Rainwater Harvesting Tank is the storage vessel — typically buried underground or housed in a basement — that holds captured rainwater between collection and reuse. UK tanks are sized to BS 8515:2009+A1:2013 and built in concrete, GRP, HDPE or corrugated steel depending on capacity and ground conditions.
Tank materials compared
Material choice for rainwater harvesting tanks depends on capacity, installation depth, ground water level and cost:
| Material | Typical capacity | Pros | Cons |
|---|---|---|---|
| Concrete (pre-cast) | 1,000–100,000 L | Heavy, no buoyancy risk, long life | Heavy lift, expensive transport |
| GRP / fibreglass | 1,000–50,000 L | Light, fast install, smooth finish | Buoyancy mitigation needed below WT |
| HDPE / Polyethylene | 1,500–25,000 L | Light, monolithic, cheap | Limited capacity, buoyancy risk |
| Corrugated steel pipe | 5,000–500,000+ L | Very large capacity, robust, deep cover | Bespoke design, longer programme |
Sizing per BS 8515
BS 8515:2009+A1:2013 provides two sizing methods. The simplified method takes the smaller of 5% of annual roof yield or 5% of annual non-potable demand. The detailed method runs a daily water balance over a representative year to optimise capacity against yield and demand patterns.
Typical UK system sizes: 1,500–4,500 litres for a single dwelling; 10,000–25,000 litres for a small commercial building; 50,000+ litres for schools, warehouses and industrial premises with large roof areas and constant non-potable demand.
Integration with SuDS
Many UK lead local flood authorities now allow the active storage volume of a rainwater harvesting tank to count towards stormwater attenuation requirements, since the tank is empty (or partly empty) at the start of most storm events. This integration can dramatically reduce the size — and cost — of additional attenuation infrastructure.
Designs must demonstrate that the harvesting tank operates with a controlled overflow to maintain attenuation behaviour, and the LLFA’s specific local guidance must be confirmed before relying on this credit.
Related ViaCon solutions
ViaCon supplies large-capacity corrugated steel rainwater harvesting tanks suitable for commercial, industrial and infrastructure projects. See our ViaCon Storm Solutions, stormwater solutions and bespoke combined attenuation-harvesting systems. Related glossary entries: rainwater harvesting, rainwater tank and SuDS.
Frequently asked questions about rainwater harvesting tank
How big should a rainwater harvesting tank be?
Tank size is calculated using BS 8515:2009. The simplified method takes the smaller of 5% of annual rainfall yield from the roof or 5% of annual non-potable demand. Typical domestic sizes are 1,500–4,500 litres; commercial systems range from 7,500 to over 50,000 litres.
What material is best for a rainwater harvesting tank?
For domestic installations, GRP and HDPE tanks dominate due to lightweight installation. For commercial and large-scale projects, pre-cast concrete or corrugated steel pipe tanks are preferred due to their robustness, depth capability and very large available capacities (50,000+ litres).
Can a rainwater harvesting tank be used for attenuation?
Yes — many UK lead local flood authorities allow the active storage volume to count towards stormwater attenuation, since the tank is typically empty or partly empty at the start of most storms. Designs must include a controlled overflow and align with the local LLFA’s specific requirements.
Where should a rainwater harvesting tank be installed?
Most UK tanks are installed underground to maximise capacity and avoid frost. Domestic tanks are typically placed in the garden, with the pump set housed in a chamber above. Commercial tanks may be placed in dedicated plant rooms or basements. Avoid tree roots, services and high water table areas without buoyancy design.
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