay the square at 12" base and the given rise as many successive times as there are base feet in the rafter to obtain the length of the rafter. (See figs. 30 and 32.) Mark on the base for the plate cut and on the rise for the ridge out. (See figs. 29, 30 and 32.) Where there is a ridge board used shorten the rafter half the thickness of the ridge. For an extension of the rafter over the plate for eave, and two or more inches left on the rafter for support, measure down perpendicularly from the spacing for the plate mark to the bottom of the rafter. Draw a line parallel with the rafter, 2" from the top edge. Set the square to where the two lines intersect, use the base for the plate cut, and the perpendicular line, down, for the outside cut of the plate. (See fig. 31.) The distance marked down from the top edge of the rafter does not change the length of the rafter. The top has the same angle, but raises the rafter and ridge the height of the thickness of the heel or support in outer line of plate. (See fig. 31.)

Fig. 30

Fig. 31

Fig. 32

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