Scupper Size Calculator
Enter the roof area draining to the scupper, the design rainfall rate and the water depth allowed at the opening. The calculator gives the flow to carry and the scupper width and height needed.
- Scupper width
- Flow
- Opening height
Water depth is how deep the water can build up at the scupper opening.
A scupper is a weir, so its flow depends on width and water depth. Flow is roof area × rainfall ÷ 96.23. A 2,000 sq ft roof in 4 in an hour of rain sheds about 83 gallons a minute. With 2 in of water at the opening, the scupper needs to be about 10 in wide.
Scupper size chart
4 in an hour, 2 in water depth. Search for a roof.
| Roof (sq ft) | Flow (gpm) | Width (in) |
|---|---|---|
| 500 | 21 | 2.5 |
| 1000 | 42 | 5.1 |
| 1500 | 62 | 7.6 |
| 2000 | 83 | 10.1 |
| 3000 | 125 | 15.2 |
| 5000 | 208 | 25.3 |
| 7500 | 312 | 38 |
| 10000 | 416 | 50.7 |
Width in inches.
Roof area and scupper
4 in an hour, 2 in depth. Slide the roof area.
The opening widens with the flow.
How to size a scupper
Roof to width. Step through it.
Area
Draining roof.
Add an overflow scupper or drain set a little higher in case the main one blocks.
Scupper, roof drain or overflow
Roof drainage. Compare them.
Through the wall.
Inside.
Backup.
Many codes require secondary drainage on flat roofs.
Before you build
Scupper checks. Tick each item.
Tick each item as you go.
Make sure the roof can carry the weight of the water depth you allow.
What you get
Scupper sizing. Each tile explains one result.
Opening.
Gallons a minute.
Opening.
Each.
Size.
One inch of water over a square foot of roof weighs about 5.2 lb.
Frequently Asked Questions
No question matches that search.
How do I size a roof scupper?
Find the flow from roof area and rainfall, then divide by two point nine times the water depth to the power of one and a half.
How much water does a roof shed?
Roof area times rainfall in inches an hour, divided by ninety-six point two three, in gallons a minute.
How tall should a scupper be?
At least twice the design water depth.
What is an overflow scupper?
A backup opening set higher than the main drain.
Does a deeper water level help?
It lets a narrower scupper carry the flow, but adds weight to the roof.
What rainfall rate should I use?
The local design rate, often from the plumbing code.
Can I use two smaller scuppers?
Yes, sharing the flow between them.
Water off the roof
Size the scupper for the storm and the roof stays dry and safe, even in the heaviest rain.
More size calculators
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