To find the diagonal for a triangle with a rise of 6 and run of 12 (for a roof with a 6-in-12 pitch), look up the first entry beneath the number 6 on the framing square’s top scale. The task now is to expand this little, 6-12-13.42 triangle into a much larger triangle that preserves the angles of the little triangle and fits the roof you are building. The hypotenuse of the triangle is provided in the decimalized number just under the number indicating the altitude.įor a 6-in-12 pitch roof, then, assume a base of 12 and look under the number 6 to find the hypotenuse, or 13.42 (see annotated framing square, below). It’s cleverly hidden in the 24-inch scale along the edge of the square.
The altitude is the number above the table. On the first line of the rafter table (length of common rafters per foot run), the base of the triangle for all the entries is 12.
#Rafter tables how to
These triangles are well-camouflaged on the square, but they are there, and learning how to decipher them in the 1970s proved to be a turning point in my own quest to learn how to frame a roof. Unlike the brace table, which uses the same isosceles triangle for the first 13 entries and the familiar 3-4-5 triangle for the final entry, the rafter table provides the base, altitude, and hypotenuse for 34 different right triangles, each with angles that are different from all the rest. The front of a standard framing square is etched with rafter tables on the wide blade and the octagon scale on the narrower tongue.
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