How to Attach a Flat Roof to Perpendicular Slanted Roofs

Attach a flat roof to a slanted roof with a ledger board, a horizontal support installed on the connecting roof. Identify the location on the roof and mark its outside edges, where the sides of the roof will be, with chalk. Locate trusses, joists or other framing on the existing roof to support the added flat roof. Get in an attic, if one is available. Drive nails through the roof along the outside trusses to mark them.

Remove shingles or other roofing from the existing roof, but leave decking and roofing paper. Set a 2-by-6-inch ledger board in place where the flat roof will attach. Level it with a level and fasten it temporarily with 16d framing nails and a hammer. Use nails through the roof as a guide to mark other support locations. Measure with a tape measure from a nail to the next truss, typically 24 inches, and mark that spot on the ledger. Mark all framing support locations on the ledger.

Drill holes with a power drill and 3/8-inch bit through the ledger into the roof trusses or other framing. Fasten the ledger with 1/2-inch lag bolts, driven with a ratchet wrench through the ledger into the frame members. Put washers under the bolt heads to create space for drainage.

Cut flat roof rafter ends to match the slope or angle of the ledger.Determine the pitch of the existing roof. Put a level against the roof, hold it level and measure the distance between the bottom of the level and the roof line 12 inches from the end of the level; a difference of 6 inches, for instance, will be a slope of 6 inches per foot. Use a speed square to mark that angle on the rafters; put the pivot point of the square at the top of a rafter and line up the "common top cut" mark on the square on the top of the board. Mark the rafter end and cut that angle with a circular saw on all flat roof rafters.

Secure ends of flat roof rafters to the ledger with framing nails or with metal rafter hangers. Drive framing nails diagonally through the rafter end into the ledger, two nails on one side, one on the other of each rafter. Nail hangers with 8d galvanized nails. Fasten the hanger to the ledger with two nails on each side, slide the rafter into place and nail it through holes in the side of the hanger.