Gutter & Downpipe Drainage Calculator
To begin using this drainage design tool, determine the largest roof area on your project plan that will be drained by a single rainwater downpipe (the worst-case scenario).
Please select your preferred gutter system:
Half Round Gutter
Deepflow Gutter
Box Gutter
Moulded Ogee Gutter
The UK Rainwater Drainage Design standard Explained (BS EN12056-3:2000).
In accordance with the eaves gutter rainwater drainage standard (BS EN12056-3), a gutter system can be checked for suitability by following the below procedure:
Calculate the effective roof area
Width of roof (m) x Length of roof (m) = Roof area (sq.m)
Roof area x Pitch factor = Effective roof area (sq.m)
| Roof Pitch | Pitch Factor | Roof Pitch | Pitch Factor |
| 10 Deg | 1.08 | 30 Deg | 1.29 |
| 12.5 Deg | 1.11 | 32.5 Deg | 1.3 |
| 15 Deg | 1.13 | 35 Deg | 1.35 |
| 17.5 Deg | 1.15 | 37.5 Deg | 1.38 |
| 20 Deg | 1.18 | 40 Deg | 1.42 |
| 22.5 Deg | 1.2 | 42.5 Deg | 1.46 |
| 25 Deg | 1.23 | 45 Deg | 1.5 |
| 27.5 Deg | 1.26 | 47.5 Deg | 1.55 |
Calculating the rainwater runoff
The BS EN12056-3 Design standard does provide specific localised rainfall data for all regions however, the UK national highest rainfall intensity is 0.021 litres per second per sq.m. Most designers use the highest figure to avoid under sizing.
Effective Roof area x 0.021 litres per second = Rainwater runoff from roof (l/s)
Selecting the correct gutter system
The table below shows the flow performance capacities of our gutter systems and the effective area each outlet is capable of draining.
| Gutter system | End Outlet | ERA | Centre Outlet | ERA |
| Half Round - 63 dia Pipe | 1.25 l/s | 59.5 sq.m | 2.5 l/s | 119 sq.m |
| Half Round - 76 dia Pipe | 1.25 l/s | 59.5 sq.m | 2.5 l/s | 119 sq.m |
| Deepflow - 63 dia Pipe | 2.1 l/s | 100 sq.m | 4 l/s | 190 sq.m |
| Deepflow - 76 dia Pipe | 2.1 l/s | 100 sq.m | 4 l/s | 190 sq.m |
| Box - 63 dia Pipe | 2.1 l/s | 100 sq.m | 4.2 l/s | 200 sq.m |
| Box - 76 dia Pipe | 2.9 l/s | 138 sq.m | 5.9 l/s | 280 sq.m |
| Box - 72x72 Pipe | 3.2 l/s | 152 sq.m | 6.5 l/s | 309 sq.m |
| Moulded Ogee - 63 dia Pipe | 2.2 l/s | 104 sq.m | 3.8 l/s | 180 sq.m |
| Moulded Ogee - 76 dia Pipe | 2.2 l/s | 104 sq.m | 3.8 l/s | 180 sq.m |
| Moulded Ogee - 72x72 Pipe | 2.2 l/s | 104 sq.m | 3.5 l/s | 190 sq.m |
If you are in need of any further assistance please contact our technical team.