Rainwater Drainage and Surface Water Management

What This Requirement Covers

Approved Document H3 requires adequate provision for the disposal of rainwater from roofs and paved areas. The hierarchy of preference for rainwater disposal is: soakaway or infiltration (where ground conditions permit), then watercourse, then surface water sewer, and only as a last resort, the combined sewer.

Key Requirements

Rainwater Disposal Hierarchy

1. Soakaways and infiltration: The preferred method where ground conditions allow; rainwater is returned to the ground
2. Watercourse: Discharge to a river, stream, or other watercourse (with Environment Agency consent if required)
3. Surface water sewer: Connection to a public surface water sewer
4. Combined sewer: Connection to a combined (foul + surface water) sewer - only where no other option exists

Gutters and Downpipes

1. Gutters must be sized to carry the expected rainfall without overflow
2. Standard domestic gutters: 75 mm half-round gutter for roof areas up to approximately 18 m²; 100 mm half-round for larger areas
3. Downpipes: 63 mm diameter for 75 mm gutters; 75 mm diameter for 100 mm gutters
4. Gutters should have a fall of approximately 1:600 to 1:350 towards the outlet
5. Leaf guards or gutter protection may be required where trees overhang the roof

Soakaways

1. Must be designed to BRE Digest 365 (Soakaway design)
2. Soil percolation tests must be carried out to determine whether the ground is suitable for infiltration
3. Soakaways must be located at least 5 metres from any building and at least 2.5 metres from any boundary
4. Soakaways must not be constructed in contaminated ground or where the water table is high

Sustainable Drainage Systems (SuDS)

1. Since 2024, new developments in England must include SuDS that comply with the mandatory SuDS standards
2. SuDS manage surface water at source, mimicking natural drainage through:

- Permeable paving - Rain gardens and bioretention areas - Swales and filter strips - Green roofs - Attenuation tanks and ponds

1. SuDS must manage both water quantity (flood risk) and water quality (pollution control)
2. Approval of SuDS is obtained through the planning process or the SuDS Approving Body (SAB)

Flood Risk

1. Where a site is in a flood risk area (Flood Zone 2 or 3), the surface water drainage strategy must demonstrate that the development will not increase flood risk elsewhere
2. A Flood Risk Assessment (FRA) may be required by the planning authority

Practical Compliance Tips

1. Carry out soil percolation tests early to determine whether soakaways are feasible; test to BRE Digest 365
2. Size soakaways to store the 1-in-100-year rainfall event plus a climate change allowance (currently +40%)
3. Separate surface water from foul water throughout the site; mixing causes pollution and sewer overflows
4. Consider rainwater harvesting as part of the surface water strategy; collected rainwater can be used for toilet flushing and garden irrigation
5. Ensure all surface water discharge complies with the local planning conditions and the Environment Agency requirements
6. Maintain SuDS components regularly; blocked permeable paving and silted ponds lose their effectiveness
7. Provide householder information on the maintenance of soakaways and SuDS components