Skylight Notes: Why They Weep in a Michigan February
The call comes in almost every February: water at the skylight, right after the coldest week of the year. Sometimes it is a flashing leak. More often it is the glass itself sweating, or an ice dam backed up against the curb. Here is how we sort the three apart before anyone touches a shingle.
A skylight is the one place on a roof where homeowners actually watch the ceiling, so it is also the place where the first sign of trouble gets noticed fastest. Every winter, once the deep cold sets in around Macomb County, we start fielding the same call: water showing up at the corner of a skylight, sometimes just a bead on the glass, sometimes a drip working its way down the inside of the shaft to the drywall below. The instinct is to assume the roof failed. Most of the time something quieter is going on, and telling the three usual causes apart is most of the job.
01 /Condensation weeping on cold glass
The most common February skylight call is not a leak at all. It is condensation. Warm, humid indoor air rises naturally, and a skylight shaft gives it a direct path to the coldest glass surface in the house. When that glass drops below the dew point of the air touching it, moisture condenses on the inside pane the same way it beads on a cold drink, and on a deep-cold morning it can run down the frame in a thin line that looks exactly like a leak from below.
A few things tend to give condensation away. It shows up on the coldest mornings of the stretch, not during or after rain or snowmelt. It is usually confined to the frame and the inside edge of the glass rather than tracking from a specific point in the flashing. And it often shows up on more than one skylight in the house at once, since the same indoor humidity is pressing on all of them. A single-pane or older double-pane unit is more prone to this than a newer, better-insulated skylight, which is a detail worth knowing if this happens winter after winter on the same unit.
02 /Ice damming against the curb
The second pattern is mechanical rather than atmospheric. A skylight interrupts the roof deck, and the curb it sits in creates a small dam of its own, a raised lip where snow can pile and meltwater can pool during a freeze-thaw stretch. If an ice dam forms just upslope of the skylight, the same way it forms along an eave, meltwater can back up against the curb and work under the flashing rather than shedding past it. This is a classic mid-winter call, usually arriving a day or two after a heavy snow followed by a partial thaw, the pattern our storm and insurance guide covers for eave ice dams generally.
Condensation shows up on the coldest, driest mornings. Ice dam backup shows up after snow and a thaw. A true flashing leak can show up any time weather drives water at the roof. Noting exactly when the water appeared is the single most useful thing a homeowner can hand us before we ever get on the roof.
03 /A genuine flashing leak
The third possibility is the one everyone assumes first: the skylight flashing itself has failed. Skylight flashing is a step-and-counter system built in layers around the curb, and it is one of the more complex flashing details on an ordinary roof, which also makes it one of the more common places for an aging or poorly installed roof to leak. A real flashing leak tends to track to one corner or one side of the unit rather than spreading evenly around the frame, and it often shows up during active wind driven rain or right as snow is melting off the roof above it, not on a still, dry, cold morning.
Sealant around a skylight curb also ages the same way it ages everywhere else on a roof, and a curb that was fine five reroofs ago can be the weak point on a roof that is otherwise sound. Where a flashing leak is confirmed, the fix is a repair to the flashing and curb detail rather than a whole-roof job in most cases, and that work falls in our published repair range of $350 to $3,200, with larger or more complex flashing rebuilds running toward the higher end.
The deeper building science of attic humidity and condensation control is its own subject, one we cover at more length in the guides library rather than here. What we can do quickly, on site, is settle which of the three is actually happening. A free roof inspection checks the skylight curb, the surrounding flashing, and the attic side of the shaft, with photos either way, at no cost or obligation. If it turns out to be storm related damage rather than condensation or an aging curb, that inspection also becomes the documentation we can walk an adjuster through on site. And if water is actively coming through right now, we answer 24/7 for emergency tarping while the real fix gets scheduled.
One winter of a weeping skylight rarely means the roof is in trouble. A pattern that repeats every cold snap, on the other hand, is worth a look before it becomes drywall damage. Start with the storm damage service page if you suspect weather is involved, or run your numbers through the 60-second cost calculator if a flashing repair looks likely; both use the same published pricing we quote on site.
- Building Science Corporation, published research and information sheets on moisture movement, air leakage, and roof assemblies in cold climates. buildingscience.com