Downpipe Size Calculator
Enter the roof plan area, the roof pitch and the design rainfall. The calculator works out the effective roof area and flow in litres a second, the single downpipe size that carries it and how many standard 68 mm downpipes you would need.
- Downpipe size
- Rain flow
- Number of downpipes
75 mm an hour is a common UK design rainfall. Check local figures for heavier rain.
Steeper roofs catch more driving rain. The design rule adds half the tangent of the pitch to the plan area. A 60 m² roof at 30° acts like 77 m². At 75 mm an hour of rain that is 1.6 L/s, more than one 68 mm downpipe carries, so use an 80 mm pipe or two 68 mm ones.
Downpipe size chart
30° pitch, 75 mm an hour. Search for a roof area.
| Plan area (m²) | Flow (L/s) | One pipe | 68 mm pipes |
|---|---|---|---|
| 20 | 0.54 | 50 | 1 |
| 40 | 1.07 | 68 | 1 |
| 60 | 1.61 | 80 | 2 |
| 80 | 2.15 | 100 | 2 |
| 100 | 2.68 | 100 | 3 |
| 150 | 4.03 | 150 | 4 |
| 200 | 5.37 | 150 | 5 |
Flow in litres a second.
Roof area and downpipes
30° pitch, 75 mm an hour. Slide the roof area.
The house shows how many 68 mm downpipes are needed.
How to size a downpipe
Roof to pipe. Step through it.
Plan area
Length × width.
Every downpipe needs a gully or drain that can take its flow.
68 mm, 80 mm or 110 mm
Downpipe sizes. Compare them.
Houses.
Larger.
Commercial.
Square downpipes carry a little more than round ones of the same width.
Before you fit
Downpipe checks. Tick each item.
Tick each item as you go.
Fix downpipes with a bracket at least every 2 m.
What you get
Downpipe sizing. Each tile explains one result.
L/s.
Effective.
Size.
68 mm pipes.
Per pipe.
A central outlet lets one gutter run drain both ways, halving the flow in each half.
Frequently Asked Questions
No question matches that search.
What size downpipe do I need?
For most houses a sixty-eight millimetre round downpipe serves up to about forty-five square metres of pitched roof.
How many downpipes does my roof need?
Divide the rain flow by what one downpipe carries, about one and a quarter litres a second for sixty-eight millimetres.
How do I calculate roof run-off?
Multiply the effective roof area by the rainfall in millimetres an hour and divide by three thousand six hundred.
Why does roof pitch matter?
Wind-driven rain hits a steep roof face, so it collects more water than its plan area.
What rainfall should I design for?
Seventy-five millimetres an hour is common in the UK. Tropical areas use much more.
Where should downpipes go?
At gutter ends or in the middle, spread evenly along the roof.
Is a square downpipe better?
It carries slightly more for the same width, but both work well.
Keep the rain moving
Size your downpipes for your roof and rainfall, and gutters stop overflowing in heavy storms.
More size calculators
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