Yes, you can put a skylight in a flat roof. It goes on a raised frame called a curb, which lifts it above the roof surface so water that sits on a flat roof cannot reach the opening. The curb and the flashing around it decide whether the skylight stays dry.
The short version
- A skylight in a flat roof is normal, and it is usually set on a curb rather than flat on the deck.
- A flat roof holds water longer than a sloped one, so the seal around a skylight does more work.
- Most leaks come from the flashing and the frame joints, not the glass.
- If someone can step on it, ask for laminated glass.
- A small sun tunnel is the easier fit for a closet or hallway.
How does a skylight go into a flat roof?
It sits on a curb, which is a short framed box built around the opening and covered by the roofing. A curb-mounted unit needs additional framing to be installed and waterproofed (opens in a new tab) before the skylight is set, according to the Pacific Northwest National Laboratory, and that waterproofing is the real job.
On a flat roof the roofing sheet, called a membrane, turns up the sides of the curb. Metal flashing then caps the top edge, so water running down the glass lands on the roof and not behind the sheet. Everything is sealed as one connected layer, not stuck on afterward.
This Old House says the better choice is a curbed skylight (opens in a new tab), and it recommends buying the flashing kit made for that exact unit. The kit adds little to the price and is meant to give a leak-free install.
Why does a flat roof make a skylight harder?
Because water sits on a flat roof instead of running off. Our flat roof installation page says it plainly: a flat roof has to keep out water that sits on it, and not only water that runs off. A sloped roof sheds a storm in seconds, and a flat one can hold a puddle for hours.
The Pacific Northwest National Laboratory notes that roofs are sloped to drain water, and that the steeper the slope, the better (opens in a new tab). It also calls a skylight a penetration through the roof’s water control layer, which makes it a weak spot to begin with. Put a weak spot where water pools and you can see the problem.
That is why the curb matters. A frame that stands up out of the puddle line keeps the seal dry between storms.
What goes wrong with skylights on flat roofs?
The usual culprits are the flashing, the sealed seams and the frame joints. The Pacific Northwest National Laboratory lists joints in the skylight frame (opens in a new tab) as a common place for leaks, and it warns that gaps or tears in the layers around the unit let water in.
Here is where we would look first:
- A curb that is too low. If it sits close to the roof surface, standing water reaches the seal.
- Membrane that was only pressed against the frame. It has to be turned up and sealed, not left butted against it.
- Dried sealant. Caulk or tar used as the main defense tends to crack in a few seasons.
- Ponding near the curb. If the roof holds water next to the skylight, the skylight takes the beating.
This Old House says leaks and condensation are common (opens in a new tab) with skylights, and regular checks of the flashing and seals catch most of them early.
Which kind of skylight works on a flat roof?
A curb-mounted skylight is the usual answer, and the choice mostly comes down to how much light, air and safety you want. Here is how the three common kinds compare.
| Kind | What it does | Good for |
|---|---|---|
| Fixed | Lets in light, does not open | Hallways, stairwells, kitchens |
| Venting | Opens to let hot air or steam out | Bathrooms, upstairs rooms |
| Sun tunnel | A small tube carrying daylight | Closets, small halls |
Our skylight service page describes the same three. A venting unit costs more and is awkward if you cannot reach it.
If you only want a little daylight, a solar tube skylight makes a smaller hole in the roof. That means fewer edges to seal, and it is worth a look before you commit to a full-size unit.
What does a skylight in a flat roof cost?
We cannot give a price for a flat roof skylight without seeing the roof, because the curb work changes the bill. What we can say is that This Old House reports that a professionally installed skylight costs between $1,000 and $3,000 (opens in a new tab) on average, and that custom units cost much more.
For a firm number, the quote is free. Pair it with our free roof inspection and you will know what the roof can take before you spend anything.
Is it worth adding a skylight to a flat roof?
It is when the room needs the light and the roof is in good shape underneath. If your membrane is near the end of its life, put the new roof first or do both together, because a new skylight on a tired roof is a new thing to leak. Our guide to the pros and cons of a flat roof covers how long these roofs tend to last.
Walking is the other thing to plan for. A flat roof is a place people step, and This Old House warns that on a flat or low-slope roof where someone might step on the skylight (opens in a new tab), fall protection such as laminated glass matters most.
Common questions
Do skylights on flat roofs leak more than on sloped roofs?
They can, because water stays on the roof longer and sits against the frame. A tall enough curb and well-sealed flashing take most of that risk away. The seal matters more than the slope.
Can you add a skylight to a room that never had one?
Often, yes. It depends on what is above the room, so we check during the free roof inspection. We will tell you what will work before you decide.
Can you walk on a flat roof skylight?
Do not count on it. Plan as if someone could step on it by mistake and ask for glass that stays in one piece if it cracks. Keep people off it all the same.
Is a sun tunnel easier to put in a flat roof?
Usually, yes. It needs a smaller opening, so there is less to frame and seal. The trade is that it gives you light, not a view or fresh air.
What to do next
If you want a skylight in a flat roof, or one on a flat roof already leaks, start with a free roof inspection. We will look at the roof and the room below it and tell you what will fit. Then see our skylight installation page for how the seal should go in.
