Guidance

Rainwater harvesting (RWH) collects rainwater, typically from roofs, for reuse. Scale ranges from simple water butts to complex systems supplying internal uses. Refer to CIRIA C753 Ch 11 and BS 8515:2009.

System Types: Water Butts (simple, garden watering only); Larger Above/Below Ground Tanks supplying external taps or internal uses (toilet flushing, laundry - requires treatment/filtration, pumps, backup mains supply, careful pipework segregation to prevent cross-contamination). Integration into new builds is most cost-effective.

Catchment Area: Typically roof areas. Calculate contributing roof area and likely rainfall yield (consider local rainfall data, collection efficiency).

Storage Tank: Size based on water demand, rainfall yield, and desired security of supply. Materials: Plastic (PE), GRP, concrete. Location: Above ground (requires space) or below ground (excavation needed). Ensure structural stability, accessibility for maintenance.

Pre-Tank Filtration: Essential to remove leaves/debris before water enters tank (e.g., leaf guards on gutters, downpipe filters, filter baskets in tank inlet).

Pump & Distribution: Required for supplying internal uses or pressurised external taps. Submersible or external pump. Pipework must be clearly labelled and kept separate from potable water supply.

Treatment (for internal use): Filtration (e.g., sediment, carbon filters) usually required. UV disinfection may be needed depending on use/risk assessment. Follow water quality regulations/guidance (e.g., WRAS).

Mains Water Top-up: Required for internal systems to ensure supply when tank empty. Must use legally compliant air gap (Type AA or AB) or equivalent backflow prevention device to avoid cross-contamination with mains supply.

Overflow: Tank must have appropriately sized overflow connected to drainage system (or another SuDS feature), ensuring safe discharge.

Maintenance (C753 S11.11): Regular checks/cleaning of gutters, downpipe filters, pre-tank filters. Annual inspection of tank interior (check for sediment build-up), pump, filters, mains top-up system. Water quality testing may be needed for internal use systems.