The first real cold snap of a Puget Sound winter has a way of exposing skylight trouble. A homeowner wakes up on a gray January morning, finds a few beads of water on the kitchen counter, glances up at the damp paint around the skylight opening and decides the roof has failed. Quite often, though, the water never came from outside at all.
That difference matters more than most people expect, because the fix for one problem does nothing for the other. A skylight that lets in rainwater needs work up on the roof, while one that sweats from indoor moisture needs changes inside the house or in the glass itself. Crews at C&A Roofing, who have been installing and replacing skylights across Kirkland, Bellevue and the rest of the Eastside since 2009, regularly see “leaks” that turn out to be condensation, and the reverse happens nearly as often. Reading the evidence first saves money and guesswork.
The Clock Often Knows Before You Do
Timing is the most honest clue you have. A true roof leak tends to show up during heavy rain or within a few hours of it, and it is usually worst when wind pushes storms in from the southwest. Multi-day atmospheric river storms are notorious for finding weak flashing.
Condensation keeps a completely different schedule. It appears on cold, clear nights and early mornings, frequently after days without a drop of rain. It gets worse after long showers or laundry drying indoors. On the coldest nights it can freeze on the glass and melt once the sun hits, so water drips on a bright, dry morning. That pattern almost always points to the room rather than the roof.
What the Water Leaves Behind
Where the drip lands tells the next part of the story. Condensation forms on the inside face of the glass and the cold frame, so it usually wets the opening fairly evenly and beads up across the pane before running down.
Rainwater behaves differently. It tends to enter at one spot, most often a corner on the uphill side, and it leaves brownish rings on drywall as tannins wash out of wood sheathing and framing. Sometimes the stain shows up a few feet away from the skylight entirely, because water travels along a rafter before it finds a place to drop. Fog trapped between two panes of glass is its own separate issue. That signals a failed glass seal, which no wiping or caulking will clear.
A few simple checks narrow things down.
- Dry the glass and frame completely in the evening, then look again at sunrise. Fresh beading on a rainless night points strongly toward condensation.
- Tape a strip of paper towel along each inside corner of the skylight shaft. A wet corner after a storm, with dry towels elsewhere, suggests a roof entry point.
- Place an inexpensive hygrometer in the room. Winter readings that stay above about 50 percent relative humidity make condensation far more likely.
- Leave hose testing to a roofer on steep or mossy roofs, since a proper test works uphill one zone at a time.
Why Northwest Skylights Sweat
Physics is not on the skylight’s side. Warm, moist air rises, and the skylight is usually the coldest surface it meets near the ceiling. A typical insulated attic might be rated R-38 or better, while an ordinary double pane skylight sits somewhere around R-3. Put those side by side in a mild but damp climate and the glass will reach its dew point long before the walls do. Tighter modern construction traps moisture that older drafty homes simply leaked away, so cooking, bathing, houseplants and even breathing add up.
The skylight shaft is the other hidden culprit. Many shafts are lined with uninsulated drywall, and warm air sneaking around the frame into the curb can condense inside the unit where nobody sees it. When that water finally drips out, it looks identical to a roof leak. Glass quality plays a role too. Skylights carry a Condensation Resistance rating on a scale of 1 to 100, and Ohio State’s overview of window and skylight ratings explains how that number works alongside U-factor when comparing units.
When the Roof Really Is to Blame
Rain gets in through a fairly predictable set of weak points, usually in the same order that water meets them on its way down the slope.
- The head flashing on the uphill side, where fir needles and moss pile up and hold water against the top of the skylight.
- The step flashing along each side, which fails when it was replaced with a smear of roof cement instead of properly woven pieces.
- The apron or sill flashing at the bottom, where water from the whole skylight collects before draining onto the shingles.
- The glazing gaskets and seals on the unit itself, which harden and shrink after enough summers of UV exposure.
- The curb framing, which can rot unseen for years once water gets behind the flashing.
Older acrylic domes yellow and eventually crack, but age is the bigger issue overall. Many skylights reach the end of their useful life around the same time as a shingle roof, sometimes sooner, and an old unit rarely holds up well once everything around it has been renewed.
Matching the Fix to the Source
Condensation problems respond best to drier indoor air and warmer glass. Running bath fans longer, using the range hood every time you cook and making sure a heat recovery ventilator is actually running can drop humidity enough to stop the dripping. Air sealing around the frame and insulating the shaft walls helps keep warm air from reaching the coldest parts of the unit. A closed shade can actually make things worse by trapping air against the glass. If the glass is old or the seal has failed, a replacement with low emissivity coated insulated glass and warm edge spacers is often the only lasting answer.
Genuine leaks call for flashing done the way the manufacturer intended. Purpose made flashing kits, self adhered underlayment wrapped up the curb and properly lapped step flashing outlast any amount of sealant. If the curb is soft or the unit is twenty years old, replacing it usually beats patching it twice.
The Smartest Moment to Make a Change
The cleanest time to deal with any skylight is while the roof around it is being replaced. The flashing ties into new underlayment and shingles as one continuous system. Leaving a tired skylight in place under a brand new roof almost guarantees someone will be pulling shingles back around it within a few years.
That moment is also a good time to ask whether every skylight still earns its place. A unit over a rarely used hallway might be better removed and roofed over, while a dim interior bathroom could gain more from a small tubular skylight than from the original box. Light from above is one of the real pleasures of living through a long Northwest winter, and it is worth having it come in without the water.