How to Build a Skylight Curb: Height, Lumber, Squaring and Fastening
A four-sided wood frame that lifts the skylight above the roof surface. Get the height and the corners right and the rest of the job is easy.
- Advanced
- 3 to 6 hours
- Updated
The short answer
A skylight curb is a four-sided frame of dimensional lumber, commonly 2 by 6 or 2 by 8, fastened to the roof deck around the opening. Size it to the skylight's outside dimensions, make it square and level on top, and build it tall enough to stand at least 4 inches above the finished roof surface on a pitched roof and 8 inches on a flat membrane roof, or whatever the manufacturer requires. Insulate the inside and seal the corners before flashing.
A curb is a simple wooden box, but it is the foundation of every curb-mounted skylight. The curb gives the skylight its height above the roof surface, gives the roofing membrane something to turn up against and keeps water from reaching the seal between the unit and the roof. A curb that is too low, out of square or badly fastened is the start of most curb skylight leaks.
Most skylight manufacturers sell matching curbs or publish exact curb dimensions. When the unit comes with a factory curb, use it. Site-built curbs are common on flat roofs and for custom or large units, and the rules below apply. The manufacturer's specified curb size and height always come first.
Once the curb is built, the unit is installed on it as described in how to install a curb-mounted skylight, and the curb is flashed as described in how to flash a skylight.
Tools
- Circular saw or miter saw
- Framing square
- Level
- Tape measure
- Drill and driver
- Framing nailer or hammer
- Caulking gun
Materials
- Dimensional lumber, commonly 2 by 6 or 2 by 8, pressure treated or kiln dried as the roof system requires
- Structural screws or framing nails
- Rigid foam or mineral wool insulation for the curb interior
- Plywood for the curb cap if the unit needs it
- Self-adhered membrane for the corners
- Metal corner brackets or straps
Safety first
- A curb is built around an open hole in the roof. Cover the opening whenever you are not working at it, and work tied off at 3 metres (about 10 feet) or more, as WorkSafeBC requires.
- Framing lumber is heavy and awkward on a roof. Stage it near the work and do not stack it near the edge.
- If the opening is larger than the space between two rafters, the framing under the curb has to carry the load. Do not cut a rafter or truss without proper headers and, where needed, an engineer.
Step by Step
Step 1: Get the curb dimensions from the skylight instructions
The curb has to match the unit. Manufacturers list the curb outside dimensions, the maximum curb width and the required clearance around the unit, often within a fraction of an inch. A curb that is too big leaves the unit overhanging and unsupported. One that is too small lets the unit slide or sit on the edge of the flashing.
Also note the required height above the finished roof surface. Many manufacturers ask for at least 4 inches, but the roofing system and the snow load can raise that.
Step 2: Work out the curb height
The curb stands on the deck. Everything built up on the deck around it, including insulation, cover board and membrane, raises the roof surface and lowers the usable curb height. Add it all up.
For example, on a flat roof with 3 inches of insulation and a 1/2 inch cover board, a curb that must stand 8 inches above the finished roof needs about 11-1/2 inches of height above the deck. A 2 by 12 has an actual depth of 11-1/4 inches, which is just short, so the curb would need a plywood cap or a stacked frame. On a pitched shingle roof with no insulation above the deck, a 4 inch minimum is met by a 2 by 6 with room to spare. Do the sums before you buy lumber.
Tip: Taller curbs cost little more than short ones and keep the glass out of pooled water and snow. In snow country, err on the tall side.
Step 3: Cut the four sides
Decide how the corners are made. Butt joints with overlapping ends, where each side ends on the face of the next, are simplest. Cut the lumber to length carefully. A saw cut that is not square makes a twisted curb.
Use lumber that is straight and dry. Wet or twisted pieces shrink and bend, and open the corners. On a flat roof, pressure treated lumber is common because the curb is in constant contact with moisture. Confirm that the membrane and sealant are compatible with treated lumber.
Step 4: Assemble the frame and square it
Assemble the frame on the deck or on the ground next to it, fastening each corner with at least two or three structural screws or nails and adding a metal corner strap or bracket where the instructions show.
Measure the diagonals corner to corner. They must match, ideally within 1/8 inch. Adjust the frame until they do, then tack it in place. A curb that is out of square makes it hard to fit the unit and the flashing.
Step 5: Fasten the curb to the deck and framing
Position the curb so its inside face lines up with the opening and the curb sits fully on the deck or on the framing around the opening. A curb that hangs partly over the opening has no support there.
Check level across the top in both directions, shimming where needed, and then fasten with screws or nails long enough to reach solid framing. Fasten all four sides, and do not rely on a single line of nails. The curb has to stay put when the snow and wind load it.
Step 6: Insulate the inside of the curb and air seal the joints
Line the inside of the curb with rigid foam, cut tight, or fill it with mineral wool. Seal gaps with spray foam or sealant so warm, moist air from inside the house cannot reach the cold wood.
Air leakage into a curb is a common cause of staining and mold on the light shaft. It looks like a roof leak, but it is condensation. See how to fix a leaking skylight.
Step 7: Check the top of the curb and add a cap if the unit needs one
The skylight sits on the top of the curb, so a high spot, a nail head or a rough edge will break the seal. Plane or sand high spots, and set any nails or screws below the surface.
Some units need a plywood top or a separate nailer to fasten into. If yours does, add it now, flush with the outside of the curb. Round over sharp outside top edges slightly so they will not cut the membrane.
Step 8: Prepare the curb for roofing and flashing
Follow the roofing system's instructions. On a membrane roof, the membrane turns up the outside of the curb and is fastened near the top. On a shingle roof, the curb is covered with self-adhered membrane and flashed with metal, lower pieces first.
See how to flash a roof curb for membrane roofs and how to flash a skylight for shingle roofs.
Why Curb Height Matters
Water, snow and debris collect around a skylight. The higher the curb, the further the seal is from all three. A low curb means the seal is under water every time the roof drains slowly, and in heavy snow country drifts and ice can sit against the glass.
Skylight manufacturers commonly ask for a curb at least 4 inches above the finished roof surface on a pitched roof. On a flat or low-slope membrane roof the roofing membrane sets the height, and its flashing commonly has to run at least 8 inches above the finished roof, so the curb has to be at least that tall. Take the number from both the skylight and the roofing system instructions, and use the higher one.
Lumber Choices
- 2 by 6 or 2 by 8: the usual sizes, chosen to give the height you need above the roof surface.
- Pressure treated lumber: common on flat roofs, where the curb is always in contact with moisture. Check compatibility with the membrane.
- Plywood cap: gives a flat, fastenable top for units that need it.
- Factory curbs: some manufacturers supply insulated curbs made to fit their units, and these are often the best choice.
Common Mistakes
- Building to the wrong size. A curb that is out by even 1/2 inch can stop the unit from seating.
- Forgetting the roofing build-up. Insulation and cover board raise the roof surface and eat curb height.
- Out-of-square corners. The unit will not sit flat and the flashing will pucker at the corners.
- Fastening into the deck only. The curb needs to tie into framing or blocking so it cannot shift.
- Leaving the curb interior uninsulated. The cold wood sweats and stains the shaft below.
- Leaving high spots or protruding nails on top. They break the seal under the skylight.
When to Call a Professional
If the curb has to span an opening where rafters are cut, carry a large unit or be tied into a membrane roof, have a roofer or carpenter build it and, where required, an engineer design the framing under it.
Mistakes in a curb are hidden by the roofing and the unit, and they show up as leaks in the first wet season.
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 Build a Skylight Curb: Questions
How tall 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 skylight manufacturer and roofing system set the real requirement, so confirm it. Measure from the finished roof surface after insulation and membrane are in, not from the deck. In snow country, a taller curb keeps drifts and ponded water off the seal.
What lumber is used for a skylight curb?
Dimensional lumber, most commonly 2 by 6 or 2 by 8, is used. Pressure treated wood is common on flat roofs where the curb is always damp, but check that it is compatible with the membrane and sealant. The size is picked to give the required curb height above the finished roof, not the other way around.
Does a skylight curb need to be insulated?
Yes. An uninsulated curb is a cold wooden box in a warm house, and moisture condenses on it and stains the light shaft. Fill the inside with rigid foam or mineral wool, seal the joints with spray foam or sealant and keep warm indoor air from reaching the cold wood.
Can I use a factory-made skylight curb?
Yes, and it is often the better choice. Factory curbs are made to fit the unit exactly, are often insulated, and come with instructions that match the skylight warranty requirements. A site-built curb makes sense for custom sizes, large units or roofs where a standard curb does not fit.
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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