A skylight is a hole in the roof with a reputation, and the reputation is only half earned: modern units with their engineered flashing kits are reliable, while the leaks attributed to them divide between a few predictable flashing points and an impostor, condensation, that is not a leak at all. Diagnosing a skylight leaking complaint starts with that split. This reference covers the impostor, the real failure points, and the repair hierarchy.
First, the condensation impostor
Water at a skylight in cold weather, without rain to blame, uncorrelated with storms and worst on humid mornings, is usually condensation: the skylight is the coldest interior surface in the room, indoor moisture films and beads on the glass and frame, collects in the bottom corners, and drips or streaks the light well, staining exactly like a leak. The tells: timing (cold snaps, showers, cooking), pattern (bottom of the glass and frame, evenly, rather than one corner after storms), and company (matching condensation on the windows). The fixes are indoor-climate fixes: humidity managed into range, bath fans actually used and ducted outdoors, and, on chronic units, the better-insulated glass of a replacement. Modern skylights add engineering to the fight, thermal-break frames and internal weep channels that drain minor condensation to the exterior, which is also why blocked weeps masquerade as leaks on aging units.
The real failure points
Actual water entry favors a short list. The flashing kit’s corners and laps: skylights seal to the roof with a manufactured step-and-saddle flashing system, lapped like all flashing, and its vulnerable spots are the corners, the uphill (head) piece where debris and snow dam, and any place an installer substituted caulk for the kit or reused old flashing under new shingles. The uphill dam: leaves, needles, and ice collecting against the head flashing pond water above the unit, the skylight playing the role of a chimney without a cricket, and seasonal clearing plus, on wide units in snow country, a diverter solves what looks like a unit defect. Gaskets and glazing seals: the perimeter seal between glass and frame ages in UV, and failed insulated-glass seals fog between panes, the same failed-seal physics as windows, announcing the unit’s age. The light well’s finish joints: water arriving at the ceiling junction sometimes started at the roof far away, traveling framing before dropping at the well, which is why the uphill leak hunt and its hose test apply to skylights exactly as to stains, and why the well’s drywall corner is an exit, not necessarily the entry.
The repair hierarchy
In escalating order: housekeeping, clearing the uphill dam and the unit’s weep channels, resolves the ponding cases free; flashing rebuild, the manufacturer’s kit installed with proper laps into the surrounding courses, is the durable answer at flashing failures, with caulk demoted to its fastener-head cameo; gasket and glass service exists for some brands, but failed seals on an aging unit usually argue for the next tier; replacement, since units run 20 to 30 years, one to two shingle cycles, and swapping a skylight during a reroof costs a fraction of a standalone job, the scheduling insight that turns every reroof quote into a skylight decision: replace it now, or commit to its remaining decade. Deck-mounted modern units with integrated flashing generally out-behave old curb-and-caulk installations, and upgrading the species during replacement is the era’s quiet reliability gain.
The bottom line
Cold-weather drips are the house’s humidity, not the roof; storm-correlated water is the flashing’s corners, the uphill dam, or a traveling leak from elsewhere; and fogged glass is the unit announcing retirement. Clear the dam, rebuild laps instead of caulking them, and let every reroof ask the 20-year question, because the only cheap time to replace a skylight is while the shingles are already off.