How to Add a Gable Overhang

Measure the length of the two rafters, leading up to the gable peak from the bottom inside of the peak to the end of the rafter. Use an angle finder to determine the pitch of the roof. Position one arm of the device along the bottom of the left roof deck and the other on the right, with the center directly under the center of the peak. Read the angle from the indicator in the center of the tool and divide by two.

Cut four 2-by-4s the length of the rafters and the angle indicated by the divided number from the previous step. Cut the boards the same at each end with the top edge of the board the same length as the original rafter to create oblong parallelograms.

Cut enough pieces of 2-by-4 9 inches long to place one every 24 inches along the length of the rafters on each side. Lay two of the rafters out with their angled ends meeting to create a peak. Mark each board every 24 inches with a rafter square, starting from the lower ends. Set up the remaining two rafters and mark them the same way.

Place a 12-inch square of 1/2-inch plywood under the joint of the peak between the rafters. Outline the joint on the plywood and cut it out with a jigsaw, then make a second. Glue and nail these gussets to the peak of each pair of rafters with wood glue and box nails.

Nail one 9-inch piece of 2-by-4 aligned with each mark on one pair of rafters. Use two 16d nails in each piece. The pieces should extend out from the face that the gusset is nailed to. Align the second pair of rafters, with the gusset to the inside, to the ends of the 9-inch rafters. Nail into the end of each short rafter through the peak rafter, using two nails in each end.

Use a chalk line to make a line 3 1/2-inches below the bottom edge of the roof decking along both angles of the gable. Cut along the line with a circular saw with the depth set to cut through the siding and sheathing. Pry the pieces from the cut area with a pry bar to expose the gable rafter.

Drill 1/4-inch holes along one pair of rafters every 24 inches between the short rafters. Have a helper lift the completed overhang up with you and hold it to the face of the gable. Drill a 3/16-inch pilot hole into the face of the gable through each hole in the overhang rafter 1 inch into the rafter behind.

Drive one 2 1/2-inch by 1/4-inch lag bolt through the overhang rafter into the face of the gable rafter through every other hole with an impact gun to support the overhang frame. Go back and install the bolts in the holes you skipped to secure it firmly. Drive 3-inch treated deck screws down through the decking into the new overhang rafter, one every 12 inches.

Cut the edge of the original roof deck back so that it ends in the center of the new overhang rafter that is attached to the face of the original gable rafter. Use a circular saw to make the cut. Cut and install decking the same thickness and material as the original, over the short rafters of the overhang, butting up to the new front edge of the original decking.