One of the most troublesome aspects of owning a home is not necessarily fixing the roof leak, but finding where it leaks. Because of the way water works in under shingles, it make take a while before a leak shows up inside the house. But by the time you do realize that the roof leaks, it might have caused some unseen damage to roof decking or rafters. Once you locate where the water gets under the shingles or flashing, stopping the leak becomes a matter of following a few basic steps.
Investigation, Demolition and Preparation
Locate the source of the leak. Remember that water flows downhill even after it is past the outer roofing material, so look for the source of the leak uphill from the spot where it penetrates the house. Search for lifted shingles, disturbed flashing, cracks around vents, skylights or other roof components above and uphill from the water leak for its source.
Remove and discard the obviously damaged shingles or flashing directly surrounding the leak. Carefully lift the flap of the shingle above the one you plan to remove to find the nails holding it in place. Pry under the nails carefully with the pry bar to lift them out while holding the shingle in place. After removing the nails, pry gently under the shingle to avoid damaging the decking beneath or subsequent courses of shingles, particularly the last one uphill that does not get removed. Work toward the nearest edge and in the opposite direction only as far as is needed to clear the entire damaged area. Once you can see the tar paper beneath the shingles, remove that as well to get at the roof decking.
Check if the roof decking was compromised by water damage. If you notice rot, remove the rotted decking and replace it with new material prior to installing a patch. This requires removing a larger section of shingles to cut the bad section out of the decking to reinstall a new cut-to-fit piece across the trusses. Look for areas of discoloration, mold and excessive softness when you poke the roof decking material with a nail or screwdriver.
Cut a 2-inch square hole into the worst section if the decking is rotten. Use a circular saw's base plate set to just over 3/4-inch. Measure in both lateral directions to locate the closest rafters and then mark the exposed decking using the framing square and carpenter's pencil. Cut out a square -- rafter center to rafter center without cutting the rafter -- and only as high and low as is necessary to remove the damaged decking. At this point, you should have a clear view of the extent of the water damage at this elevation in the house. If you discover too much rotten decking, rafters or trusses, call a professional to fix these issues while you have the roof open.
Patching the Affected Area
Measure between the rafters on the top and bottom edge of the repair, then cut blocking pieces out of the 2-by-4 to this size and nail them into place, scooting the blocking a bit under the remaining deck so that it covers half the thickness of the 2-by-4 and leaves half showing. This provides support for the cut edge of remaining decking and for the roof decking patch. Cut out a matching piece of new plywood and nail it in place with 8d ring-shank nails every 4 inches around the perimeter of the patch, and then nail the pre-existing decking into the blocking following the same nailing pattern.
Cut out a section of tar paper and set it over the entire new decking board, tucking the top and bottom edges under the existing shingle courses. Staple it in place.
Secure the new shingles with roofing nails starting at the bottom of the repair, following the previous course of shingles by overlaying the new shingles atop the remaining course. If you did not need to remove and replace decking prior to this point, then add roof caulk or asphalt cement to every nail before driving them into the shingle. Repeat as needed for the remaining shingle courses. Tuck the last shingle course under the remaining shingles very carefully and use tar on these nails as well, along with adding a continuous bead of caulk or asphalt cement under the edge of the top course of shingles. Take care as you nail the last course in, as you don't want to break the shingles above. If you had to remove and replace flashing, add a continuous bead to seal the edges where the shingles overlap the flashing.