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# Roof rafter calculations

Updated February 21, 2017

There are essentially four ways to do calculations for roof rafters. You can do the basic math, which can be complex. You can refer to professional rafter tables, which show lengths, angles, even variations of types of wood. You can use a basic framing square, which has all the information you need. Or you can use a speed square, a variation of the framing square. The basic framing square is easiest for a homeowner.

## Pitch and Run Define Rafters

Rafters are defined by length and pitch, how great the distance each one must support (the run) and the angle of slope of the roof, figured in inches of rise per foot, as in 5/12 for a 5-inch rise every foot of run. Common rafters, the basic type, start with calculating a top or plumb cut, where two rafters meet at a peak. You figure that by knowing the pitch and putting the point of the framing square at the bottom of a rafter board with the pitch mark on the thin leg of the square and the 12-inch mark on the fat leg at the top of the board. That forms a plumb cut angle at the end of the board.

## Use a Rafter Table for Length

Calculating length requires using a table on the fat leg (blade) of the square. It has six lines, starting with "length of common rafters per foot of run." Checking the pitch will show how long a rafter must be to cover each foot of run. For a 5 pitch roof, that's 13, or 13 inches for every foot of run. It will vary with the pitch -- a 4 pitch is 12.65 inches per foot, a 6 pitch 13.42 inches. Multiply that number by the run (half the width or span of the roof) to get the length for a rafter from the peak to the outside wall.

## Figure a "Birdsmouth"

Each rafter also must have a "birdsmouth," a triangle cut into the bottom of the rafter to fit exactly over the 2-by-4-inch cap board on the top of the wall. That's calculated by measuring the run length from the bottom of the plumb cut line. A 5 pitch rafter on a 12-foot run, for instance, will be 13 feet or 156 inches. The birdsmouth is calculated from that point on the bottom of the rafter to 3 1/2 inches back up the board and 1 inch up into the board, to make the triangle.