How to Install a Curb-Mounted Skylight and Flash the Curb
The unit sits on a raised frame, so the roof is flashed to the curb and the skylight is fastened and sealed on top.
- Advanced
- 4 to 8 hours
- Updated
The short answer
A curb-mounted skylight is set on a raised frame, the curb, that is already built and flashed into the roof. Set the unit on the curb with its gasket or sealant bead, fasten it through the flange or brackets as the manufacturer specifies, and cap the curb with its cap flashing. The roofing or membrane runs up the curb sides, and the curb is commonly at least 4 inches above a pitched roof surface, and about 8 inches above a flat membrane roof.
A curb-mounted skylight sits on a raised frame, called the curb, which lifts the unit above the roof surface. The curb is what makes the whole system work on flat and low-slope roofs, because the roofing membrane can turn up the curb sides and water can never reach the joint between the skylight and the curb unless it rises several inches. It is also used for large units and on metal roofs.
The skylight itself is a fairly simple part of the job. Most of the work is the curb, which is built and flashed first, and the connection between the curb and the unit. This guide covers setting the unit, fastening it and finishing the curb flashing. The curb itself is covered in how to build a skylight curb.
Manufacturers specify the curb size, the height, the fastening and the sealant for each model. Their installation instructions govern. For flat roofs, also read how to install a skylight on a flat roof.
Tools
- Drill and driver
- Level
- Tape measure
- Caulking gun
- Hook blade utility knife
- Tin snips
- Hand seamer
- Framing square
Materials
- Curb-mounted skylight unit
- Curb cap or counterflashing, matched to the unit
- Gasket tape or sealant specified by the manufacturer
- Fasteners specified for the unit and the curb material
- Self-adhered membrane or roofing membrane for the curb
- Roofing nails or fasteners for the flashing
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury. A skylight unit itself is not a safe thing to lean on.
- Skylight units are awkward, heavy and catch wind like a sail. Lift them with two people and do not carry them on a windy day.
- Do not step on skylight glass or domes. Mark or cover them so nobody walks on them by accident.
Step by Step
Step 1: Check the curb is the right size, square and solid
Measure the top of the curb and compare it to the unit. Most manufacturers give the curb outside size and the allowable tolerance, which is usually small. Measure both diagonals; they should match within about 1/8 inch.
Lay a level across the top in both directions. An out-of-level curb means the unit sits unevenly, stresses the seals and can trap water. Shim or re-fasten as needed. If you are building the curb from scratch, see how to build a skylight curb.
Step 2: Confirm the curb height
Curb height is measured from the finished roof surface, not from the deck. Add the thickness of any insulation, cover board and membrane before you judge whether there is enough curb left above it.
A curb that is too low lets snow and ponded water reach the glass seal. On the higher ground near Hope, snow can bury a low curb for weeks. If the curb is too low, it has to be raised before the unit goes on.
Tip: The required height varies by product and roofing system. Confirm it in the skylight manufacturer's instructions, not from memory.
Step 3: Complete and inspect the curb flashing
On a low-slope membrane roof, the membrane turns up the side of the curb and is fastened and sealed at the top according to the membrane manufacturer. On a shingle roof, curbs are flashed with a combination of membrane and step or apron flashing pieces, lower pieces first.
Run your hand around the curb and look for gaps at the corners. Corners are the usual leak point, so look at how the membrane or flashing is folded there. For the overall method see how to flash a skylight and, on a membrane roof, how to flash a roof curb.
Step 4: Prepare the seal between curb and unit
Clean the top of the curb of dust and debris. Lay the gasket or the sealant as the instructions show, in one continuous line with no gaps, including at the corners. Overlap the ends where they meet.
Use only the sealant that the manufacturer names. Some sealants damage plastic domes or gaskets, and some do not stick to treated lumber.
Step 5: Set the unit on the curb
Check the overhang on all four sides. The unit should sit square on the curb, and the frame should overhang evenly. Do not slide it across the sealant, because that smears the bead and leaves gaps.
If the unit has a pre-attached flashing skirt or cap, make sure it is not folded or crushed underneath. Keep your hands clear of the glass edges and handle the unit by the frame.
Step 6: Fasten the unit as instructed
Fasteners usually go through mounting brackets or a flange into the curb. The spacing and screw type are given on the installation sheet, and screws that are too long can poke into the light shaft or through the curb.
Snug the fasteners evenly. Overtightening can distort the frame, crack a dome or squeeze the seal out from under the unit.
Step 7: Install the curb cap or counterflashing over the joint
Some units are supplied with a cap that covers the joint between the frame and the curb. Others need a site-made counterflashing, which is lapped over the membrane or flashing turned up the curb. Metal caps and counterflashings should be lapped so the upper piece is always over the lower one.
Seal only where the instructions call for it, and do not rely on sealant alone to shed water. Finish by checking every joint, corner and fastener head.
Step 8: Finish the interior shaft and test for leaks
Insulate the shaft and add a vapour barrier on the warm side. Cold shafts make condensation that looks like a leak. See how to fix a leaking skylight.
Have a helper watch from inside while you run a hose over the lower part of the unit first, then the sides, then the top, each for several minutes. Stop if water shows inside.
Domes, Glass and Plastic Units
Curb-mounted skylights come as plastic domes, usually single or double layered acrylic, and as framed glass units. Domes are common on flat roofs because they are lightweight, cheap and easy to replace. Glass units have better insulation and less condensation but are heavier. Whichever you use, the curb and the flashing matter more than the glazing.
Snow and Fraser Valley Conditions
Heavy wet snow and long, rainy winters are hard on skylight seals. A curb that is tall enough, well flashed and draining freely around it will outlast one that holds water. See how to install a skylight on a flat roof for drainage behind the curb.
Common Mistakes
- A curb that is too low. Snow or ponded water reaches the seal and gets in.
- Gaps in the gasket or sealant. One missed corner is enough to leak.
- Using the wrong sealant. Some sealants craze plastic domes or fail on treated lumber.
- Overtightening the fasteners. It cracks domes, warps frames and squeezes out the seal.
- Flashing that stops at the curb top. The counterflashing or cap must cover the joint, not just meet it.
- Skipping the water test. A leak found with a hose is a ten minute fix. One found in January is much worse.
When to Call a Professional
If the roof is flat with a membrane that has to be heat welded, torched or adhered to the curb, or the curb has to be built and tied into the framing, call a roofer. The membrane work and the curb are the parts that keep water out.
Large and heavy units also need proper lifting equipment and more than one person on the roof.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Skylights service or the towns we cover.
How to Install a Curb-Mounted Skylight: Questions
How high should a skylight curb be?
A common minimum is at least 4 inches above the finished roof surface on a pitched roof and about 8 inches on a flat membrane roof, but the manufacturer and the roofing system decide the actual number, so confirm it in the instructions. Measure from the finished surface after insulation and membrane. Taller curbs keep snow and ponded water below the seal, which matters in heavy snow areas.
What do you put between a skylight and the curb?
A gasket tape or a sealant bead that the manufacturer specifies, laid in an unbroken loop on top of the curb. It seals the joint and cushions the frame. Use only the product named in the instructions, because some sealants damage plastic domes or do not adhere to treated lumber.
Can a curb-mounted skylight go on a shingle roof?
Yes. It is used for large or heavy units, or when a deck-mounted kit does not suit the location. The curb is flashed into the shingles with step flashing and a head flashing. Deck-mounted units with a matched kit are usually simpler on a pitched shingle roof.
How do you seal a skylight dome?
The dome is sealed to the curb with the gasket or sealant specified by the manufacturer, then the curb cap or counterflashing covers the joint so water sheds over it. Sealant alone is a backup, not the main defence. Avoid sealants that can craze acrylic.
Related Guides
All Skylights guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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