Flat roof skylights are skylights installed on roofs pitched below 3:12, and building code requires them to sit on a raised curb instead of mounting flush to the deck. The curb lifts the whole seam several inches above standing water, which is the single detail that keeps a flat roof skylight from leaking. Below is what the code actually requires, how curved glass and domes differ, what each type costs installed in 2026, and how the job is done.
Can you put a skylight on a flat roof?
Yes, you can put a skylight on a flat roof, but only a curb-mounted unit qualifies. A skylight cannot sit flat on a low-slope roof because it would trap water, snow, and debris against its seals. The fix is a curb: a raised frame built around the roof opening that lifts the skylight above the water line. Deck-mounted skylights, which bond directly to the roof plane, are not rated for pitches below 3:12.
The visual difference matters too. On a flat roof you either accept a raised profile or choose a curved-glass unit that shrugs off rain. Both approaches solve the same problem: water has to be moved off the glass and away from the seams before it can pool.
Why flat roof skylights must be curb-mounted (the code rule)
For any roof pitched below 3:12 (under about 14 degrees), curb-mounting is not just good practice, it is a code requirement. The 2024 International Residential Code requires unit skylights on roofs flatter than 3:12 to sit on a curb that rises at least 4 inches above the plane of the roof. That 4-inch minimum is the floor, not the target.
The numbers below are the ones inspectors and manufacturers actually hold you to.
| Condition | Requirement | Basis |
|---|---|---|
| Roof pitch below 3:12 | Curb-mounted skylight required, curb at least 4 in above the roof deck | 2024 IRC |
| Snow and ice regions | Curb 6 to 8 in above the deck to clear accumulation | Field practice |
| Flat glass unit on a flat roof | Curb pitched to at least 1:12 so water sheds | Manufacturer spec |
| Any skylight glazing | Roughly 2 to 5 degrees of slope so water, snow, and dirt shed | Industry guidance |
The reason is mechanical, not bureaucratic. By raising the seam several inches, water would have to pool inches deep before it could ever reach a joint. On a dead-flat mount, a quarter inch of ponding sits directly against the seal.
Curved glass, domed, or flat glass on a pitched curb?
Flat roof skylights shed water in one of three ways: a convex dome, a curved sheet of glass, or flat glass set on a curb that is itself built with a slope. The right pick depends on the room below, the view you want, and budget. Utility spaces tolerate a dome; a kitchen or living room usually justifies curved glass.
| Glazing style | How it sheds water | Best for | Trade-off |
|---|---|---|---|
| Acrylic or polycarbonate dome | Convex shape runs water off the top | Budget jobs, garages, utility rooms | Plastic can yellow and cloud over 10 to 15 years |
| Curved tempered glass (VELUX CurveTech) | Edge-to-edge curved glass disperses rain | Kitchens, living spaces, clear overhead view | Highest unit cost |
| Flat glass on a pitched curb | Sloped curb pushes water toward one edge | Matching a flat, modern glass look | Needs precise curb carpentry to hit 1:12 |
Types of flat roof skylights
Flat roof skylights come in four working types, separated by whether they open and how much light they move. All four are curb-mounted on a low-slope roof. Fixed units are the cheapest and most leak-resistant because there are no moving parts. Vented units add ventilation. Tubular units bring daylight to small interior rooms without a full opening.
- Fixed: Does not open. Fewest failure points, lowest cost, best for daylight only.
- Vented or fresh-air: Opens by electric motor or solar power, often with a rain sensor that closes it automatically. Good for kitchens and baths that need to shed heat and moisture.
- Tubular (sun tunnel): A small domed unit that pipes daylight down a reflective tube to a ceiling below. The lowest-cost way to light a windowless flat-roof room.
- Roof access or exit: A hinged unit sized for a person to climb through, used where a flat roof doubles as a deck or service platform.
What flat roof skylights cost in 2026
A flat roof skylight unit runs about $150 to $1,800 on its own, and installed cost on a flat roof runs roughly $2,100 to $4,800 for a standard fixed or vented model. Flat-roof installation adds about $300 to $700 over the same skylight on a pitched roof, because the crew has to build and flash the curb. Tubular units are far cheaper than glazed units.
| Type | Unit price | Installed on a flat roof |
|---|---|---|
| Tubular (sun tunnel) | $200 to $600 | $750 to $1,500 |
| Fixed curb-mounted (dome or glass) | $150 to $1,000 | $2,100 to $3,500 |
| Vented or fresh-air (solar or electric) | $500 to $1,500 | $2,800 to $4,800 |
| Curved glass (CurveTech) | $600 to $1,800 | $3,000 to $5,000 |
These are reported ranges, and the final quote moves with roof height, membrane type, drywall and paint to finish the shaft, and local labor. A skylight on a torch-down or gravel built-up roof costs more to flash cleanly than one on a smooth TPO or EPDM membrane. Get the cost broken out by unit, curb, flashing, and interior finish so you can compare bids line by line.
VELUX flat roof skylight models
VELUX is the most widely stocked flat-roof skylight brand in the United States, and its flat-roof line is built around curb mounting. The core models cover fixed daylight, powered ventilation, roof access, and small-room tubular lighting. Each is designed to shed water either through a curved cover or a domed top, so none relies on the roof itself having slope.
- CurveTech Glass: A curved, edge-to-edge glass cover on a PVC curb that runs rainwater off the surface.
- No Leak Fixed Curb-Mounted: The economical fixed option for dark, closed-in rooms.
- No Leak Solar Powered Fresh Air Curb-Mounted: Opens on solar power with an integrated rain sensor that closes it at the first drops.
- Flat Roof Exit: A hinged access unit for reaching a flat roof from inside.
- TCR Sun Tunnel: A low-profile acrylic or polycarbonate dome with one-piece curb-mounted counterflashing for small rooms.
How a flat roof skylight is installed
Installing a flat roof skylight is a curb-first job: the raised frame goes on before the skylight ever touches the roof. The sequence below is what a competent crew follows. Cutting the opening and flashing the curb are the steps that decide whether the unit leaks, so they take the most care.
- Mark and cut the roof opening from above, sized to the skylight and curb.
- Frame the rough opening below with headers between the joists or trusses.
- Build the curb (usually plywood or dimensional lumber) to at least 4 inches, or 6 to 8 inches in snow country.
- Wrap and flash the curb into the roof membrane, lapping flashing under the upslope side and over the downslope side.
- Set the skylight on the curb, fasten it, and air-seal the joint between unit and curb.
- Finish the interior light shaft with drywall, then insulate and paint.
Do flat roof skylights leak?
A properly curb-mounted flat roof skylight is highly leak-resistant, because the seam sits inches above any water that collects on the roof. Leaks come from a flat mount, a curb built too low, sloppy flashing, or ponding water that never drains. The curb height and the flashing detail, not the skylight itself, are where flat-roof installs succeed or fail.
Ponding is the specific flat-roof risk. If the roof holds water after rain, that water works at every seam it can reach. Keep the skylight away from low spots, confirm the roof drains toward its scuppers or drains, and air-seal between the unit and curb so wind-driven negative pressure cannot pull water under the flashing. If your roof already ponds, fix the flat roof drainage before adding a penetration. When a skylight does leak, the source is usually the flashing or the curb, which is worth knowing before you start a skylight leak diagnosis.
For a wider view of unit prices across roof types, see how skylight costs break down by type, compare the VELUX skylight range before you spec a model, and browse the rest of our roofing guides for the surrounding details.
Frequently asked questions
Can you put a skylight on a flat roof?
Yes. A skylight can go on a flat or low-slope roof as long as it is curb-mounted. The curb is a raised frame that lifts the skylight above the roof surface so water drains away instead of pooling against the seals. Deck-mounted skylights that bond flat to the roof are not rated for pitches below 3:12 and should not be used on a flat roof.
Do skylights leak on flat roofs?
A correctly curb-mounted skylight rarely leaks on a flat roof, because the seam sits several inches above any standing water. Leaks trace back to a flat or too-low mount, poor flashing, or ponding water that never drains. Building the curb to at least 4 inches (6 to 8 inches in snow regions) and flashing it into the membrane is what keeps a flat roof skylight dry.
What is the minimum pitch for a flat roof skylight?
For roofs below 3:12, the 2024 IRC requires the skylight to sit on a curb at least 4 inches above the deck. Flat glass units need the curb itself pitched to at least 1:12 so water sheds. As a rule, the glazing surface should have roughly 2 to 5 degrees of slope so water, snow, and dirt run off rather than sit on the glass.
How much does a flat roof skylight cost to install?
Installed cost on a flat roof runs about $2,100 to $4,800 for a standard fixed or vented model, with the unit alone at $150 to $1,800. Flat-roof mounting adds roughly $300 to $700 over the same skylight on a pitched roof because the crew builds and flashes a curb. Tubular sun tunnels are cheaper, often $750 to $1,500 installed.
What is the best skylight for a flat roof?
For living spaces, a curved-glass curb-mounted unit such as the VELUX CurveTech sheds water well and keeps a clear overhead view. For budget or utility rooms, a domed acrylic unit works and costs less. For a small windowless room, a tubular sun tunnel is the cheapest way to add daylight. All three are curb-mounted, which is the non-negotiable part on a flat roof.
Can you open a skylight on a flat roof?
Yes. Vented or fresh-air flat roof skylights open by electric motor or solar power, and many include a rain sensor that closes the unit automatically when it starts to rain. Venting units suit kitchens and bathrooms that need to shed heat and moisture. Fixed units do not open and have fewer failure points, so they are the more leak-resistant choice where ventilation is not needed.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.