Most houses today are built with prefabricated roof trusses, but many are still built with rafters that span the roof. Framing a roof with rafters is far more difficult than framing walls because the roof pitch must be calculated and rafter angles cut correctly. Online rafter tables or a roofing calculator can make this job easier. A framer's square has the basic information on it, but learn how to use it from an experienced roofer or a building supply store.
Measure your roof area and decide its pitch (the angle at which it slopes from the peak). A pitch might be 5-to-12, meaning 5 inches of rise for every 12 inches of run. Determine the run by measuring the width to be roofed and dividing in half; a 24-foot building would have a run of 12 feet. Mark your first 2-by-4-inch rafter by putting your framing square with the point on the bottom of the board, the thin edge on 5 inches and the fat side on 12 inches, then drawing a line. The line must be diagonal to fit flush against the ridge board. Move down the board, putting the tongue on the previous body mark; do this 12 times to the end of the rafter, and mark for the outer edge of the building.
Add the length of overhang you want to the final mark you made, and draw a line there for rafter length. Go back to the outside wall edge mark and mark a "bird's mouth" or notch, which will fit over the cap of the wall. Go back to the top and take off half the thickness of the ridge board; a 2-by-6 will actually be 1 1/2 inches wide, so take off three-quarters of an inch. Cut one rafter as a pattern and put in place to make sure it fits properly, flush against the side of the ridge board (test it with a short piece), the notch firmly on the wall cap and the overhang the right length.
Cut all rafters, carefully aligning the pattern for precise cuts. Set your first two end rafters in place, alongside a joist, with bird's mouth notches securely on the wall cap, and nail them temporarily to the joist. Move to one rafter space from the other end (24 inches) and repeat this. Slide the ridge board into the opening between the opposing rafters from underneath and nail it in place. Measure, cut and install a centre gable stud between the top of the wall and the bottom of the ridge board on each end. Plumb the stud and square the ridge board, then nail rafters firmly into place.
Move down the wall, installing pairs of rafters 24 inches apart until you have a full set from end wall to end wall. Then install braces. There are two types: purlin and collar tie. Purlins are set diagonally from a rafter to a joist on alternating sides, so a purlin on one rafter goes to one wall, and a purlin on the next goes to the opposite wall. Collar ties are nailed squarely across both rafters below the ridge board; each pair of rafters has a tie.
Rafter roof framing requires at least three people on a team.
Use caution working on a roof.