How to Extend Roof Joists to Build a Soffit

Snap a chalk line 4 feet above the current edge of the roof to create an overhang of 2 feet to support a soffit. Pry up the shingles and the plywood roof decking to this chalk line.

Measure the existing roof joists or rafters. If they are 2-by-8 rafters, for example, cut 4-foot lengths of 2-by-6 rafters to create your extensions.

“Sister” the 2-by-6s to the existing 2-by-8s by pressing them together while extending the 2-by-6 boards 2 feet past the top plate, the horizontal framing on top of the exterior wall where it meets the rafters. This extension creates “rafter tails” to build the soffit. Work on the two end rafters first. Clamp the sister rafter so that its top edge is 3/4-inch below the top edge of the 2-by-8 rafter. Screw the sister rafter in place with 2 1/2-inch wood screws in a staggered pattern.

Tack a straight 2-by-4 between the end rafter tails so that the intermediate rafter tails touch it and are in alignment. Install sister rafters on the intermediate rafters. Mark a plumb -- vertical -- line on each rafter tail with a carpenter's level at a uniform distance from the wall. Cut along these lines with a circular saw.

Bevel the top of a 2-by-4 board to match the angle of the top of the rafter and create a fascia header. Use galvanized 2-inch deck screws to attach the fascia header to the rafter tails so that its beveled top forms a continuous surface with the rafter top.

Mark the wall level with the bottom of the fascia header at several spots with the level and a carpenter’s pencil. Snap a chalk line between these marks. Nail a 1-by-4 board to act as a cleat against the wall, with its lower edge even with the chalk line. Screw a primed 1-by-6 board to the fascia header with deck screws installed from the back of the fascia header into the 1-by-6 board to hide the screws.

Cut a piece of 3/8-inch plywood to fit from the wall to the bottom of the fascia header. Prime it and paint its cut edges with a waxy sealer. After it dries, screw it in place with 3/4-inch galvanized deck screws to the fascia header, the 1-by-4 cleat and the bottoms of the rafter tails.

Run several rows of horizontal 1-by-6 tongue-and-groove boards over the 2-by-6 rafter tails to bring their top edges in line with the existing 2-by-8 rafters. Nail larger sheets of OSB board in place to replace the sheathing removed in Step 1 and to cover the additional surface created by the extended rafter tails. Cover the sheathing with builder’s felt then re-shingle the roof.