There is water under your skylight and you want to know whether you have a roofing problem or a moisture problem. These are the two calls we get most in October, they look identical from the floor, and they need completely different work. The good news is that you can usually tell them apart yourself in about a day, without anybody going on the roof.
The short version: condensation turns up on cold mornings and dries as the day warms. A leak turns up with rain and keeps going in the sunshine. If you want to work through it properly, the leak diagnostic asks six questions and narrows it down.
Watch when the water appears, not where it appears. That one habit settles most of these before anybody climbs a ladder.
It is probably condensation if the glass is wet first thing on a cold morning and dry by lunchtime. If it is worse after a cold clear night than after a night of rain. If the water sits on the inside face of the glass, or beads along the bottom edge of the frame where the glass meets it, and you can wipe it off. If it is worse in a bathroom, a kitchen, or a room where somebody dries laundry. If it started when the heating went on.
It is probably a leak if the water only shows during rain or in the few hours after it. If it comes with wind driven rain from one particular direction and not the others. If it is running rather than beading, or dripping from one spot. If the stain on the ceiling is away from the skylight rather than directly under it. If it carries on in mild weather when nothing could be condensing.
Keep a note for a week. Write down the date, whether it rained, roughly how cold the night was, and whether there was water. A week of that is better evidence than any single visit, and it is the thing most people have never done.
Condensation needs two things at the same time: air carrying moisture, and a surface cold enough to take it back out of the air. For most of the year a New York skylight is not cold enough. In October that changes.
The heating goes on, so the air indoors gets warmer, and warm air carries a lot more water vapour than cold air does. At the same time the nights get cold and the glass over your head becomes the coldest surface in the room by a wide margin, because it is the only part of the ceiling with nothing but outside on the other side of it. Water vapour leaves the air at the coldest point it can reach. That is the skylight.
This is also why it is worse in bathrooms and kitchens, worse in a room where washing is dried, and worse in a well sealed modern apartment than a draughty old one. Every one of those puts more moisture into the air, and none of them changes how cold the glass is.
There is a third thing people find, and it gets mistaken for both of the above. If you can see haze, milky patches, trapped droplets or little bubbles that never move and never wipe away, the water is between the two panes rather than on the room side of them.
That means the seal around the edge of the glass unit has failed. A double glazed unit is two sheets of glass with a sealed gap between them, and that gap is meant to stay dry forever. Once the perimeter seal opens up, damp air gets in, and every cold night it condenses in there where you cannot reach it. It usually leaves a faint tide mark on the glass over time.
Nothing done to the room will fix that one, because the problem is inside the sealed unit. Ventilation will not touch it and neither will a dehumidifier. The glass unit has to be replaced, which on most skylights can be done without replacing the whole thing. That is covered on skylight glass replacement.
One thing worth adding, because it comes up more than it used to. Heat blocking and solar control film applied to the inside of a sealed unit makes the glass run much hotter than it was built for, and heat is hard on the edge seal. Film will not break a healthy unit by itself, but it will finish off one that was already near the end. It also normally voids the glass maker's warranty.
The reason people get this wrong in the other direction is that water does not fall straight down once it is inside a roof. It runs along the underside of the deck and along the framing until it meets something that stops it, and then it drops. The framed opening around a skylight is very good at stopping it.
So the skylight is often where the water appears and not where it got in. Water that entered at a parapet, a coping, a chimney or a seam ten feet up the roof will arrive at the skylight opening and come through there. Sealing around the skylight will do nothing, which is why these are the leaks that come back after two or three repairs. There is more on that on why a skylight keeps leaking after it was repaired.
It also means a stain that sits a foot or two to one side of the skylight, rather than directly beneath it, is a point in favour of a leak rather than condensation. Condensation forms on the glass and the frame and runs down the shaft. It does not travel sideways through the ceiling first.
In this order, because the cheap ones work more often than people expect.
Get the moisture out of the room. An extractor fan that vents outside, used during and for a good while after showers and cooking, removes more water from a house than anything else on this list. A lot of the fans we see either vent into a void or are not run long enough to do anything. If laundry is dried indoors under the skylight, move it.
Let air move across the glass. Warm damp air that sits still against a cold surface condenses. Air that keeps moving mostly does not. A light shaft boxed in tight, or a blind kept closed all night, traps a pocket of air right against the coldest thing in the room. Leaving the shade open overnight through the cold months changes more than people expect, and costs nothing to try.
Raise the temperature of the glass. This is the one that actually ends it, and it is the one that costs money. A single glazed skylight, an old acrylic dome, or a double glazed unit with a failed seal will keep sweating no matter how well the room is ventilated, because the inside face of the glass is simply too cold. A modern insulated unit runs warm enough on the room side that moisture stops leaving the air there.
Something worth saying plainly: a venting skylight helps with this, because the cause of condensation is damp air that has nowhere to go. Opening a skylight for ten minutes clears a room faster than any fan, since the wet warm air is already at the top of the room and wants to leave. That is a real reason to choose a venting unit over a fixed one in a bathroom or a kitchen, separate from the daylight.
We look at the glass and the frame first, because a failed sealed unit announces itself and settles the question on the spot. Then we look at where the staining actually is, and whether it is on the glass side of the frame or coming through the plaster. Then we look at the room, the extraction, and what the shaft is built out of.
If it is condensation we will tell you so, and we will tell you which of the three fixes above we think will do it. Plenty of those visits end with us saying run the fan properly and leave the shade open, and no work at all. We would rather do that than sell you a skylight you did not need.
If it is a leak, we find where the water is actually entering before anybody quotes anything. If it turns out to be a travelling leak from elsewhere on the roof, we will say that too, even when the repair is not ours to do.
The assessment is free either way, across all five boroughs, Long Island and Westchester.
Related reading. If the water only ever comes with rain, start at skylight leak repair. If it has been sealed before and came back, read skylight keeps leaking. If the moisture is trapped inside the glass, see skylight glass replacement.
Timing tells you almost everything. Condensation shows up on cold mornings, sits on the inside face of the glass or along the bottom edge of the frame, and dries off as the day warms up. A leak shows up during rain or in the hours just after it, and it keeps going when the sun is out. If it is wet on a clear freezing morning and dry after a day of rain, it is condensation.
Because condensation needs a cold surface and warm damp air, and October is when you get both at once for the first time. The heating goes on, the air inside gets warmer and holds more moisture, and the glass overhead is the coldest surface in the room. Moisture leaves the air at the coldest point it can find, which is the skylight. In July the glass is not cold enough for it to happen.
It is, but it is a different problem and it cannot be wiped off. Moisture inside the sealed cavity means the edge seal of the glass unit has failed and damp air is getting in. You see it as haze, milky patches, trapped droplets or bubbles that never move. Nothing you do to the room will fix it. The glass unit itself has to be replaced.
It can. Solar control and heat blocking film makes the glass run considerably hotter than it was designed to, and that extra heat works on the edge seal of a sealed unit. Film does not break a healthy unit on its own, but it will finish off one that was already tired. It also normally voids the glass manufacturer warranty, which is worth knowing before anybody applies it.
Yes, if it runs for long enough. Condensation that forms night after night runs down the frame and soaks into the shaft lining and the plaster at the bottom. You get staining, peeling paint and sometimes mould in the corners of the light shaft. People often call that a leak, and it looks exactly like one, but sealing the roof will not change it.
Three things, in this order. Get the moist air out of the room, which usually means extraction in bathrooms and kitchens that actually vents outside. Stop warm air sitting against the glass, which means not boxing the shaft in tight and leaving the blind or shade open at night. Then raise the temperature of the glass itself, which is where a modern insulated unit makes the real difference. An old single glazed or a failed double glazed unit will keep sweating whatever else you do.
No. The assessment is free. It is worth having somebody look before you spend money, because condensation and a leak need completely different work and the wrong one costs you the price of the job and the problem is still there afterwards.