Skylight Curb Perimeter Flashing Length Calculator

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Skylight Curb Length

The length of the skylight curb.

Measure the outside length of the built curb the skylight unit sits on, not the glazed opening size.

Skylight Curb Width

The width of the skylight curb.

Measure the outside width of the built curb, perpendicular to the length.

Flashing length needed

14 ft

High confidence

Add a waste allowance for corner cuts and laps per the flashing manufacturer's installation instructions — this calculator gives the raw perimeter length only.

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Perimeter = 2 × (Curb Length + Curb Width) — a straightforward material takeoff for skylight curb flashing, distinct from this site's existing skylight rough-opening (framing) and glazing-bar (glass support spacing) calculators.

Inputs used

Skylight Curb Length: 4 ft
Skylight Curb Width: 3 ft

Final result14 ft

Confidence note: Add a waste allowance for corner cuts and laps per the flashing manufacturer's installation instructions — this calculator gives the raw perimeter length only.

What this calculation does not cover

One number for four different pieces. Curb flashing is an apron at the downslope side, a side pan or step flashing up each slope, and a head piece with a counter over it upslope, and they are not interchangeable lengths — on a shingle roof the side dimension converts into a COUNT of step pieces keyed to the shingle exposure rather than a run of material.
Gives no girth, and girth is what makes the joint watertight. How far the metal turns up the curb face and how far it laps out over the roof covering sets the coil width or the profile of the pre-formed piece; a perimeter says nothing about either, and the right length of the wrong-width stock is the usual way this order goes wrong.
A curb much wider than 600 mm (2 ft) across the slope generally needs a cricket or saddle behind it, and that is separate metal with its own framing. Without one, water and debris bank against the flat back of the curb and stay there — the flashing is not failing, it is being asked to hold a pond.

Estimated cost — your price

Price per ft ($)

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 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Perimeter = 2 × (Curb Length + Curb Width) — a straightforward material takeoff for skylight curb flashing, distinct from this site's existing skylight rough-opening (framing) and glazing-bar (glass support spacing) calculators.

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 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.
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 5 tools this job needs

Essential

Cordless drill/driver
Hand saw
Spirit level
Tape measure

Recommended

Utility knife

Also needed as materials: drill and driver bits, replacement blades.

what each roofing tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Fitting a Roof Window Into a Pitched Roofuses this calculator

A roof window is ordered twice: once for the unit, once for the flashing kit that has to suit the covering it lands in and the pitch it lands on.

Flashing a Roof Curbuses this calculator

Eleven boxes on a roof are eleven perimeters, eleven blank widths and forty-four corners. Here is how all three reach the shop in one order.

Framing a Skylight Openinguses this calculator

One catalogue code decides the opening, the header, the curb and the saddle above it — worked in that order, from the unit outward.

Skylight Curb Length	Flashing length needed (ft)
2 ft	9.91
3 ft	11.9
4 ft	13.9
5 ft	15.9
6 ft	17.9
7 ft	19.9

Skylight Curb Length

Flashing length needed (ft)

2 ft

9.91

3 ft

11.9

4 ft

13.9

5 ft

15.9

6 ft

17.9

7 ft

19.9

Frequently asked questions

What sets the height of the curb, if not the skylight?

The water, and the snow that sits in it. A curb is a small wall standing proud of the roof deck, and the skylight rides on top of it precisely so that the waterproofing can be turned UP the curb and terminated above anything that will ever run across the roof. Its height is therefore a drainage and climate decision rather than a product one: shallower pitches move water more slowly and pond it deeper against an obstruction, and anywhere that holds snow needs the flashing to finish above the drift line rather than above the deck. A curb built to the minimum on a low-pitch roof in a snow region is the classic leak that only appears in February, and no amount of sealant at the head fixes it, because the water is arriving above the termination rather than through it.

How is skylight curb flashing length calculated?

Flashing length is the perimeter of the curb: 2 × (Curb Length + Curb Width), a straightforward material takeoff distinct from the rough-opening (framing) size.

Does this include waste for corner cuts and laps?

No — this calculator gives the raw perimeter length only. Add a waste allowance for corner cuts and laps per the flashing manufacturer's installation instructions.

Should I measure the curb or the rough opening?

Measure the built curb's outside dimensions, not the framed rough opening or the glazed opening size — flashing wraps around the curb itself.