Eaves gutter performance & Eaves drainage design - BS EN12056-3
The UK Rainwater Drainage Design Standard Explained (BS EN 12056-3:2000)
In accordance with the eaves gutter rainwater drainage standard (BS EN 12056-3:2000), a gutter system can be checked for suitability by following the procedure outlined below. This guide is designed for users with no prior expertise, providing a step-by-step explanation of the key principles. Our updated calculator incorporates regional rainfall intensities across the UK and accounts for frictional resistances in longer gutter runs, ensuring more accurate and tailored results for your project.
Calculate the Effective Roof Area
The effective roof area is a critical factor in determining the drainage requirements, as it accounts for the roof's pitch, which increases the surface area exposed to rainfall.
Width of roof (m) x Length of roof (m) = Roof area (sq.m)
Roof area x Pitch factor = Effective roof area (sq.m)
The pitch factor adjusts for the slope of the roof, as steeper pitches collect more rainwater per unit of horizontal area. Use the table below to find the appropriate factor based on your roof's pitch.
| 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 |
For example, a roof with a 30-degree pitch has a pitch factor of 1.29, meaning the effective area is 29% larger than the flat projection to account for the slope.
Calculating the Rainwater Runoff
The BS EN 12056-3 design standard provides localised rainfall intensity data for different UK regions, reflecting variations in weather patterns. Previously, many calculations used a uniform national highest intensity of 0.021 litres per second per square metre to avoid under-sizing. Our calculator now allows selection of region-specific values for greater precision:
- South-East England: 0.022 l/s/m²
- South-West England: 0.020 l/s/m²
- Midlands: 0.018 l/s/m²
- North England: 0.015 l/s/m²
- Scotland: 0.010 l/s/m²
- Wales: 0.017 l/s/m²
- Northern Ireland: 0.014 l/s/m²
These intensities represent the expected rainfall rate during a storm event with a return period suitable for standard domestic and commercial applications.
Effective Roof Area x Rainfall Intensity (l/s/m²) = Rainwater Runoff from Roof (l/s)
For instance, in South-East England, an effective roof area of 100 sq.m would require handling 2.2 l/s of runoff (100 x 0.022).
Selecting the Correct Gutter System
The table below shows the base flow performance capacities of our aluminium gutter systems and the effective roof area each outlet can drain, based on independent testing. These values assume level installation and standard conditions.
To ensure accuracy for real-world installations, our calculator applies a frictional resistance factor for longer gutter runs. This adjustment reduces capacity when the gutter length exceeds 50 times its depth, accounting for flow resistance due to friction. The reduction is calculated as F_l = 1 - (L_d / depth - 50) * 0.00333, with a minimum of 0.5, where L_d is the effective drainage length per outlet.
| 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 |
These capacities are for outlets positioned at the end or centre of the gutter run. Central positioning typically doubles the effective drainage area, as it splits the flow. If your calculated runoff exceeds the system's capacity, consider adding more outlets, repositioning them, or selecting a higher-capacity profile.
Additional Considerations for Accurate Design
- Outlet Position and Number: End-positioned outlets drain from one direction, while central ones handle flow from both sides, improving efficiency.
- Gutter Slope: The standard assumes level gutters; slight slopes can enhance flow but are not factored here.
- Safety Factors: For high-risk areas (e.g., public buildings), a capacity factor of 1.2–1.5 may be applied, though our tool uses the standard 1.0 for typical UK installations.
- Professional Verification: While this tool aligns with BS EN 12056-3, always consult a structural engineer for complex projects or non-standard roofs.
If you require further assistance or have questions about our aluminium rainwater systems, please contact us. Our technical team is available to provide guidance on product selection and installation.