Always try to match the house roof pitch angle and overhang for your shed roof. Some shed roofs are designed to be nearly flat. Flat roofs are more susceptible to leaking than pitched roofs. Shed roofs project as a single plane roof pitched away from the house. If you are tying the roof directly to a house wall, you can install a ledger board with flashing and install rafter ties to attach the rafters at the appropriate angles or use engineered trusses. The opposite side of the porch roof will be supported by a header beam that is installed parallel to the house on top of 6×6 support posts. Support posts should be spaced less than 8′ apart and be located on top of properly sized frost footings. Rafters should be attached to the header beam using hurricane ties.
If you are tying the shed roof into an existing roof on the house, you will need to remove the shingles and sheathing to expose the roof framing. You will need to attach the porch rafters to the house roof rafters and replace the roofing and flashing.
Many building departments require a local architect or engineer to design plans for porch roofs because of the increased snow, roof and wind loads created by adding a roof.
Building a Shed Roof Over A Deck: Step By Step
Install post base connectors to attach the support posts to the deck frame. Make sure the frame has the appropriate blocking to ensure a solid connection. Do not attach the post base to only decking.
Install your 6×6 support posts to the post base connectors.
Install temporary bracing to keep the support posts level and in place. Notice that the tops of the 6×6 support posts are notched to hold the header beam.
Install the 2-2×10 header beams to the notched 6×6 support posts. Use thru bolts for a solid and strong connection.
Installing the header next to the house
Attach the header to the support post next to the house. Do not attach the header directly to a house cantilever.
Flash and bolt the ledger board in place to the house wall. This board will support the rafters.
Use a speed square to mark and cut your rafter angle to the roof pitch.
Lay out your rafters. Most hand-framed porch rafters are installed at 16″ on center along the ledger board.
Toe nail the rafters in place to the ledger board.
Attach the rafters to the header
Install the birdsmouth of the rafter to the header with framing nails.
Install the roof sheathing
Install the plywood roof sheathing over the rafters to create the subroof.