What is a skylight?
Updated August 2026
A skylight is a window set into the roof plane to bring daylight into a room from above. Three versions cover nearly every house: fixed units that never open, manual venting units cranked open with a pole, and motorized or solar venting units that shut themselves when a rain sensor gets wet. Current units use insulated glass; older ones are acrylic domes. The small cousin, a tubular daylight device, funnels light down a 10-inch to 14-inch mirrored tube into a ceiling lens.
Where it is
What matters is not where the glass sits but how it meets the roof. A curb-mounted skylight stands on a wood frame built 4 to 6 inches above the deck, so water has to climb a step to reach the seal. A deck-mounted unit fastens flush to the sheathing under a factory flashing kit. Either way the waterproofing is a layered system of step flashing, head flashing, and an apron, never a bead of sealant. Indoors, the opening usually flares into a drywall light shaft crossing the attic, and that shaft is where a leak announces itself as a stain running down one corner.
What it does
Daylight is the point, and there is a sizing rule worth knowing: the U.S. Department of Energy suggests holding skylight area under 5 percent of a room's floor area where there are plenty of windows, and under 15 percent where there are few. Leaks are what people remember, and the glass is almost never guilty. Blame belongs to 20-year-old flashing, a rushed installation, or leaves and shingle grit damming behind the uphill edge. The unit runs on two clocks of its own. The insulated glass seal gives out between years 10 and 20, and once it does, the pane fogs permanently between the layers and cleaning does nothing. Acrylic domes yellow, craze, and crack in 10 to 20 years. Budget 20 to 30 years from a quality glass unit, which on most houses is less than the roof around it.
Maintenance
Twice a year, clear leaves, needles, and grit from the uphill side, since that small dam is the usual reason a skylight that never leaked suddenly does. Once a year, inspect the flashing, the exterior seals, and the gasket on any unit that opens, then run a hose test if you are suspicious. Exterior joints want attention roughly every 10 years (how often to reseal a skylight), and the rest of the routine sits in the skylight checklist. A postponed drip gets expensive fast: the cost of ignoring a leaking skylight.
What it costs to replace
A replacement skylight runs $900–2,500 installed for a fixed or manual venting unit, $1,500–3,500 for a solar or electric venting model, and $300–800 to reflash a sound skylight that leaks only at the edges.
OnOtto's photo scan identifies parts like this in your home and builds the maintenance schedule for you. See how the scan works.
Frequently asked questions
Do all skylights eventually leak?
No, but old flashing does. Units sold today ship with engineered flashing kits and factory-bonded gaskets, and a curb-mounted install on a properly shingled roof commonly goes 20 years without a drop. The bad reputation belongs to 1980s bubble domes, site-built curbs sealed with tar, and any skylight flashed by someone in a hurry. Roof age matters more than brand: a skylight sitting in worn shingles leaks eventually no matter what the label promised.
Is the water around my skylight a leak or condensation?
Watch when it appears. Condensation shows up on cold winter mornings, collects on the glass and the frame edge, and has nothing to do with rain; the cure is indoor humidity held near 30 to 40 percent, a bath fan run 20 minutes after showers, and better attic ventilation. A genuine leak arrives with or just after rainfall and stains the drywall shaft instead of beading on the glass. If both happen, repair the flashing first and see what is left.
Should I replace my skylights when I reroof?
If a unit is over about 10 years old, yes. Flashing comes off and goes back on regardless, so the labor already sits in the roofing bid, and adding a new unit at that moment runs $600–1,500 apiece instead of $900–2,500 as a standalone job. Reusing a 20-year-old skylight under a 30-year roof guarantees a second, costlier visit five years out, with fresh shingles cut open to do it. See how long a skylight lasts.
Why is my skylight foggy between the panes?
The insulated glass seal failed and let moist air into the sealed cavity. It typically happens 10 to 20 years in, sooner on a roof that bakes, and no cleaning or drilling trick fixes it for long. Some manufacturers warranty the glass unit for 10 to 20 years, so read the label stamped in the corner of the pane before paying anyone. Otherwise the choice is replacing the glass unit or the whole skylight.