Rainwater Harvesting Systems: Regulations, Tank Sizing & Installation

Summary

Rainwater harvesting (RWH) captures roof runoff, filters it, stores it in an underground or above-ground tank, and then pumps it to non-potable end uses — predominantly WC flushing and garden irrigation. A well-designed system can reduce household mains water consumption by 30–50%, depending on household size and garden use.

Growing interest in water efficiency, driven partly by SUDS requirements for new developments and partly by increasing water bills, has made RWH a viable consideration for new builds and some retrofit projects. However, the regulatory requirements are frequently misunderstood, and incorrect installation — particularly regarding the separation between harvested rainwater and the mains supply — can create serious public health risks through contamination of the mains water supply.

The Water Supply (Water Fittings) Regulations 1999 prohibit any fitting arrangement that could allow backflow from a non-mains source into the public water supply. This is the primary regulatory constraint on RWH design. BS 8515 is the British Standard that translates the legislative requirement into practical design guidance and provides the methodology for tank sizing.

Key Facts

• Primary standard — BS 8515:2009+A1:2013 (Rainwater Harvesting Systems — Code of Practice)
• Backflow prevention — Type AA air gap required between mains top-up supply and tank; no direct pipe connection permitted
• Water company notification — required under Water Fittings Regulations before installation; some companies require inspection
• Roof catchment — only clean roof surfaces; avoid roofs with lead flashings, asbestos cement, or bitumen surfaces unless filtration is upgraded
• Runoff coefficient — concrete/clay tiles 0.75; metal roofing 0.9; green/sedum roofs 0.5
• First flush diverter — diverts first 0.2–0.5mm of rainfall to drain; removes most contaminants from roof surface
• Tank position — underground tanks preferred for temperature stability (reduces algae growth); above-ground acceptable
• Minimum tank capacity — must be calculated to BS 8515; typically 1,500–7,500 litres for domestic
• Pump and controls — submersible pump in tank or external pump; required where underground tank is below point of use
• Overflow — must drain to soakaway or surface water sewer; never to foul sewer
• Distribution pipework — must be identified as non-potable (purple pipe, BS 6920 colour); labelled at all outlets
• Permitted development — above-ground tanks may require planning permission if over 1m³ and in front of the building
• Not for drinking — harvested rainwater must never be used for drinking, cooking, food preparation, or personal hygiene without full treatment

Quick Reference Table

Detailed Guidance

BS 8515 Tank Sizing Method

BS 8515 provides a simplified calculation method suitable for most domestic applications. The inputs are:

• A = annual average yield from roof (litres/year) = roof area (m²) × annual rainfall (mm) × runoff coefficient × filter efficiency (0.9 typically) × first flush losses (deduct 2%)
• D = annual demand for non-potable uses (litres/year) = daily demand (litres/person/day) × occupants × 365
• Tank volume = the lesser of A/12 or D/12 (i.e., approximately one month's supply or one month's demand)

Worked example:

• 80m² usable roof area; SE England (600mm/year rainfall); concrete tiles (runoff coefficient 0.75); family of 4
• Annual yield A = 80 × 600 × 0.75 × 0.9 × 0.98 = 31,752 litres/year
• WC flushing demand: 4 people × 36 litres/day × 365 = 52,560 litres/year (36 litres/person/day is the standard WC flushing allowance)
• Governing value = A (31,752) as it is less than D (52,560)
• Tank volume = 31,752 ÷ 12 = 2,646 litres → specify 3,000 litre tank

For more precise calculations, the full BS 8515 method uses monthly demand and yield profiles with a mass balance approach.

System Components

1. First flush diverter Fitted to the downpipe before water enters the filter. The first 0.2–0.5mm of rainfall flushes the roof, washing away dust, bird droppings, and debris. This contaminated water is diverted to the drain. Only after this initial volume has been diverted does water route to the filter and tank. Self-cleaning versions refill the standpipe between rain events.

2. Calmed inlet filter Water enters the tank through a floating-inlet calmed entry point that introduces water below the tank surface to avoid disturbing settled sediment. Built-in filter (typically 0.44mm mesh) removes particulate matter. Must be cleaned annually.

3. Overflow syphon An overflow syphon at the top of the tank prevents tank overflow from discharging at surface level in a surge. The syphon breaks the flow and prevents backflow via the overflow pipe. Overflow must run to an appropriate outlet — soakaway, surface water sewer, or watercourse (EA consent may be needed).

4. Submersible pump and controls A submersible pump with dry-run protection feeds the distribution system. A control unit manages the pump, the mains top-up valve, and low-level indicators. The mains top-up must be via a Type AA air gap — a break tank with a ball valve feeding the main tank, with a physical air gap between the valve outlet and the maximum tank water level.

5. Distribution pipework All pipework carrying harvested rainwater from the tank to the point of use must be clearly identified. Purple pipe (BS 6920 colour) and labels reading "CAUTION: NON-POTABLE WATER — DO NOT DRINK" at all visible points, at each outlet, and at each service valve.

Backflow Prevention — The Critical Safety Element

The Type AA air gap is non-negotiable. It means there is a visible, unobstructed gap between the water outlet (the float valve or inlet pipe) and the maximum water surface level in the tank. The air gap prevents any scenario where contaminated water could be siphoned back into the mains supply.

What is NOT acceptable:

• A mains pipe connected directly to the tank with a non-return valve only (non-return valves are not fail-safe)
• A mains pipe connected to the distribution pipe from the tank
• Any arrangement where a loss of mains pressure could create back-siphonage from the tank into the mains

Water companies take contamination risk very seriously. Incorrect installation can result in prosecution and enforcement action.

Installation Sequence (Underground Tank)

1. Obtain water company notification acknowledgement before starting
2. Excavate to correct depth — bottom of tank plus 200mm for granular bed; ensure adequate cover for tank (minimum 600mm, or to manufacturer's specification)
3. Lay 200mm granular bed; compact
4. Lower tank into position (heavy equipment needed for tanks over 1,500 litres)
5. Connect inlet pipework from roof downpipes; install first flush diverter at this stage
6. Connect overflow pipework to appropriate outlet
7. Install pump and control unit; connect distribution pipework
8. Install break tank for mains top-up with Type AA air gap
9. Install purple pipe distribution to WCs (and any other end uses)
10. Label all non-potable outlets
11. Backfill in layers, compacting carefully to avoid deforming the tank (follow manufacturer's backfill specification — some tanks need partial water fill before backfilling)
12. Commission and test

Frequently Asked Questions

Do I need planning permission for a rainwater harvesting tank?

Above-ground tanks: if they are within the curtilage of a dwelling and under 1m³, they are generally permitted development. Larger above-ground tanks, or tanks in front of the principal elevation, may require planning permission. Underground tanks are generally permitted development for domestic use, but always check with the local planning authority for listed buildings or conservation areas.

Can I use rainwater harvesting water in a washing machine?

Technically yes, but with caveats. Rainwater is soft and low in minerals, which is good for washing machines. However, it may contain organic matter or low-level contamination that could affect sensitive skin, particularly for young children's clothing. Manufacturers' warranties may be affected. Most RWH specialists recommend limiting machine use to garden irrigation and WC flushing as the primary applications.

What maintenance does a rainwater harvesting system need?

• Annual: clean calmed inlet filter; inspect and clean first flush diverter; check tank interior for sediment (pump out if more than 50mm accumulated); inspect overflow for blockage; check all pipework labels are in place
• 2–5 yearly: inspect pump impeller; check control unit function and test mains top-up operation including air gap integrity
• 10 yearly: full system inspection by competent person

Does a rainwater harvesting system need Building Regulations approval?

The system itself does not fall under Part H (which covers disposal of surface water, not harvesting). However, if the system involves structural work (an underground tank requires excavation near a building foundation), Part A may apply. The water fittings regulations are a separate legislative requirement that applies regardless of building control. Notify your water company before installation — this is the primary regulatory action required.

Regulations & Standards

• BS 8515:2009+A1:2013 — Rainwater Harvesting Systems — Code of Practice; the primary design standard

• Water Supply (Water Fittings) Regulations 1999 — prohibits fittings that could allow backflow; defines fluid categories and backflow prevention requirements

• WRAS Guidance Document 09-02-05 — WRAS (Water Regulations Advisory Scheme) guidance on RWH systems; backflow requirements

• Building Regulations Approved Document H (H3) — rainwater drainage from roofs

• BS EN 805 — water supply requirements for systems outside buildings; hydraulic design

• CIRIA Report C539 — Rainwater and greywater use in buildings (older but still referenced)

• WRAS: Rainwater and Greywater — regulatory requirements for installation

• BSI: BS 8515 — purchase the standard for full sizing methodology

• Environment Agency: Water efficiency in new developments — policy context

• UKRHA (UK Rainwater Harvesting Association) — trade body with installer guidance and approved supplier lists

• soakaways — overflow disposal from harvesting tanks; surface water management

• underground drainage — underground pipe work connecting to harvesting system

• water regulations — Water Supply Regulations 1999 and WRAS requirements