What to Do:
Make an attempt locating the leak from inside
1. Perhaps the most obvious start is to attempt looking for a roof leak directly above the leak in an exterior wall or ceiling. Get a flashlight or adequate lighting. Inspect the attic floor while it's raining over the leak. Attempt to spot any water buildup, mold, water stains or wet insulation.
2. Study the roof's underside for areas of wetness or mold near areas of penetration like plumbing vents, chimneys, or where different planes of the roof intersect such as valleys, and dormers. These examples signify holes in the flashing or bad installation.
3. A leak away from these areas usually indicates troubles in the roofing material. Always keep in mind that water can travel sideways before passing through a joint in the roof sheathing, and can travel in a horizontal joint before falling to the ceiling or floor.
4. It's usually very helpful to record measurements from inside that you can use to locate points of reference from the outside. Take a measurement down from a ridge and horizontally from the center of a sidewall or valley. Or measure distances from other points of penetration or chimneys.
5. For roof rafters that are attached to the ceiling is take measurements that can help you locate a leak externally and try to control them internally.
Managing the Damage
1. Water is well capable to travel on the underside of sheathing and down roof rafters before dropping into one or more areas. Water management can be made simply by putting a string into the stream of water and routing the string to a bucket. Water will usually tend to run along the string into the bucket.
2. You can also drill a hole into your ceiling to allow the water to run through it. This technique will keep the water from spreading to other areas causing more damage. It does not allow the ceiling to become further saturated. It will reduce the chance of your ceiling collapsing and will allow you to use the string to bucket method to collect the water.
Locate the Leak Outside
1. Using some of the measurements you got from inside, scope out your observations using a ladder and / or a pair of binoculars to make it easier on you. Refrain from walking on roofs that are wet, or during rain. Especially wood roofs as they will be incredibly slick.
2. Look for leaves and other debris that could be blocking water flow. It can happen in valleys adjacent to, above any roof penetration, or dormers. If snow is on the roof, a possible ice dam could have formed at the lower edge of the roof causing water to back up under overlapping layers of roofing material. Remove any debris or obstruction if you can safely get to it.
3. If you're able to get close enough, examine metal flashings for corrosion or joints that connect to a chimney or other roof penetration. You can patch these metal flashing temporarily. Replacing them is the only permanent solution to it though. Usually with dried out rubber gaskets on the plumbing vents you can easily replace them.
4. Pay close attention to any areas covered with the black flashing cement. That indicates previously repaired leaks. Try spotting any pinholes or cracks that often occur as the material ages. You can make temporary repairs to them with some flashing cement using a drywall or putty knife.
5. When and if you're able to try inspecting attachments like antennas, a satellite dish, or anything else that might have been nailed, or screwed into the roof. A small flashing that is in good condition should always cover each fastener. Ideally, the best thing to do is not fasten anything onto your roof in the first place.
6. If you determined while inside that your leak was mid-roof not relating to any flashing, look for shingles that are missing or damaged. On wood roofs, look for any shingles or shakes that are badly warped, cupped shaped, or cracked. Look for any joints in one course that fall less than 1 1/2 inches to the right or left side of a joint in the course below. Flat or nearly flat roofs usually require very close inspection to find any deteriorating, worn out, or damaged areas.