How to Level Out Roof Rafters

Check the level of the wall caps, the top boards on the wall, on each side of the roof. You usually won't find your wall caps are out of level with new construction, but you might with an older house in which walls could have shifted. Lay a 4-foot level on the wall cap boards to check the level lengthwise. Put a long board diagonally on side and end walls at every corner to check the level between walls. Use shims under the cap boards to raise any wall caps that need leveling.

Lay plywood or oriented strand board over the joists for temporary supports.

Determine the pitch or slope of the roof, expressed in inches of rise per foot: 5/12 denotes a roof that slopes 5 inches per linear foot. Find the run, or distance each rafter must support from wall to peak; get this from a house plan or by measuring the span or width of the roof and dividing by two.

Mark rafter boards with a framing square, which has a wide side called a blade and a thin side called a tongue which meet at a point, a tape measure and a pencil. Put the point of the square at the bottom of one end of a 2-by-4-inch board. Align the 12-inch mark on wide blade at the top of the board with the inch-mark for the pitch, 5-inch for a 5/12 roof, on the thin tongue. Mark that angle with a pencil on the side of the tongue.

Find the differential in length needed for the rafter dimension on the table on the blade; a sloping rafter must cover more space than a horizontal measurement. Look under the pitch mark, 5-inch in this example; it shows 13, meaning the rafter must be 13 inches long for every foot from the peak to the bearing point on the wall. Measure to that point with a tape measure; that would be 156 inches for a 12-foot run on a 24-foot wide roof in this example.

Make a triangle "bird's mouth" at that point, 1 inch up into the rafter bottom and 3½ inches up the bottom of the board; this will fit exactly on top of the wall cap board. Add an overhang, usually 18 inches on a house roof, and mark a "seat cut" angle the opposite of the top cut; put the point of the square at the top of the board and and align the blade and square at the bottom of the board. Go back to the top cut and take off another ¼ inch to allow for a ridge board.

Use a circular saw to cut all rafters; get the number needed by dividing the length of the roof by 24 and doubling that number for a rafter on each side. Mark rafter locations on both wall caps. Get on a ladder and measure 1½ inches from the end wall; draw a line across the board. Measure 23¼ inches from the end and draw another line. Measure 24-inch spaces based on that line, and mark all the other rafters' positions.

Prepare bracing boards at the end of the wall, fastened to stakes in the ground. Lay them on the end wall. Lift the first pair of rafters into position. Use a level to get both rafters plumb and nail them to the braces with 16d framing nails and a hammer, with the rafter tops resting loosely against each other. Repeat this on the other end of the roof.

Set a 2-by-6-inch ridge board between the end rafters. Lift it between the rafter tops from the bottom. Use a 4-foot level to make sure it is level the length of the roof, then nail the rafters to each end with framing nails, making sure to keep the top of the ridge board level with the tops of the rafters on each side. Install other rafters in pairs to the other end of the roof. Use a level to make sure that each rafter pair is plumb, that the ridge board stays level and that the top of the ridge is aligned with the top of the rafters.