Adding on to existing structures always poses challenges. How to attach new construction to old is always a part of the equation. When it comes to attaching a sloping roof to an existing wall, the device most commonly used is a ledger board. This is a horizontal lumber beam attached to the frame of the original structure to provide support for the rafters that should be supported at the lower end by the top cap of a new wall. This support wall must be framed before the ledger and rafters can be installed.
Measure up to the top face of the new roof on the existing wall. Subtract 1 inch and use a string level and chalk line to snap a line across the wall at that height, spanning the entire width of the new roof. Measure down 5 1/2 inches and mark a second line. Cut the siding and sheathing from this rectangle with a circular saw, set deep enough to cut through and skim the face of the studs beneath.
Pry the siding and other material from the slot you created in the wall, exposing the original studs. Mark the centre of each stud with a pencil on the face of the siding above and below the cut area for easy location. Cut 2-by-6 treated lumber to span the width of the new roof. Fit this beam into the slot you created. This is your ledger board.
Mark the ledger with a square to indicate the centre of each stud behind. Drill two 3/16-inch holes, 3 inches deep along each stud mark. Drive 1/4-by-3-inch lag bolts into each hole, using an impact driver. Drive the heads in flush.
Mark the ledger, starting from the left end, every 24 inches. Mark the support wall below, starting from the same end, every 24 inches. Use 2-inch treated deck screws and a drill to install a 2-by-6 rafter joist hanger on each mark on the ledger. Attach the left side, leaving the right side loose to fit the rafter. Install a hurricane tie on each mark on the wall cap to receive the end of the rafter.
Look over your blueprint to find the rafter end angle and length for your roof. Cut the angles at each end of every rafter, using a mitre saw. Adjust the mitre saw turn table to the correct angle by releasing the latch, either by pulling up on a trigger, or twisting the handle counterclockwise, depending on the saw, and align the indicator with the correct angle on the mitre gauge.
Set one cut rafter into each joist hanger, hurricane tie bracket set and install the remaining screws to fasten the brackets to the rafters. Use one 2-inch treated deck screw in every hole.