Skylights are supposed to bring light into your life, not rain on your parade--but sometimes they do spring a leak. In the event that your skylight starts leaking, here's how to analyze the problem and make a few of the simpler repairs yourself.
Verify that the supposed leak is not actually excessive condensation that has moved along the underside of the glass toward the edge and then dripped down the side of the interior opening. This is most likely in very cold climates, in single-glazed skylights, and in damp locations such as bathrooms, kitchens and greenhouse rooms. You will probably need a ladder.
Verify that the skylight is fully closed and that any weather seals are in good condition (pliable, not deformed or torn). Problems typically occur when skylights are located in very high ceilings and it's hard to see if they are fully closed.
Ask yourself: Does the leak occur only in winter, in fall, or when there is snow or ice on the roof? If the answer to any of these questions is yes, look behind the skylight for leaves, ice, snow or other debris that might be blocking the downhill flow of water and causing it to back up under the shingles.
Look carefully at the roofing or flashing on vents and other penetrations further up the roof. A leak there can travel quite a distance before finding its way to an indoor opening at the skylight.
Most skylights have integral flashing or a special flashing kit that is specifically designed for the skylight and roofing material. Obtain the installation instructions for the skylight's make and model.
Refer to the installation literature to verify that the unit is installed according to instructions. Inspect with a flashlight and mirror, and pry up a few shingles (but don't damage them).
If it is not installed correctly, and you know the company or builder that did the job, contact that company to fix it. Otherwise correct the installation yourself or hire a professional recommended by the manufacturer's local dealer.
If the visible portion of the installation appears to match the instructions, inspect the flashing for defects or damage.
Inspect a custom skylight (typically nonopening) for proper flashing details, including the following:
Soldered top flashing wraps around the corners and extends at least 6 inches (15 cm) under the roofing at the top and 4 inches (10 cm) at the sides.
Soldered (or folded) base flashing wraps around the lower corners and extends at least 4 inches (10 cm) under the shingles at the sides and 4 inches (10 cm) over the shingles below.
Step flashing interwoven with shingles on the sides extends at least 3 inches (7.5 cm) up the curb sides and 4 inches (10 cm) onto the roof, and overlaps the next piece by at least 2 inches (5 cm).
Counter flashing that's sealed or bonded to the skylight and soldered at the corners extends down all sides to within 1 inch (2.5 cm) of the roof.
Roofing should extend to the flashing on the sides and top. Gaps tend to collect debris that traps and slows water flow.
Examine exposed flashing carefully for corrosion, pinholes and other damage. Carefully bend up the roof to see as far under it as possible with the help of a mirror and flashlight.
Examine previous "temporary" repairs made with caulk, flashing cement or other sealants. Look for cracks and holes, dry or brittle sealants and peeling. Either remove old sealant and apply a new layer, or make a more reliable repair.
Patch damaged flashing as described in How to Patch a Gutter Leak.
If you are an experienced do-it-yourselfer with roofing skills and feel comfortable working on roofs, remove the roof and flashing as necessary to correct a faulty installation or replace damaged flashing. Otherwise hire a competent professional.