How to Construct a Shed Roof for a Porch Addition to an Existing Roof
- 1). Remove the shingles from the existing roof in the area where the new roof will attach. Leave the wood decking on, but locate the rafters or joists to support the added roof. Snap a chalk line across the old roof deck to mark the position for a ledger board, to hold rafters for the roof addition. Install a 2-by-6-inch ledger board the width of the roof addition, fastened with lag bolts to joists in the existing roof. A ledger board is one fastened to a wall or roof to act as a support for rafters or joists. It is a common item in remodeling construction. Use a level to make sure it is level.
- 2). Cut the outer supports of the shed so that they will rise to a height below that of the ledger board. This will help create a downward slope away from the house once the shed roof is installed. The outer wall of the shed should be at least 1 inch lower than the ledger board for every foot the shed roof extends away from it. Therefore, if the roof extends 10 feet away from the house, the outer height of the shed should be 10 inches lower. If the ledger board is 9 feet up from the floor of the porch, the shed's outer wall should rise no higher than 8 feet 2 inches. Remember to factor in the height of a 4-by-4-inch beam that will lay across the top of the support columns. In this scenario, your outer columns installed would rise to a height of roughly 7 feet 10 inches.
- 3). Install the posts at the outside edges of the porch, if none were placed when the porch was added. Dig holes with a post-hole digger and set 4-by-4-inch posts in concrete at the outside edges of the porch. Set at least two posts, and more if the shed roof area is very wide. If your shed is on a raised porch, install columns on top of the porch's planking. To do this, put wood or steel posts in metal holders that fasten to the porch deck. Then you can build wood or masonry columns around them.
- 4). Place your 4-by-4 inch top beam across the posts or columns you have installed. Secure it to the posts or columns with nails or metal brackets, depending on the materials you have used to build the posts or columns. Place a level on the beam to make sure it doesn't slant in any direction. Measure the height of the beam to double check it forms the proper slope from the top of the ledger board.
- 5). Install rafters to connect the ledger board and outer beam. Use metal rafter hangers, which nail to the ledger board and to the end beam. These U-shaped hangers hold the rafter in place. They are nailed to the ledger or beam and the rafter fits inside and is nailed into the hanger from the side. Space rafters 24 inches apart the width of the porch. Cover the rafters with oriented strand board, or OSB, and nail it to the rafters. Your shed now has a rough roof.
- 6). Staple roofing paper over the shed roof's OSB board. Add shingles to match the original roof. Nail the first row of shingle upside down, with the tabs up, so the shingles form a straight line at the edge of the roof. Add shingles to the top of the shed roof, overlapping the tabs according to the shingle manufacturer's directions. Install metal flashing where the shed roof meets the old roof to seal the seam between the roofs. Cover it with roofing paper and put shingles over it; mate the new shingles and the old at the connecting point. Replace any needed shingles on the old roof below the new roof line.
