Water is running down the inside of your skylight and pooling on the frame. It is obviously leaking. Except that in a Seattle winter, there is a substantial chance nothing is leaking at all, and the difference determines whether you need a roofer or a bathroom fan.
A skylight is a large piece of glass mounted in a roof, which makes it the coldest interior surface in the room by a wide margin. The room below is warm and, in a Seattle winter with the house closed for months, carries a fair amount of moisture from showers, cooking, and occupancy.
When that moist air contacts glass sitting below its dew point, water condenses on it — the same process as a cold drink glass on a summer afternoon. It then runs down the glass and collects at the lowest point, which is the frame or the bottom of the light well.
Skylights over bathrooms and kitchens are the most affected, for obvious reasons. Older single-glazed units and units with aluminium frames that lack a thermal break are dramatically worse, because the frame itself becomes a cold bridge.
Timing against weather. The most useful test. Condensation is worst on cold nights and clear mornings, and it appears whether or not it has rained. A leak correlates with rain, and especially with wind-driven rain from a particular direction. If you are getting water during a dry cold snap, it is condensation.
Where the water starts. Watch the unit while it is happening. Condensation forms across the whole glass surface as a film or as droplets and runs downward from the middle of the pane. A leak enters at a perimeter — at the head, the sides, or where the frame meets the roof — and runs from the edge inward or down the well.
The light well. Staining on the drywall of the light well, particularly at the top corners, points to a leak at the flashing. Water sitting on the frame with a clean well above it points to condensation.
Correlation with occupancy. Worse after a long shower, after cooking, or after a weekend when the house was full? That is interior humidity.
Fogging between the panes. If a double-glazed unit shows moisture between the panes rather than on the inside surface, the insulated glass seal has failed. That is neither a leak nor room condensation — it is the unit itself at end of life, and no amount of flashing work changes it.
The fix is moisture management, not roofing. Run bathroom and kitchen exhaust fans during and for a good while after use, and confirm those fans actually vent outside rather than into the attic — a startling number do not, and one that terminates in the attic is creating a second problem above the ceiling while failing to solve this one.
Beyond that: measure indoor humidity with an inexpensive hygrometer, and if it sits above roughly 50 percent through the winter, address the sources. Improving insulation around the light well reduces the cold bridging. And on an older single-glazed or non-thermally-broken unit, replacement with a modern insulated unit — many of which include a small internal channel designed to catch and evaporate condensate — may simply be the answer.
Skylight leaks come from a small number of places.
Sealed instead of flashed. The most common serious error. A skylight must be installed with the manufacturer's flashing kit, which comprises separate sill, step, and head components integrated into the shingle courses. A unit bedded in sealant with shingles butted to it is a leak on a timer, and no amount of additional sealant fixes it.
Head flashing failure. The upper edge carries everything running down the slope above it. On a long run above the unit, that volume is significant, and a cricket may be warranted on wider units for the same reason chimneys need them.
Seal failure at the unit. The gasket between glass and frame degrades with age, typically in the same twenty to thirty year range as the unit itself.
Curb versus deck mount. Curb-mounted units sit on a raised built-up curb and are generally more forgiving of imperfect detailing. Deck-mounted units sit lower and depend more heavily on precise flashing integration.
If the unit is old and the flashing needs redoing anyway, replacing the skylight at the same time is usually the better spend, because the labour overlaps almost entirely. Reflashing a twenty-five year old unit means paying for the access twice when the glass fails five years later.
And if a re-roof is coming, that is unambiguously the moment. New flashing kits go in as part of the work, and if the units are near end of life, replacing them while the roof is open costs a fraction of doing it as a separate project later.
This post is part of our in-depth coverage of roof repair & leak detection in Seattle.
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