Skylight Leaking? How to Tell If It Is the Skylight or the Roof

Water on the floor under a skylight is one of the more misleading leaks in a house. It could be the skylight itself, the flashing around it, or nothing more than condensation on a cold morning, and each one has a different fix. Here is how to tell them apart before anyone reaches for a caulk gun.

Three problems that look the same from the couch

A skylight is a hole in the roof with a window in it. Water can get to the ceiling three ways: it can form on the glass from inside (condensation), it can get past the seal between the glass and the frame or through a cracked dome (the skylight failing), or it can get past the flashing where the roof meets the skylight (the roof failing around a perfectly good skylight). In our experience the third is the most common on roofs more than a few years old, the first is the most common false alarm, and the second is what people assume every time.

The good news is that each one leaves different evidence, and you can read most of it from a stepladder with a flashlight.

Condensation: the leak that is not a leak

If the drip shows up on cold mornings, in a bathroom or kitchen, or the morning after a long rain when the house is humid, and the glass is wet on the inside, you are looking at condensation. Warm, moist indoor air rises into the skylight well, hits the cold glass, and turns back into water, which runs down and collects at the bottom edge of the frame. It can drip enough to stain drywall and look exactly like a leak.

The test is simple: does it happen when the weather is dry? A roof leak needs rain. Condensation needs a temperature difference and humidity, and it ignores the forecast. Fixes are indoor fixes: run the bath fan, improve air circulation in the well, and in a tall shaft, insulation around the shaft walls. One related sign is fog or haze trapped between the two panes of glass that never clears. That is a failed insulated glass seal. It is not a leak either, but the glass unit has lost its insulating value and will keep fogging, and the only real fix is replacing the skylight.

Flashing: the roof problem

A skylight on a shingle roof is flashed like a small chimney: an apron at the bottom, step flashing woven into each shingle course up the sides, and a head flashing or saddle across the top that sends water around. Any of those pieces can be installed short, nailed through the wrong spot, rusted, bent by a boot, or buried under a bead of caulk that has since cracked. Shingles laid tight against the curb with no gap for water to run trap debris and hold water against the frame.

Flashing leaks show up during or shortly after rain, often on the uphill side or at a corner of the skylight well, and the stain tends to spread on the ceiling away from the skylight rather than straight down from the glass. Look in the attic if you can get beside the well: water tracks on the framing, dark staining on the underside of the deck above the skylight, or wet insulation on one side tell you the water is entering outside the skylight itself. This is the same detective work as any other leak, and our guide on leaks when the shingles look fine walks through the method.

Seal and frame failure: the skylight problem

Older skylights with acrylic bubble domes crack and craze with age, and hail cracks them outright. The gasket between the dome and the frame dries out, and the corners of the frame can open up. Some older units were built with a glass unit set in a frame that simply does not seal anymore. If the water is clearly coming from inside the frame, from the glass edge or the corner joint rather than from the surrounding ceiling, the skylight is the problem.

After a hail storm, check the dome or glass before you assume the roof leaked. A cracked skylight is a common piece of hail damage that gets overlooked because everyone is looking at shingles, and when it is part of a storm claim it belongs in the scope. We cover the rest of the overlooked items in what else hail damages. Coverage depends on your policy, and our adjuster's job is to document it properly, not to promise an outcome.

Curb-mounted versus deck-mounted

There are two ways a skylight sits on a roof, and it changes how it is repaired and replaced. A curb-mounted skylight sits on a wooden box, typically a few inches tall, framed up from the roof deck. The skylight caps the curb, and the curb is flashed like a chimney. These are common on older homes and low-slope roofs, they are easy to re-flash, and the skylight can be swapped without touching the curb if the sizes match.

A deck-mounted skylight has a low profile and a flange that fastens directly to the roof deck, with the manufacturer's flashing kit and an adhesive underlayment integrating it into the roof. Done right, it is a tighter, better-looking installation. The catch is that it depends on the kit being installed exactly as designed, with the shingles, so a repair usually means removing shingles around it, and a replacement is best done when the roof is open anyway. Neither type is inherently leakier; the installation is what matters.

Finding the source without guessing

If the attic does not settle it, a hose test will. One person on the roof with a garden hose, one inside watching the well. Start low, wetting only the apron below the skylight for several minutes, then the sides, then the top, then the glass itself, moving uphill and giving each zone time before moving on. The moment water appears inside tells you which zone is leaking. It is slow, it is unglamorous, and it beats the alternative, which is running a bead of caulk around everything and waiting to see if it worked.

Caulk is not flashing. It is fine as a temporary measure on an active leak; it is not a repair, and a contractor whose skylight fix is a tube of sealant has scheduled the next leak for you.

The replace-at-reroof rule

In our experience a skylight's seals, gaskets, and flashing age on roughly the same schedule as the shingles around it, and the manufacturer's flashing is designed to be installed with new shingles, woven course by course. Put a new roof around a fifteen-year-old skylight and in a few years you will be cutting into new shingles to replace it, paying for the flashing work twice and disturbing a roof that was just made watertight.

That is why we quote new skylights with every roof replacement. You can decline, and if the unit is recent you probably should, but you should see the number and make the call with it in front of you rather than find out later. Replacement day is also the moment to change the type, the size, or to add a vented unit if the room needs it, because the framing is exposed and the labor is already on the roof. Our guide on what gets fixed on replacement day covers the other items that belong in that conversation.

If you have a drip under a skylight and you are not sure which of the three it is, Robust Roofing's free inspection will tell you with photos, from the roof and the attic, and quote the roof repair only if one is needed. Call (470) 887-6637.