A shed roof is not necessarily on a shed. Shed is a style of roof, a single slope from one end of a building to another. Shed roofs are typically low slope, usually with a rise of 4 inches per foot, or less. They are ideal for attaching to another roof because of their single direction and low angle of slope. The shed style is often used for dormers on one side of a gable roof, over porches or patios, or over small house additions such as game rooms.
Determine a pitch or slope, which will affect the placement of the connection to the existing roof. The end of the roof will affect that; a framed addition will have a fixed height outside wall, a carport or patio should have end posts or supports. Install posts or end supports if there are none by setting 4-by-4 inch wood posts or metal posts in concrete in the ground. Adjust the heights of these if necessary to make a proper attachment point on the old roof.
Work from the height of the outside of the new shed roof. A shed roof should slope at least 1 inch per foot, but 2 or 3 inches will be better. If the outside edge is 8 feet high and the area being covered is 15 feet, attach the shed roof addition at least 9 feet 3 inches above the level of the porch or other area being roofed or 10 feet 6 inches for a 2-pitch.
Mark the proper height on the existing roof and snap a chalk line across the width of the area for the new shed roof. Remove all shingles and roofing paper from that section. Leave the wood roof decking in place, but mark locations of all rafters or joists to which it is fastened. Measure with a tape measure and cut with a circular saw a 2-by-6 inch ledger board for that width. Fasten it to the rafters or joists of the existing roof with lag bolts--long strong bolts that screw through the ledger into the joists.
Install shed rafters from the ledger board to the outer wall, using metal rafter hangers, which attach to the ledger board with screws or nails. Fit rafter board into the U-shaped hangers and nail it in place. Add decking atop the roof and nail on metal flashing where the shed roof joins the existing roof. Staple on roofing paper and shingle the new shed roof starting at the bottom and working up. Reinstall the removed shingles or use new shingles to cover the area of the old roof which was exposed.