How to Build a Shed Roof on Top of a Gabled Roof

A shed roof is often put over a gable roof to cover an addition, like a dormer or a porch. Such shed roofs don't cover the entire gable roof---they typically start about halfway between the peak and the eave of the existing roof, then extend over the dormer or a porch. They are built at a much lower pitch or slope and are tied into the existing roof at the top and fastened to a dormer wall or to posts at the corners of the porch. They rarely are the full width of the existing roof.

Things You'll Need

• Tape measure
• Ladder
• Framing square
• 2-inch-by-4-inch framing lumber
• 3-inch framing nails
• Oriented strand board (OSB) decking
• Roofing nails
• Metal flashing
• Metal drip edge
• Roofing paper
• Shingles

Instructions

1. Measure the area you will be roofing to get the exact width and length of the roof. Decide on a pitch or rise---most shed roofs are very low pitched while most gable roofs are fairly steep. A 4/12 pitch would have a rise of 4 inches in 12 feet. The length will measure from the existing gable roof to the end of the dormer or the end of the porch, called the run of the rafter. Use a framing square to determine the length. It has a table on one side, which will give the length of common rafters per foot run.

2. Nail a 2-foot-by-6-inch ledger board to rafters in the the existing roof where the new roof will start and to the width of the new roof. This will be a ledge or start to fasten the new rafters to. You may remove existing roofing, but it is not necessary. Put your framing square on a full rafter board, with the small side of the square at the top of the board on the "4" mark and the bottom on the length of the rafter that says 12 feet. That will give you the angle to cut to fasten to your ledger. Measure to the end of your dormer or porch and mark a spot for a "bird's mouth" notch that will fit over the dormer end or porch beam. Then add length for any overhang.

3. Cut a test rafter, with angled end to meet the ledger board and to the length of overhang. Cut a bird's-mouth notch, at the point the rafter meets the dormer wall or porch beam, and cut a wedge out so the rafter will fit snugly on top of the wall or beam. Test that rafter to make sure it fits, then cut all other rafters to match. Figure on a rafter every 24 inches across the width of the new shed roof. Nail rafters into place, to the ledger board and to the dormer wall or porch beam. Make sure the bird's-mouth notch fits snugly against the dormer wall cap or porch beam.

4. Measure and cut short "jack" rafters to fit between the top rafter and the existing porch roof. These will decrease in length as you move down the roof. Nail them in place to existing roof rafters and the new top rafters. A porch shed roof will extend beyond the end of the existing roof. If joists are not already in place between the house wall and the porch posts, install them by nailing another ledger board to the end of the existing roof and mounting joists between that and the beam at the end of the porch. You can use metal joist hangers for these; they are easy to use and very secure. Make jack rafters to fit between these joists and the new roof rafters.

5. Cover the new shed roof, top and sides, with oriented strand board (OSB) sheathing. Finish the sides with house siding or leave them open (on a porch). Nail metal flashing where the ledger board meets the old roof and metal drip edge along all sides of the new shed roof. Put down roofing paper, overlapping generously at the seams, then nail on shingles to match the old roof.

6. .

Tips & Warnings

• Be extremely cautious when working on roofs.

References

• Fine Homebuilding: Tying New Roof to Old
• Vance Hester: How to Cut Roof Rafters
• Joseph Fusco: Cutting Roof Rafters
• Online Tips: How to Attach a Roof
• Carpentry Pro: Cutting Rafters