All roof leaks are caused by a flaw in the progression of water flow. Roofing systems are designed so that the water starts at the highest possible point and flows downward to the next highest point, and so on until the water reaches the edge of the roof.
Finding the Leak
Sometimes it can be difficult to locate the exact spot of a leak because the water spot on the ceiling inside your house may not be in the same location as the actual leak in the roof. Water can run down rafters or trusses and show up inside the house a good distance away from the actual source of the problem. The best way to find the true source of the leak is to climb up into the attic and look for water damage on the underside of the roof deck. This should give you the best idea as to the location where repair work will be necessary.
Once the leak has been located, the repair process is fairly simple. Assuming that there is no structural damage and that the roof and decking do not need to be replaced, a little bit of tar, some flashing, and a few roofing nails will take care of most small, annoying leaks.
Leaks in the Roof Surface
If the water is coming in through the flat surface of the deck, there may be some wind damage to a shingle, or one may be split because of a direct blow. Things like branches falling out of a tree can cause these kinds of problems. If you happen to have extra shingles that match the color, the easiest option is to simply replace the damaged shingle.
In many cases, however, homeowners don't have extra shingles just lying around. In this instance, take a small pry bar and carefully remove the 3 or 4 roofing nails that hold down the shingle in question. Be careful not to tear the shingle anymore than it already is. After finding the damaged area, lift the tabs and apply a generous coat of tar underneath the tear, then nail the shingle back down to the deck. Apply a heavy layer of tar directly over the tear and a spot of tar over each nail head. If you were unable to use previous nail holes, make sure you cover these with tar as well. Another thin line of tar on the underside of the shingles that overlap the damaged one prevents ice from backing up under the shingle in the winter. A new shingle has a line of tar on the underside already, but you may have broken this seal when lifting it up to make the repairs, so it's a good idea to apply a little more.
Leaks around Vents
If the leak is coming in from around a chimney or a vent of some kind, then it is likely that the seam where the flashing meets the chimney or vent pipe has developed a gap. This can happen if the caulk that was used begins to dry out, or if the pipe was hit and shifted slightly. In most cases, a fresh coat of tar at this seam will solve the problem.
Occasionally, however, the flashing may have been pierced by a falling tree branch or a wayward footstep, and will need to be replaced. You can buy rolls of aluminum flashing at most home improvement stores, and it is flexible enough to be cut and worked by hand and with a pair of simple tin snips. Gently loosen the shingles that cover the old piece of flashing and remove it. When you tack down the new flashing with roofing nails, be sure to seal around the edges with a generous coat of tar. Lay the shingles back down around the new flashing in the same pattern that they were originally laid and make sure that the flashing is tightly sealed to the pipe or chimney that it protects.
As with all roofing repairs, safety is paramount. Before beginning any roofing project, make sure you have a ladder that is the proper size to reach the area that you are working on. Another thing to briefly consider is footwear. Make sure that your shoes will not be too slippery on the roofing surface. With any job involving some kind of risk, it is always a good idea to have a partner as well. Make a good plan to get yourself out of a sticky situation before you get yourself into it.