How to Make Rafters for a Shed

Get the width and length of the shed. Measure the shed with a tape measure or use a building plan. The width determines the span of rafters from wall to wall; half that distance is the rafter run, the key dimension and the space each rafter must cover from peak to wall. The length will determine the number of rafters needed.

Decide on a pitch or slope. If you're building a shed yourself, you can set your own pitch. Most shed roofs will be 4/12 or 5/12, sloping up 4 or 5 inches per foot from wall to peak. Use a low pitch for an average shed, a higher pitch for one that requires more inside space or is in an area of heavy snow accumulations.

Mark a pattern rafter with a 2-by-4-inch board longer than the run. A 10-foot wide shed will have a run of 5 feet, so start with an 8-foot board, which is a standard length. Put the point or heel of a framing square at the bottom of one end of the board, laid flat with the 4-inch face up. Align the pitch mark (4 in this example) on the thin tongue of the square and the 12-inch mark on the wide blade at the top of the board. That forms an angle along the tongue called the top or plumb cut.

Calculate the length of rafter needed from the top line of the table on the blade of the square, marked "length of common rafter per foot of run." Look under the pitch mark (4); it will show 12.65, so each rafter must be 12.65 inches for every foot of run. Multiply that by the run -- for a 5-foot run that equals 63.25, meaning a rafter must be 63 1/4 inches from the peak to the wall.

Measure that length on the bottom of the board from the bottom of the plumb cut and mark the spot. Measure 1-inch up into the board at that point, then measure 3 1/2 inches from the first point back toward the plumb cut. Draw a line between the two marks to make a triangle for a notch, called a birdsmouth, to fit exactly over the cap board on the wall.

Add any length desired for an overhang or eave on the sides of the shed, typically 12 inches. That makes the total rafter length 75.5 inches. Now make a decision. If you want a ridge board atop the rafters to make a peak, go back to the plumb cut and take off another 3/4-inch to make space for the ridge. If you just want the rafters to join for a peak, use metal gusset plates to secure them at the top and add collar ties, 2-by-4 horizontal braces between the two rafters just below the peak.

Cut all the angles on the pattern with a circular saw. Put the pattern on the wall cap to make sure the birdsmouth is tight and the plumb cut is at the center of the roof. Use that rafter as a pattern for all others. Divide the length of the shed in feet by two to get the number of rafters needed; for a typical 12-foot-long roof that would be six pairs of rafters plus a set for the far end.