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The roof area draining to this scupper or secondary drain.
Use the full tributary roof area that sheds water to this drain point, including any area draining toward it from crickets or sloped sections.
The opening area required per unit of roof area, set by local design rainfall intensity and code drainage tables.
This factor comes from your local plumbing code's roof drain sizing table (e.g. IPC Appendix), which varies by design rainfall intensity — heavier rainfall design storms require a larger factor. BOTH HALVES OF THIS RATE ARE METRIC AND NEITHER FOLLOWS THE UNIT TOGGLE: the field wants cm² of opening per m² of roof even when the tributary area above is showing sq ft. A US table states the same quantity as square inches of opening per square foot of roof, and converting one to the other takes BOTH conversions, not just the area one — 1 in² = 6.4516 cm² and 1 m² = 10.7639 sq ft, so multiply a per-square-foot figure by 6.4516 × 10.7639 = 69.44 (exactly 10000/144). The default 2.5 cm²/m² is 0.036 in² per sq ft. Typing 0.036 does not merely shrink the answer: it is below this field's minimum of 1, and bounds here clamp rather than refuse, so it is rewritten to 1 and sizes the opening 2.5 times too small — 200 cm² instead of 500 cm² on a 200 m² roof. The field does report that it clamped, but it reports a number out of range, not a number in the wrong unit. Applying only the area factor of 10.7639 gives 0.39 and clamps to 1 as well; the numerator has to convert too.
Required scupper opening area
77.4 in²
The sizing factor depends on your local design rainfall intensity and applicable plumbing code roof drainage table (e.g. IPC) — confirm the correct factor for your jurisdiction and roof drainage design (primary vs. secondary/emergency) before sizing scuppers.
They open the calculator with your figures already in it
Roof Scupper/Secondary Emergency Drain Sizing Calculator: 77.4 in² — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Required opening area = roof area × a sizing factor derived from the local design rainfall intensity, following the same tributary-area method used by plumbing code roof drainage sizing tables (e.g. IPC Appendix roof drain sizing)
Inputs used
- Tributary Roof Area
- 2150 sq ft
- Sizing Factor (cm² of opening per m² of roof, per local rainfall intensity/code table)
- 2.5
Confidence note: The sizing factor depends on your local design rainfall intensity and applicable plumbing code roof drainage table (e.g. IPC) — confirm the correct factor for your jurisdiction and roof drainage design (primary vs. secondary/emergency) before sizing scuppers.
What this calculation does not cover
- Returns an AREA, and a scupper is not sized by area alone. It spills as a weir, so what it passes depends on how WIDE the opening is and how deep the water stands above its invert, with flow climbing as roughly the 1.5 power of that depth. A tall narrow slot and a wide low one of the same area are not interchangeable — the wide one drains the roof, the narrow one needs far deeper standing water before it moves the same flow.
- Says nothing about how high to set the invert, and that is the number deciding how much water the structure has to hold. An overflow set high leaves a deeper layer standing over the whole tributary area before it spills, and that depth is a real load on deck and framing that has to be checked for ponding — an opening area cannot fix it.
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This result is a specification — 77.4 in² — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
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
- Required opening area = roof area × a sizing factor derived from the local design rainfall intensity, following the same tributary-area method used by plumbing code roof drainage sizing tables (e.g. IPC Appendix roof drain sizing)
Which documents these citations point at
- International Plumbing Code (United States)Plumbing systems — fixtures, water supply, sanitary drainage, venting and storm drainage.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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