BS EN 12056-3 Explained: Roof Drainage Design Standards
The main building regulation that applies to guttering in England and Wales is Approved Document H (Drainage and Waste Disposal), which references BS EN 12056-3 as the design standard for gravity roof drainage systems. BS EN 12056-3 sets out how to calculate effective roof area, determine the required gutter capacity, select downpipe sizes, and design a system that handles the expected rainfall for your location. In Scotland, the equivalent guidance is in Section 3 of the Technical Handbooks. In Northern Ireland, it is Part N of the Building Regulations. All reference BS EN 12056-3 as the calculation methodology.
For most domestic installations, you do not need to read the full standard — the key principles are straightforward and are summarised in this guide. For complex commercial projects with multiple roof levels, valley gutters, and internal drainage, a drainage engineer should carry out a full BS EN 12056-3 calculation.
What Is BS EN 12056-3?
BS EN 12056 is a European standard adopted across the UK that covers gravity drainage systems inside buildings. It has five parts:
| Part | Title | Scope |
|---|---|---|
| Part 1 | General and performance requirements | Overall framework |
| Part 2 | Sanitary pipework, layout and calculation | Waste and soil pipes inside the building |
| Part 3 | Roof drainage, layout and calculation | Gutters, downpipes, and rainwater disposal |
| Part 4 | Waste-water lifting plants | Pumped drainage |
| Part 5 | Installation and testing | Commissioning and verification |
Part 3 is the section relevant to guttering. It provides the methodology for:
- Calculating the effective roof area
- Determining the design rainfall intensity
- Sizing gutters to handle the calculated flow rate
- Sizing downpipes to drain the gutters effectively
- Designing the full rainwater system from roof to ground-level discharge
How BS EN 12056-3 Relates to Building Regulations
BS EN 12056-3 is not legislation in itself — it is a technical standard. Building Regulations in England and Wales (Approved Document H, Section 3: Rainwater Drainage) reference it as the accepted method for designing rainwater systems.
What Building Control Expects
For domestic work:
- New builds, extensions, and significant alterations that require a Building Regulations application must include rainwater drainage in the design
- Building Control will check that gutters and downpipes are adequately sized for the roof area
- The system must discharge to an appropriate point — soakaway, surface-water sewer, or watercourse
- Connection to the foul sewer is generally not permitted for rainwater
For like-for-like gutter replacements on existing properties (same profile, same positions), Building Control approval is not usually required. However, if you are changing the drainage discharge point, adding new downpipes to different locations, or working on a listed building, you may need approval.
The Core Calculation: Effective Roof Area
The starting point of BS EN 12056-3 is determining how much water the roof captures. This is not simply the plan area of the roof — it must account for the roof pitch, which increases the effective catchment.
The Standard Formula
Effective Roof Area = (W + H/2) × L
| Term | Definition |
|---|---|
| W | Horizontal width of the roof section draining to the gutter (metres) |
| H | Height from gutter level to the ridge or highest point (metres) |
| L | Length of the gutter run (metres) |
Why Pitch Matters
A flat roof (0° pitch) and a steeply pitched roof (60°) over the same footprint have very different effective areas. The steeper the pitch, the larger the surface exposed to wind-driven rain, and the greater the effective catchment.
| Roof Pitch | Multiplier Effect on Effective Area |
|---|---|
| Flat (0°) | Minimal — W × L only |
| 30° | Moderate — adds ~30% to horizontal area |
| 45° | Significant — adds ~50% |
| 60° | Substantial — nearly doubles the horizontal area |
Worked Examples
Example 1: Simple two-storey semi-detached
| Parameter | Value |
|---|---|
| Roof span (W) — half the total ridge-to-ridge | 4.5 m |
| Height gutter to ridge (H) | 3.0 m |
| Front elevation gutter length (L) | 7.0 m |
Effective area = (4.5 + 3.0/2) × 7.0 = 6.0 × 7.0 = 42.0 m²
Example 2: Large detached with steep pitch
| Parameter | Value |
|---|---|
| Roof span (W) | 6.0 m |
| Height gutter to ridge (H) | 5.0 m |
| Front elevation length (L) | 12.0 m |
Effective area = (6.0 + 5.0/2) × 12.0 = 8.5 × 12.0 = 102.0 m²
This detached house needs a hi-cap or deep-flow gutter — standard profiles are undersized.
Rainfall Intensity
BS EN 12056-3 uses a design rainfall intensity measured in litres per second per square metre (l/s/m²), which translates to millimetres per hour (mm/h).
UK Design Values
The standard uses regional rainfall data. For most of England and Wales, the design intensity is:
75 mm/h (0.0208 l/s per m²)
This represents a moderately heavy rainfall event with a return period of approximately 2 minutes (i.e., the intensity that is exceeded on average once every 2 minutes during a storm). It is the accepted standard for domestic gutter design.
For higher-risk locations:
| Location | Design Intensity |
|---|---|
| Most of England and Wales | 75 mm/h |
| Western highlands, Snowdonia | 100 mm/h |
| Lake District, western Scotland | 100–150 mm/h |
| Exposed coastal sites | Up to 150 mm/h |
The Met Office and local authority drainage departments can provide site-specific rainfall data. For projects in high-rainfall zones, using the higher intensity is essential.
Flow Rate Calculation
Once you have the effective roof area and the design intensity, the required flow rate is:
Q (l/s) = Effective Area (m²) × Rainfall Intensity (l/s/m²)
At 75 mm/h, the intensity in l/s/m² is 0.0000208 m/s, or more practically:
Q (l/s) = Effective Area × 75 ÷ 3,600,000 × 1,000
Simplified: Q = Area × 0.0208
| Effective Area (m²) | Flow Rate at 75 mm/h (l/s) | Flow Rate at 100 mm/h (l/s) |
|---|---|---|
| 20 | 0.42 | 0.56 |
| 40 | 0.83 | 1.11 |
| 60 | 1.25 | 1.67 |
| 80 | 1.67 | 2.22 |
| 100 | 2.08 | 2.78 |
| 150 | 3.13 | 4.17 |
Gutter Sizing to BS EN 12056-3
The standard specifies that the gutter must have a tested flow capacity (in l/s) that meets or exceeds the calculated required flow rate. Gutter manufacturers test their products according to BS EN 607 (eaves gutters and fittings) and publish the flow capacity for each profile.
Common PVC-U profiles and their tested capacities:
| Gutter Profile | Tested Capacity (l/s) | Max Area at 75 mm/h (m²) |
|---|---|---|
| Mini (76 mm) | 0.5 | 24 |
| Half round (112 mm) | 0.9 | 43 |
| Square line (114 mm) | 1.1 | 53 |
| Ogee (116 mm) | 1.1 | 53 |
| Deep flow | 1.7–2.5 | 82–120 |
| Hi-cap (150 mm) | 2.0–2.5 | 96–120 |
| Industrial | 2.5+ | 120+ |
Select the profile whose capacity meets or exceeds your calculated flow rate. Always build in a safety margin — especially in high-rainfall areas or near trees where partial blockage is likely.
Browse the full Kalsi rainwater systems range with tested capacities for every profile.
Downpipe Sizing
BS EN 12056-3 also covers downpipe sizing. The downpipe must drain the gutter as fast as the gutter fills. A common error is fitting an adequate gutter with an undersized downpipe.
| Downpipe Size | Capacity (l/s) |
|---|---|
| 50 mm round | 0.4 |
| 65 mm round | 0.7 |
| 68 mm round | 1.0 |
| 80 mm round | 1.5 |
| 65 × 65 mm square | 1.0 |
| 80 × 80 mm square | 1.8 |
For gutter capacities above the downpipe’s limit, use a larger downpipe or add additional outlets.
Gutter Fall and Outlet Positioning
BS EN 12056-3 does not mandate a specific fall for eaves gutters, but a fall of 1:350 to 1:600 is recommended in the associated guidance. This equates to approximately 1.7–3 mm per metre.
Outlet Positioning Rules
- Maximum gutter run before an outlet: 12 m (domestic)
- For longer runs, use two outlets with a central high point
- Outlets should be at the lowest point of the fall
- Additional outlets are needed if the calculated flow rate exceeds one downpipe’s capacity
Other Relevant Standards
| Standard | What It Covers |
|---|---|
| BS EN 607 | Eaves gutters and fittings — material testing and dimensions |
| BS EN 12200 | PVC-U rainwater piping systems — material specification |
| BS EN 1462 | Gutter brackets — testing and load requirements |
| Approved Document H | Building Regulations guidance for drainage in England and Wales |
| BS 8515 | Rainwater harvesting systems — design and installation |
| BS EN 752 | Drain and sewer systems outside buildings |
Compliance in Practice
For Domestic Installers
For straightforward domestic gutter replacement:
- Calculate the effective roof area
- Check the gutter capacity against the required flow rate
- Ensure the downpipe is adequate
- Install with correct fall and expansion gaps
- Discharge to an appropriate drain, soakaway, or watercourse
You do not need to submit a formal BS EN 12056-3 calculation for a like-for-like replacement. But understanding the principles protects you from undersizing — and from callbacks.
For Specifiers and Architects
For new builds and extensions:
- Full BS EN 12056-3 calculation as part of the drainage design
- Include in Building Control submission
- Specify gutter and downpipe profiles with tested capacities
- Document rainfall intensity based on site-specific data
- Coordinate with underground drainage design (BS EN 752)
For Building Control
Building Control officers check that:
- The rainwater system is adequate for the roof area
- Discharge is to an acceptable point (not the foul sewer)
- The system is installed to a reasonable standard
- Complex systems (valley gutters, internal drainage, commercial) have a supporting calculation
Frequently Asked Questions
What building regulations apply to guttering?
In England and Wales, Approved Document H (Drainage and Waste Disposal), Section 3 covers rainwater drainage. The calculation method references BS EN 12056-3. Like-for-like gutter replacements generally do not require Building Control approval, but new builds, extensions, and changes to drainage discharge points do.
Do I need to follow BS EN 12056-3 for a gutter replacement?
For a like-for-like replacement on an existing domestic property, you are not required to submit a formal calculation. However, the principles of BS EN 12056-3 should guide your sizing — choosing a gutter that can handle the roof area at the local design rainfall intensity. For new builds and extensions, a calculation is part of the Building Control submission.
What rainfall intensity should I use in the UK?
75 mm/h for most of England, Wales, and lowland Scotland. 100–150 mm/h for high-rainfall areas — western Scotland, the Lake District, Snowdonia, and exposed coastal sites. If in doubt, check with your local authority or use the higher figure.
What is the maximum gutter run before a downpipe?
The general guidance is 12 m maximum for domestic installations. Longer runs should have two outlets with the gutter falling from a central high point. This also reduces the total fall needed at each end, improving the visual line.
Does BS EN 12056-3 apply to sheds and outbuildings?
The standard applies to all buildings, but in practice, Building Control does not inspect sheds and outbuildings unless they are habitable (e.g., garden rooms with electrical installations). Regardless, following the sizing principles ensures your shed guttering works properly.
