Rainwater Tank — StormWater Solutions

Rainwater Tank is any storage vessel — above ground or buried — used to hold rainwater collected from roofs or hard surfaces. Tanks serve two related but distinct purposes in the UK: harvesting rainwater for non-potable reuse, and attenuating stormwater run-off to meet discharge limits.

Above-ground and below-ground tanks

Above-ground rainwater tanks — water butts, slimline tanks, modular stackable units — are common for domestic gardens and small irrigation use. Capacities range from 100 to around 10,000 litres. They are cheap, easy to install and require no excavation, but capacity is limited and the tank must be drained in winter to avoid frost damage.

Below-ground tanks are the standard solution for any system above 5,000 litres or where the tank is part of a SuDS scheme. Materials include concrete, GRP, HDPE and corrugated steel pipe. Below-ground installation eliminates frost risk, provides cool storage that preserves water quality, and frees up valuable site area.

Single-purpose versus combined tanks

UK practice now distinguishes three tank functions:

• Harvesting only — sized per BS 8515 against demand. Tank typically stays partly full to ensure supply availability.
• Attenuation only — sized per CIRIA C753 for the design storm. Tank is normally empty between events to maximise available storage.
• Combined harvesting and attenuation — split-level tanks where the lower portion stores water for reuse and an upper, controlled overflow zone provides attenuation. Increasingly popular as LLFAs allow harvesting volume to count towards attenuation requirements.

Regulations and standards

Below-ground rainwater tanks for harvesting must comply with the Water Supply (Water Fittings) Regulations 1999, including a Type AA air gap separation between mains top-up and stored rainwater to prevent backflow contamination of the public supply. Tank installation also needs to satisfy Building Regulations Approved Document H for any drainage connections.

For attenuation function, the tank is part of the broader SuDS scheme and must be designed per the SuDS Manual (CIRIA C753) and the locally applicable Non-Statutory Technical Standards.

Related ViaCon solutions

ViaCon supplies large-capacity corrugated steel rainwater tanks for harvesting, attenuation and combined systems. See our ViaCon Storm Solutions, stormwater solutions and bespoke combined-function tanks. Related glossary entries: rainwater harvesting, rainwater harvesting tank and attenuation tank.

Frequently asked questions about rainwater tank

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