A skylight lasts 10 to 20 years per the InterNACHI life-expectancy chart, its leaks come from failed flashing rather than the glass, and a watertight unit depends on a flashing kit matched to the mounting type and roof covering. VELUX America credits lasting waterproofing to that matched kit, not to caulk that breaks down.
Understanding the service life, the real source of leaks, and the flashing system behind a skylight tells a homeowner what to expect and what to verify.
How Does a Skylight Stay Watertight?
A skylight stays watertight through an engineered flashing kit matched to both the mounting type and the roof covering, which sheds water without relying on caulk that breaks down over time, per VELUX America. The kit layers metal pieces over and around the unit so water passing the frame drains back onto the roof, and the manufacturer engineers a different kit for each combination of mounting type and roofing material rather than a single universal detail.
Skylights install in one of two mounting types, per VELUX America. A deck-mounted unit fastens its frame directly to the roof deck for a lower profile, while a curb-mounted unit sets on a built-up curb and suits flat and low-slope roofs. Each mounting type takes its own matched flashing kit, so a deck-mount kit and a curb-mount kit are not interchangeable, and the kit also varies with the covering beneath it.
Cracked, dried, or peeling caulk around the curb signals a sealant-only installation breaking down, which is the failure an engineered kit avoids. Rust stains, lifted or separated flashing edges, and meltwater backing up at the uphill edge during an ice dam point to the same flashing problem rather than the glass. The fix restores the matched flashing kit, not another bead of caulk.
When Does a Skylight Need Repair Versus Replacement?
A skylight inside its 10-to-20-year InterNACHI service life that develops a flashing leak calls for flashing repair, while a unit past that range, or one with fog between the panes, favors replacement. The InterNACHI Estimated Life Expectancy Chart sets the 10-to-20-year figure that separates a repairable unit from one near the end of its useful service.
Fog, haze, or trapped moisture between the glass panes indicates a failed insulated-glass seal, a failure that cleaning cannot fix because the moisture sits inside the sealed airspace. VELUX America covers that fogging under a 20-year insulated-glass-seal warranty, separate from leak coverage, and the repair for a fogged unit is glass-unit or skylight replacement rather than a reseal.
Coordinating skylight work with the roof covering avoids redoing the penetration twice, so a unit near the end of its service life often gets replaced during a re-roof while the surrounding shingles or membrane are already open. That timing keeps the flashing kit, the unit, and the covering as one continuous system installed together.
What About Condensation, Glass, and Low-Slope Roofs?
Water at a skylight in cold weather that clears as humidity drops is condensation from excess indoor humidity on cold glass, not a roof leak, and Low-E warm-edge glass reduces but does not eliminate it, per VELUX America. The diagnostic cue from VELUX America is direct: condensation comes and goes with temperature and humidity, while a true leak tracks with rain and storms.
A skylight on a roof under 3:12 slope sits on a curb at least 4 inches above the roof plane under IRC R308.6.8, unless manufacturer instructions specify otherwise; that 4-inch curb requirement applies only to roofs under 3:12, not to every skylight. Low-slope roofs also need positive drainage, since the NRCA minimum design slope of 1/4 inch per foot keeps water from ponding at the penetration, where standing water past 48 hours counts as a defect per NRCA and ARMA guidance.
On a detached one- and two-family dwelling, replacing or repairing the roof covering and its penetrations counts as ordinary maintenance that requires no construction permit under N.J.A.C. 5:23-2.7. On a commercial building, repairing more than 25 percent of total roof area within a 12-month period requires a permit under the same rule.
What Should a Homeowner Verify Before Hiring?
Verify that the installer matches the flashing kit to the brand, the mounting type, and the roof covering, and confirm that the manufacturer No-Leak warranty attaches only to a spec installation. VELUX America applies its No Leak installation warranty only when the unit is installed to spec with a matching VELUX deck-mount or curb-mount flashing kit, and Fakro USA conditions its 10-year leak-proof guarantee on using original Fakro flashing kits.
Brand names such as VELUX and Fakro identify the units and kits an installer puts on the roof, not a credential the contractor holds. A grounded conversation confirms which mounting type the roof calls for, which flashing kit pairs with the covering, and whether the manufacturer warranty stays intact through a spec installation. Those answers separate a watertight install from a sealant-only one that fails inside the service life.
A skylight lasts 10 to 20 years, leaks at the flashing rather than the glass, and stays watertight only through an engineered kit matched to its mounting type and roof covering. Verifying that match, and the warranty that depends on it, is the homeowner's surest protection.