How to Add a Shed Roof

Identify the two end points against the house and at the outer edge of the porch, deck or other addition. Determine a slope, usually based on the height of the columns or other supports on the outside, and locate an attachment point on the house. Allow a slope of at least 2 inches per foot, more if possible, so the roof will drain well.

Remove the house siding with a pry bar at the attachment point the width of the roof being added and at least one row of siding above. Mark both ends of the attachment point, an inch wider than the roof, and snap a level chalk line to mark the top of a ledger board to be fastened to the house to support the shed roof joists or rafters.

Locate supports in the house to attach the ledger, a rim joist between floors, the double cap boards on a wall or wall studs, or floor joists. Mark those spots along the ledger line. Nail a 2-by-6-inch ledger board in place temporarily with 16d framing nails and a hammer. Drill pilot holes with a power drill through the ledger into the wall attachments and secure the ledger with lag bolts long enough to go through the ledger and into the house framing.

Attach another 2-by-6-inch board the same length as the ledger across the posts, columns or other supports at the end of the roof. Fasten this with framing nails or 3-inch galvanized screws and a screw gun. Add outer bands between those two ends. Place a long board between the two ends and mark the angles on either end or use a framing square to calculate pitch angle cuts.

Fasten the side bands to the ends of the ledger and outer band with framing nails or 3-inch galvanized screws. Install metal rafter hangers on both end boards, spaced 24 inches apart, to support rafters or joists. Fasten hangers to the ledger and outer band with 8d galvanized nails, set the rafters inside, and nail them through the holes in the sides of the hanger.

Cover the roof with plywood or oriented strand board decking, with panels set perpendicular across the rafter joists. Fasten decking with galvanized nails. Lay roofing paper over the decking and staple it in place with a construction stapler. Overlap it at least six inches at seams. Install metal drip edging on the roof edges, nailed under the roofing paper at the end and over it on the sides. Nail step flashing at the ledger to go up under the siding and down the roof decking, under the roofing paper.

Finish the roof with shingles, corrugated metal panels or vinyl sheets. Run shingles right up to the sheathing on the house. Reinstall house siding to come down over the edge of shingles. Use special end caps to seal the junction of metal or vinyl panels.