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Rooftop Unit Curb Flashing Calculator

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

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

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 flashing needed

28.35 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.

With the figures above, the curb flashing needed comes to 8.64 m. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

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.

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 — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

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

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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.