By The Roofing Brief Team. Last reviewed: October 2026.
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. Section R308.6.8 of the International Residential Code says unit skylights on a roof pitched below 3 units in 12 “shall be mounted on a curb extending not less than 4 inches (102 mm) above the plane of the roof, unless otherwise specified in the manufacturer’s installation instructions.” Installed, a flat-roof skylight runs $2,100 to $4,800 per HomeGuide’s skylight cost guide (August 29, 2025).
Can you put a skylight on a flat roof?
Yes, you can put a skylight on a flat roof, but it needs a curb-mounted unit or a model the maker rates for 0 degrees. The VELUX 2025 residential catalog rates its deck-mounted skylights for 14 to 85 degrees (3:12 and steeper), its curb-mounted VCS, VCE, VCM and FCM units for 0 to 60 degrees (0:12 to 20:12), and its Flat Roof Skylights for 0 to 15 degrees (0:12 to 3:12). A deck-mount unit on a 1:12 roof is outside the maker’s rating.
The visual difference matters too. On a flat roof you either accept a raised curb profile or choose a curved-glass unit such as the VELUX Curved Flat Roof Skylight, whose CurveTech glass sheds rain on roofs from 0 to 15 degrees. Both approaches move water 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 a code requirement. IRC R308.6.8 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, with one exception: where the manufacturer’s installation instructions specify otherwise. That 4-inch minimum is the floor, not the target.
| Condition | Requirement | Source |
|---|---|---|
| Roof pitch below 3:12 | Curb at least 4 in (102 mm) above the roof plane, unless the maker’s instructions say otherwise | IRC R308.6.8 |
| VELUX deck-mounted skylights (FS, VS, VSE, VSS) | Roof pitch 14 to 85 degrees (3:12 and up) | VELUX 2025 catalog |
| VELUX curb-mounted skylights (FCM, VCM, VCE, VCS) | Roof pitch 0 to 60 degrees (0:12 to 20:12) | VELUX 2025 catalog |
| VELUX Flat Roof Skylights (CurveTech and flat glass) | Roof pitch 0 to 15 degrees (0:12 to 3:12) | VELUX 2025 catalog |
| VELUX 10-year installation warranty | Requires VELUX flashing on deck- and curb-mount systems | VELUX 2025 catalog |
| Snow regions | Taller curb where drifts exceed 4 in; confirm height with the maker’s instructions | Editorial; IRC allows maker instructions to govern |
The reason is mechanical. A curb of 4 inches or more means water would have to pond 4 inches deep before reaching the joint between skylight and curb. On a dead-flat mount, even shallow ponding sits directly against the seal.
Curb-mounted vs deck-mounted skylights: the difference
The difference between curb and deck mounted skylights is where the unit sits. A curb-mounted skylight sits on a raised box framed on the roof, at least 4 inches tall under IRC R308.6.8 on roofs below 3:12, and the VELUX catalog rates its curb units for 0 to 60 degrees. A deck-mounted skylight fastens directly to the roof deck with step flashing and is rated by VELUX for 14 to 85 degrees only. HomeGuide notes that curb-mounted units sit above the roof line and are easier to install, while deck-mounted units are flush with the roof but take longer to install.
| If your roof is | Choose | Why |
|---|---|---|
| Flat or below 3:12 (under 14 degrees) | Curb-mounted or a VELUX Flat Roof Skylight | IRC R308.6.8 4 in curb; deck mount rated only from 14 degrees |
| 3:12 to 20:12 with shingles | Deck-mounted or curb-mounted | Both inside VELUX ratings; deck mount gives a flush look |
| Steeper than 20:12 (over 60 degrees) | Deck-mounted | Curb units rated only to 60 degrees |
| Replacing an old curb unit | Curb-mounted sized to the outside curb | VELUX lists outside curb sizes from 17 1/2 x 33 1/2 in (FCM 1430) |
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 built with a slope. Utility spaces tolerate a dome; a kitchen or living room usually justifies curved glass, which costs $943 for a 60 x 60 cm VELUX CurveTech fixed unit at SolarSkylights.com (October 2026). For how the plastic options compare, see our guide to acrylic or polycarbonate dome skylights.
| 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 with UV exposure |
| Curved tempered glass (VELUX CurveTech) | Edge-to-edge curved glass disperses rain | Kitchens, living spaces, clear overhead view | Highest unit cost: $943 to $1,571 by size |
| Flat glass on a pitched curb | Sloped curb pushes water toward one edge | Matching a flat, modern glass look | Needs precise curb carpentry |
Types of flat roof skylights
Flat roof skylights come in four working types, separated by whether they open and how much light they move. On a roof below 3:12 all four are curb-mounted or built on their own curb. HomeGuide prices fixed skylights at $1,500 to $4,000 installed, ventilating units at $1,600 to $4,700 and tubular sun tunnels at $500 to $1,000.
- Fixed: Does not open. Fewest failure points, lowest cost, best for daylight only.
- Vented or fresh-air: Opens by motor or solar power; VELUX says each solar and electric venting unit has a built-in rain sensor.
- Tubular (sun tunnel): A small dome, 10 to 22 inches across per HomeGuide, that pipes daylight down a reflective tube.
- Roof access or exit: A hinged unit sized for a person to climb through, such as the VELUX Roof Access Skylight.
What flat roof skylights cost in 2026
HomeGuide’s August 2025 guide puts a flat-roof skylight unit at $150 to $1,100 and the installed cost on a flat roof at $2,100 to $4,800, against $1,600 to $4,200 for a pitched roof, so the flat-roof premium is about $500 to $600 at each end of the range because the crew has to build and flash the curb. Tubular units are far cheaper than glazed units.
| Type | Unit price | Installed | Source |
|---|---|---|---|
| Tubular (sun tunnel) | $150 to $500 | $500 to $1,000 | HomeGuide, Aug 2025 |
| Fixed skylight | $150 to $800 | $1,500 to $4,000 | HomeGuide, Aug 2025 |
| Ventilating skylight | $300 to $2,000 | $1,600 to $4,700 | HomeGuide, Aug 2025 |
| Standard skylight on a flat roof | $150 to $1,100 | $2,100 to $4,800 | HomeGuide, Aug 2025 |
| VELUX CurveTech fixed, 60 x 60 cm | $943 | Unit only | SolarSkylights.com, Oct 2026 |
| VELUX CurveTech fixed, 120 x 120 cm | $1,571 | Unit only | SolarSkylights.com, Oct 2026 |
The final quote moves with roof height, membrane type, drywall and paint for the light shaft, and local labor. HomeGuide lists drywall installation at $1.50 to $3.50 per square foot and interior painting at $1.00 to $3.00 per square foot for the shaft. A skylight on a torch-down or gravel built-up roof is harder to flash cleanly than one on a smooth TPO or EPDM membrane, so get the bid broken out by unit, curb, flashing and interior finish.
VELUX flat roof skylight models
VELUX is a widely stocked flat-roof skylight brand in the United States, and its flat-roof line is built around curb mounting. Its 2025 catalog lists Flat Roof Skylights in sizes from 060060 (rough opening 23 11/16 x 23 11/16 in) to 200100 (78 3/4 x 39 7/16 in), all rated for 0 to 15 degrees. None relies on the roof itself having slope.
- Curved Flat Roof Skylight: CurveTech edge-to-edge glass; $943 to $1,571 for fixed units at SolarSkylights.com.
- Fixed Flat Roof Skylight: The economical fixed option for dark, closed-in rooms.
- VCS Solar-Powered Fresh Air curb-mount: Opens on solar power, rain sensor included, rated 0 to 60 degrees.
- Roof Access Skylight: 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.
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 touches the roof. Cutting the opening and flashing the curb decide whether the unit leaks, and VELUX ties its 10-year installation warranty to using VELUX flashing.
- 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 from plywood or dimensional lumber to at least 4 inches above the roof plane (IRC R308.6.8).
- 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 leak-resistant because the seam sits at least 4 inches above any water that collects on the roof. Leaks come from a flat mount, a curb under 4 inches, 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 still holds water long after a rain, that water works at every seam it can reach, so a 4 inch curb is the minimum, not a cure. Keep the skylight away from low spots, confirm the roof drains to its scuppers or drains, and air-seal between the unit and curb. 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, see how skylight costs break down by type, compare the VELUX skylight range, and browse our roofing guides.
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 or rated by its maker for 0 degrees. IRC R308.6.8 requires a curb at least 4 inches above the roof on pitches below 3:12 unless the maker’s instructions say otherwise. VELUX rates its deck-mounted skylights only for 14 to 85 degrees, so they 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 at least 4 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 IRC R308.6.8 and flashing it into the membrane with the maker’s flashing is what keeps a flat roof skylight dry.
What is the minimum pitch for a flat roof skylight?
There is no minimum pitch for a curb-mounted unit: VELUX rates its curb-mounted skylights for 0 to 60 degrees and its Flat Roof Skylights for 0 to 15 degrees. For roofs below 3:12, IRC R308.6.8 requires the curb to rise at least 4 inches above the roof plane. Deck-mounted VELUX skylights need at least 14 degrees, or 3:12.
How much does a flat roof skylight cost to install?
HomeGuide’s August 2025 guide puts a flat-roof skylight at $2,100 to $4,800 installed, with the unit alone at $150 to $1,100, compared with $1,600 to $4,200 installed on a pitched roof. Tubular sun tunnels cost $500 to $1,000 installed. A VELUX CurveTech fixed unit lists at $943 to $1,571 by size at SolarSkylights.com.
What is the best skylight for a flat roof?
For living spaces, a curved-glass unit such as the VELUX Curved Flat Roof Skylight, rated for 0 to 15 degrees, sheds water and keeps a clear overhead view. For budget or utility rooms, a domed acrylic unit costs less. For a small windowless room, a tubular sun tunnel at $500 to $1,000 installed (HomeGuide) is the cheapest way to add daylight.
Can you open a skylight on a flat roof?
Yes. Venting curb-mounted units such as the VELUX VCS solar model, rated for 0 to 60 degrees, open by solar or electric power, and VELUX says each solar and electric venting unit has a built-in rain sensor that closes it in bad weather. Venting units suit kitchens and bathrooms. Fixed units have fewer failure points where ventilation is not needed.
Choosing a unit for a low-slope roof? Compare manufacturers in our guide to the best skylights and brands for 2026.