Because skylights interrupt and penetrate the surface of a roof, they are particularly prone to leaking. Rainwater flows down the roof, collects at the top of the skylight, builds up, and eventually finds a place around the edges where it can come drooling inside.

Skylights are typically manufactured with their own integral metal flashing--a flat metal flange that extends from the body of the skylight and tucks under the roof shingles.

In some cases, a skylight is set up on a curb that is flashed with continuous flashing along the base, step flashing up the sides, and saddle flashing across the top, similar to the way a chimney is flashed.

If your skylight leaks, chances are good that the problem is the seal between the flashing and the roof. To fix this, lift the shingles next to the skylight one at a time, and brush out any dirt and debris.

Use a caulking gun loaded with a tube of roofing cement to force the mastic up under the shingle where the metal flashing meets the roofing felt (the thick black paper under the roofing material). Then press down on the roofing shingles to seal them down tight.

Seal any obvious holes, open seams or joints in the flashing, too. For this, you can use the same roofing cement.

Check the seal where the skylight's glass meets the frame. If you see any signs of leaking, seal this with clear silicone caulk.

Note: Some skylights, notably those in bathrooms are very humid rooms, can drip moisture from condensation.

Obviously, this water is not coming from the top of the room, it is occurring when the moist room air meets the cold surface of the skylight and condenses. To fix this problem, improve the ventilation in the room.