The first and perhaps most obvious place to look for a roof leak is directly above the leak in a ceiling or exterior wall. Use a flashlight to inspect the attic floor over the leak while it's raining. Look for standing water, water stains, mold, wet insulation or other exposed insulation.
Examine the underside of the roof for wetness or mold around points of penetration (plumbing vents, chimneys), wherever different roof planes intersect (valleys) and near dormers. These symptoms indicate holes in the flashing or faulty flashing installation.
A leak away from such locations suggests a problem in the roofing material. Keep in mind that water may travel sideways before passing through a joint in the roof sheathing, and may travel in a horizontal joint before falling on the floor or ceiling.
Take measurements from points inside that you can also locate from outside. Measure down from a ridge and horizontally from the center of a valley or sidewall; or measure distances from a chimney or other point of penetration.
If your ceiling is attached to roof rafters, as would be the case for a cathedral ceiling, all you can do from inside is take the measurements that will help you locate the leak externally, and attempt to control the damage internally.
Water can travel on the underside of sheathing or down roof rafters before dropping off in one or more places. To control where it falls, tack a piece of string into the stream of water and let it hang into a bucket. The water will tend to follow the string.
Poke or drill a hole in your ceiling to let the water through. This technique prevents the water from spreading across the top of the ceiling to other areas; it prevents the ceiling from becoming saturated, eliminating the chance of collapse and often the need for replacement; and it allows you to collect water from below using the string-and-bucket method.
A roof inspection should be done at least twice a year, preferably in the spring particularly right after a harsh winter, and in the fall just before winter. Hot and cold weather can turn minor problems into major complaints.
Roof inspections can locate potential leaks and damage before they become real problems. The cost to replace a roof is measured in dollars compared to the cost to inspect a roof which is measured in pennies. Thus a roof inspection is a very cost effective way of avoiding potentially further damage and associated costs.