SettingsSettings for this calculationUS
The standard allowance most suppliers and estimating guides assume for ordinary work.
The long dimension of the framed hole, measured up the slope.
This is the opening in the deck after the trimmers are in, not the glass size and not the unit's outside frame. The default is 1,400 mm by 780 mm, a size sold worldwide, which is why it reads as a round figure in millimetres and an awkward one in inches — take your own numbers off the unit's instructions rather than off the frame.
The dimension across the slope, between the trimmed rafters.
Measured between the cut rafters or between the trimmers, whichever bounds the hole. It is the pair of pieces cut to this dimension that get shortened by the stock thickness, because they fit between the two long ones rather than running past them.
The actual thickness of the board, not its nominal name.
38 mm covers both common cases: it is the dressed thickness of nominal 2x lumber (1½ in) and of UK CLS. Use the figure a tape gives you on the actual board — the gap between a nominal size and a dressed one is exactly the sort of error that shows up as a curb that will not close on its last corner.
How far the board stands off the deck — this is what sets the curb height.
The curb is boards stood on edge, so the board's WIDTH becomes the curb's height above the deck. 184 mm is nominal 2x8; 140 mm is 2x6 and is where most curbs that turn out too short began. Pick the board first and let this page tell you what height it actually buys, rather than choosing a height and hunting for a board.
Everything between the deck and the weather surface at the curb.
Underlayment plus the covering itself, measured where it dies into the curb rather than at its thinnest point. Asphalt shingles with an ice-and-water membrane under them are about 30 mm; a batten-and-tile roof can be four or five times that, and it is the single figure that turns a curb that looked generous on the deck into one that is nearly buried.
The clearance your unit's instructions call for, measured from the weather surface.
It is asked rather than assumed because the markets differ and so do the coverings. North American instructions commonly call for at least 4 in (100 mm) above the finished roof; 150 mm is the usual upstand in UK and European practice. Both are minimums for an ordinary pitch — a shallow roof, a long run of tile above the skylight, or a site that holds snow all wants more, and the instructions for your unit are what govern.
How many curbs of this size are being built.
Only useful where the openings are identical. A run of different sizes is worth running one at a time, because the cut list is the part you take to the saw and a merged total hides which piece belongs to which hole.
The margin for saw kerfs, a split end and one piece cut the wrong way.
A curb is short pieces cut from long stock, so the real waste is the offcut left when a length will not yield another piece. Ten per cent covers an ordinary job; push it up if the opening dimension divides badly into the stock lengths your merchant actually sells.
Curb stock needed
15.22 ft
The cut list is exact arithmetic on the dimensions you entered. The height is only as good as the covering thickness you gave it, which is the figure most often taken at the roof's thinnest point rather than where it meets the curb.
- Two long pieces per curb, cut to
- 55 in
- Two short pieces per curb, cut to
- 28 in
- Stock per curb before waste
- 13.83 ft
- Height this board stands above the finished roof
- 6 in
- Clear of the height you asked for by
- 0 in
They open the calculator with your figures already in it
Skylight Curb Timber Calculator: 15.22 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- A curb is built to the rough opening's outside dimensions and stood on the deck, so two pieces run the full opening and two fit between them: stock length per curb = 2 × opening length + 2 × (opening width − 2 × stock thickness)
- The height that matters is measured from the FINISHED roof surface, not the deck — height above the roof = board depth − the thickness of the covering build-up running up to the curb
- Curb height is set by the skylight manufacturer's installation instructions and by the covering. North American instructions commonly call for at least 4 in (100 mm) above the finished roof; 150 mm is the usual upstand in UK and European practice, and a low pitch or a snow site wants more than either
Inputs used
- Rough Opening Length
- 55 in
- Rough Opening Width
- 31 in
- Curb Stock Thickness
- 1.5 in
- Curb Stock Depth (Board Width, On Edge)
- 7.25 in
- Thickness of the Covering Running Up to the Curb
- 1.25 in
- Height Wanted Above the Finished Roof
- 6 in
- Number of Skylights
- 1
- Waste Allowance (%)
- 10
Intermediate steps
- Two long pieces per curb, cut to
- 55 in
- Two short pieces per curb, cut to
- 28 in
- Stock per curb before waste
- 13.83 ft
- Height this board stands above the finished roof
- 6 in
- Clear of the height you asked for by
- 0 in
Confidence note: The cut list is exact arithmetic on the dimensions you entered. The height is only as good as the covering thickness you gave it, which is the figure most often taken at the roof's thinnest point rather than where it meets the curb.
What this calculation does not cover
- It does not decide whether your skylight needs a curb at all. Deck-mounted units are flashed straight to the roof plane and have no curb; curb-mounted units need one, and which you have is on the unit's own instructions.
- It does not size the framing around the opening. The trimmers and headers that carry the cut rafters are a structural question and a different page.
- The height check compares your board against a target YOU enter. It is arithmetic, not approval — the number that governs is in the manufacturer's instructions for your unit and your covering, and a shortfall reported here is a prompt to read them rather than a verdict.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-20 · v1.0.0
Regulatory standards & verification citations3
- A curb is built to the rough opening's outside dimensions and stood on the deck, so two pieces run the full opening and two fit between them: stock length per curb = 2 × opening length + 2 × (opening width − 2 × stock thickness)
- The height that matters is measured from the FINISHED roof surface, not the deck — height above the roof = board depth − the thickness of the covering build-up running up to the curb
- Curb height is set by the skylight manufacturer's installation instructions and by the covering. North American instructions commonly call for at least 4 in (100 mm) above the finished roof; 150 mm is the usual upstand in UK and European practice, and a low pitch or a snow site wants more than either
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.
Tools and safety for this job
To cut framing lumber and fit a rooflight or roof window. Generic types, no brands, no prices.
Protection this work requires
- Hardwood dust is a recognised carcinogen: extract at the tool and wear a P2/N95 respirator when cutting or sanding indoors.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
- Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
Fit a rooflight or roof window: Fitting a rooflight between rafters, or on a kerb on a flat roof, is carpentry and roofing to its maker's instructions. Where a rafter or joist has to be cut to make an opening wider than one bay, the trimming that carries the cut ends is sized by an engineer or from accepted tables, never by this site.
Show the 8 tools this job needsHide tools
Essential
Circular saw
Combination or framing square
Cordless drill/driver
Hand saw
Spirit level
Tape measure
Recommended
Mitre saw
Utility knife
Also needed as materials: circular saw blades, drill and driver bits, mitre saw blades, replacement blades.
what each carpentry tool is for and what each roofing tool is for, and the spec that decides which to buy where one does.