Why does a curb need a cricket on the upslope side?

Because a curb sitting across the fall is a dam. Water running down the roof collects against its upslope face, ponds there, and finds the weakest point in the flashing. A cricket is a small ridge behind the curb that splits the flow and sends it around both sides. Above a modest curb width it stops being optional.

How high should the curb be?

High enough that the membrane upstand terminates well clear of any water that can stand or back up against it — commonly a minimum stated in the membrane manufacturer's details, measured from the finished roof surface rather than from the deck. Insulation thickness eats into that height, and a curb detailed from the deck level is frequently too short once the build-up is on.

Can the flashing be run continuously round the corners?

Not reliably in sheet metal. Corners are formed or welded units because a folded corner in a continuous run leaves a pinhole at the fold that no amount of sealant closes permanently. In single-ply membrane systems the corners are preformed moulded pieces for the same reason.

What fails first on a curb detail?

The upslope corner, almost always — it is the point where ponding, a change of direction and a fabricated joint all coincide. It is worth inspecting there first on any roof with a leak near a rooftop unit, and worth detailing there most carefully on a new one.

Materials & Quantities

Rooftop Unit Curb Flashing Calculator

Calculate the flashing material needed around a rooftop mechanical unit curb, with waste allowance.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

Waste is set to 8% by hand. Pick a tier above to replace it, or keep your own figure.

Curb Perimeter

The full perimeter of the rooftop unit curb.

Measure around all four sides of the curb that the mechanical unit sits on, at the level where flashing will wrap it.

Waste Allowance (%)

Extra material to allow for corner cuts, overlaps, and offcuts.

Curb flashing requires cutting and folding at each corner, so a higher waste allowance than a straight run is typical.

Curb flashing needed

28.08 ft

High confidence

Curb flashing is measured on the curb's perimeter, but its performance depends on the curb's height above the finished roof surface and on whether the upslope side sheds water around the curb rather than into it.

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

Show calculation logic

How this was calculated

Formula source(s)

Total curb flashing = curb perimeter × (1 + waste allowance), the standard method for estimating continuous flashing around a roof curb penetration

Inputs used

Curb Perimeter: 26 ft
Waste Allowance (%): 8

Final result28.08 ft

Confidence note: Curb flashing is measured on the curb's perimeter, but its performance depends on the curb's height above the finished roof surface and on whether the upslope side sheds water around the curb rather than into it.

What this calculation does not cover

Excludes the cricket or saddle needed on the upslope side of any wide curb, which is a fabricated item sized from the curb width and the roof slope.
Corner pieces are fabricated units rather than cut from a run and are priced individually.
Does not verify curb height. A curb too low for the covering system leaves nowhere for the upstand to terminate above standing water.

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 2026-08-26 · in the site-wide review of 2026-09-06 · v1.0.0

Regulatory standards & verification citations1

Total curb flashing = curb perimeter × (1 + waste allowance), the standard method for estimating continuous flashing around a roof curb penetration

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.

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.

Detailing a Parapet and the Cricket Behind a Curbuses this calculator
A parapet gets weather on three faces and a cricket is the only slope you build by hand — this is what both cost in metal, board and flashing height.
Installing a Standing Seam Metal Roofuses this calculator
Standing seam panels expand and contract every single day, and every clip, seam and trim detail either allows that movement or fights it.
Leadwork Around a Chimneyuses this calculator
Four separate pieces wrap a stack and none of them seals. Which detail suits your covering, what code of lead to buy, and how far each piece runs.

Called something else where you work? Flashing at abutments and penetrations — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate rooftop unit curb flashing in 3 steps

Curb PerimeterThe full perimeter of the rooftop unit curb.
Waste Allowance (%)Extra material to allow for corner cuts, overlaps, and offcuts.
Curb flashing neededThe tool computes the curb flashing needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Curb flashing needed by curb perimeter

Page defaults, not your figures above.

Curb Perimeter	Curb flashing needed (ft)
20 ft	21.6
30 ft	32.4
40 ft	43.2
50 ft	54

Frequently asked questions

Why does a curb need a cricket on the upslope side?: Because a curb sitting across the fall is a dam. Water running down the roof collects against its upslope face, ponds there, and finds the weakest point in the flashing. A cricket is a small ridge behind the curb that splits the flow and sends it around both sides. Above a modest curb width it stops being optional.
How high should the curb be?: High enough that the membrane upstand terminates well clear of any water that can stand or back up against it — commonly a minimum stated in the membrane manufacturer's details, measured from the finished roof surface rather than from the deck. Insulation thickness eats into that height, and a curb detailed from the deck level is frequently too short once the build-up is on.
Can the flashing be run continuously round the corners?: Not reliably in sheet metal. Corners are formed or welded units because a folded corner in a continuous run leaves a pinhole at the fold that no amount of sealant closes permanently. In single-ply membrane systems the corners are preformed moulded pieces for the same reason.
What fails first on a curb detail?: The upslope corner, almost always — it is the point where ponding, a change of direction and a fabricated joint all coincide. It is worth inspecting there first on any roof with a leak near a rooftop unit, and worth detailing there most carefully on a new one.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.