How to Make Attic Rafters

Determine the total length of the rafter (r) by substituting the height (h) and the width (w) of the roof into the following equation: r^2 = h^2 + m^2, where ^2 means squared, or multiplied by itself. For example, if the height of the roof from the top of the joists to the top of the ridge board is 5 feet, then h = 5, and if the width of the roof from the center line to the outer end of the joists is 15 feet, then w = 15. By substituting the values into the equation you get r^2 = 5^2 + 15^2, or 250. Since r^2 equals 250 it follows that r equals the square root of 250, or 15.81.

Convert the decimal fraction part of the result from Step 1 into inches by dividing it by 0.0833. The result is the decimal fraction expressed as inches. Using the example data from Step 1, 15.81 feet becomes 15 feet, 9-3/4 inches, because 0.81 divided by 0.0833 is 9-3/4.

Use a suitable wood saw to cut a length of rafter to the length determined in Step 2. Again, using the example data, you would cut a length of rafter 15 feet, 9-3/4 inches long.

Determine the angle at which to cut the top end of the rafter, so that it fits flush against the side of the ridge board. The top angle is found using this equation: angle = arc-tangent of the roof width / roof height. The width and height, w and h, are both known from Step 1. Substituting them into the equation, using the example data, gives angle = arc-tangent of15/5, or 3. The arc-tangent of 3 is 71.5, so the angle to cut is 71.5 degrees.

Measure the angle from the top corner of the rafter, assuming that the top side of the rafter represents zero degrees and moving down from there. Mark the angle by drawing a line across the face of the rafter. Use a sharp wood saw to cut along the line and remove the unwanted wood from the rafter end.

Cut the bottom end of the rafter to the correct shape after it has been fitted to the roof. Use a plumb line and draw a vertical line down the side of the rafter in line with the other rafter ends. Cut along the line to complete the construction.