How to Build a Gable Roof for an Outside Patio

Set posts to outline the gable patio roof. Use 4-by-4 or 6-by-6-inch posts, depending on the size of the patio. Put one at each corner and at intervals in between if the patio roof covers more than 12 feet. Dig holes with a posthole digger a third of the depth of the finished height, for instance, 3 feet deep for a roof with 9-foot posts. Set the posts plumb with a level and fill the holes with concrete mixed in a wheelbarrow with a prepared mix.

Frame the outside of the roof with 2-by-4 or 2-by-6-inch boards nailed with 16d framing nails and a hammer to the tops of the posts, with the wide face up. Miter the corners at a 45-degree angle to make a perfect rectangle. Make sure the boards are level for both the length and width of the patio.

Install a "king" post vertically on each end with a ridge board, the same size as the frame lumber, between posts. Miter the tops of the king posts and ridge board to make a 90-degree angle. Fasten the king post to the center of each end frame board with 16d framing nails, toenailed into the frame. Make sure the ridge board is level and nail it to the king posts. Size the king posts for the pitch or slope of the roof, so the ridge board will form a peak.

Make rafters to connect the sides of the frame and the ridge board. Use a framing square to figure the top angle. Determine the pitch, the rise in inches per foot from outside to peak, by the height of the ridge and the distance between outer band and peak; if the ridge board is 50 inches higher than the outside and the distance is 10 feet, the roof rises at 5 inches per foot and is called a 5/12 pitch.

Put the point of a framing square at the bottom of one end of a 2-by-4-inch board. Set the 5-inch mark, for a 5-12 roof, on the thin tongue of the square and the 12-inch mark on the wide blade at the top of the board and mark the angle at the end along the tongue. That is a top or plumb cut to fit against the ridge board.

Use the "length of common rafter" table on the blade to find the length needed to connect the ridge to the outside; under the 5-inch mark that would be 13 or 13 inches for every foot from peak to outside. Multiply that by the run, half the total width of the roof. Mark a triangle to fit over the outside beam. Measure with a tape measure an inch up into the bottom of the board, then back up the board 3 1/2 inches, for a 2-by-4, or 5 1/2-inches, for a 2-by-6, to make the triangle. Add a foot to provide an overhang and mark another angle with the square point at the top of the board.

Cut all the rafters with a circular saw. Nail them in place to the outside frame and the ridge board with framing nails, two nails per rafter at each joint. Use a level to ensure they are plumb vertically. Install rafters in pairs, one set at a time the length of the roof.

Cover the roof with oriented strand board decking and shingles or with corrugated metal or vinyl. Nail OSB to rafters; fasten corrugated panels with special screws with plastic washer caps. Finish either type of roofing with ridge caps over the peak of the roof.