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The outside diameter of the roof drain pipe.
Measure the drain pipe's outside diameter at the point where it passes through the roof deck.
The clearance required on all sides between the pipe and the sump pan edge.
This clearance provides room for water to collect and flow into the drain without ponding excessively around the pipe.
Required sump pan area
1.396 sq ft
- Pan diameter
- 16 in
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Roof Drain Sump Pan Sizing Calculator: 1.4 sq ft — 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)
- Sump pan diameter = pipe diameter + 2 × clearance; pan area = π × (pan diameter ÷ 2)², standard circular sump pan sizing to provide adequate clearance around the drain pipe for water collection
Inputs used
- Drain Pipe Diameter
- 4 in
- Required Clearance Around Pipe
- 6 in
Intermediate steps
- Pan diameter
- 16 in
What this calculation does not cover
- Sizes the pan and says nothing about getting water to it. A sump drains only what reaches it, which means the insulation around it has to be tapered down into the pan and crickets have to run between drains and behind any curb up-slope of one. On a dead-flat deck this pan is simply the lowest 0.13 m² (1.4 sq ft) of a field that ponds everywhere else.
- Takes clearance as a hydraulic figure when it is usually a hardware one. The width around the pipe is driven by the drain body's clamping ring diameter and by the membrane lap needed outboard of it to make the flashing — check the drain manufacturer's ring dimension before this number, because a pan that does not clear the ring cannot be sealed to the membrane at all.
- Cutting the recess is a structural decision. The opening has to fall clear of the joists, purlins or deck flutes underneath, or the deck needs framing around it — and a drain landing on the centreline of a structural member cannot be sumped where the drawing puts it, whatever plan area this returns.
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
- Sump pan diameter = pipe diameter + 2 × clearance; pan area = π × (pan diameter ÷ 2)², standard circular sump pan sizing to provide adequate clearance around the drain pipe for water collection
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