Adding a roof overhang can provide for added protection from rain while opening or closing the door as well as shade windows from the sun, helping to lower cooling bills. Most overhangs can be added without major structural change by simply installing rafter extensions that fasten to the existing rafters to create the desired space.
Cut rafter extensions for the overhang that are equal to the desired length of the overhang plus two feet. Make these out of the same size material as the existing rafters, usually 2x8 or 2x10.
Mark the rafter extensions two feet from one end along one edge of the extension using a pencil.
Remove the fascia, if present, from the edge of the existing roof edge using a hammer and pry bar. The fascia is the board that covers the end of the rafters. It is usually almost, if not completely, perpendicular to the ground.
Install the extensions by lining up the marks made in Step 2 with the end of the existing rafter. Nail them to the existing rafter. Start by installing the extensions on either end of the new overhang and then stretch a string between the ends of the extensions and use this string line as a guide to install the rest of the rafter extensions in a straight line.
Nail 1/4 inch plywood to the bottom of the rafter extensions to form the soffit, if desired. The plywood should fit tightly against the edge of the existing soffit and be flush with the outside edges of the rafter extensions.
Install the fascia trim board to cover the ends of the rafters, if desired. This trim should be made of a standard 3/4 inch thick board, and it should hang past the bottom of the soffit at least an inch. Check the size of the fascia on the rest of the house and match it to that.
Cut the rafter extensions to the desired length, plus 18 inches.
Remove the fascia as in Step 3 above.
Install one rafter extension on either edge of the new overhang area by toe nailing it, or nailing at an angle through the rafter into the top plate, into the top plate of the exterior wall. Also nail it into the existing rafter end.
Place the rest of the rater extensions by using a string line as a guide just like in Step 4 above.
Securely fasten the rafter extensions by nailing a rafter tie, sometimes called a hurricane tie, into the side of the extension and the top of the wall plate.
Reinforce the rafter extensions by nailing a 2x4 on top of the ceiling joists in the attic, making sure that it butts the back end of the rafter extensions.
Install rafter ties by nailing them on top of the 2x4 and into the side of the rafter extensions. Nail a rafter tie on both sides of the extensions.
Install soffit and fascia, if wanted, as in Step 6 in the previous section.