Use a flashlight to look at the underside of the roof decking. The top center of water spots show the general area where the water is breaking through the roofing materials. Measure the distance from the nearest gable wall and the distance from the intersection of the roof with the wall.
Set a ladder against the house. Firmly plant the feet of the ladder on the ground, and extend the top at least 3 feet above the roofline. On your way to the top of the roof, stop and measure the distance from the wall to the edge of the roof. Add this measurement to those from inside the attic.
Use the corrected measurements to find the approximate location of the roof leak. Look for curled, damaged or missing shingles in the area. Other potential leaks exist around plumbing vents and chimneys. Valleys, where two different rooflines meet, are other places leaks tend to form.
If you are unable to find any potential sources of the leak, call for professional help. Leaks caused by inadequate flashing are harder to find. In some cases, the roofer may recommend replacing the entire roof instead of making a repair.