Rainwater harvesting at home: what a system can and cannot supply

A water butt is the simplest form of rainwater harvesting. A full system goes further: it collects rain from the roof into a larger tank, filters it, and pumps it back into the house to flush toilets and sometimes run a washing machine. It can cut the amount of mains water a household uses. It also brings a second, non-drinking water supply into the house, and that is where the rules come in.

If you only want water for the garden, start with our guide to water butts and garden watering.

How a domestic system works

The details vary by maker, but most systems have the same parts:

• Collection. Gutters and downpipes from the roof feed into the system instead of, or as well as, the surface water drain.
• Filtering. A filter on the way in removes leaves and grit. Many need cleaning from time to time.
• Storage. A tank, often buried in the garden, sometimes above ground or in a garage or basement.
• Overflow. When the tank is full, excess water goes to a drain or soakaway.
• Pump and controls. A pump sends water to the points that use it, either directly or via a header tank in the loft.
• Mains backup. In a dry spell the tank runs low, so the system tops itself up with mains water to keep the toilets working.
• Separate pipework. Rainwater runs in its own pipes, marked so nobody mistakes them for drinking water pipes.

What it can and cannot supply

Harvested rainwater is not treated to drinking water standards. Systems are typically used for:

• WC flushing;
• a washing machine, where the maker and installer say it is suitable;
• outside taps for the garden and car washing.

It should not supply the kitchen tap, drinking water, cooking, bathing or showering. If you have questions about the quality of your mains supply, see water quality and hard water areas.

Why the mains connection matters

This is the part that is not casual DIY.

Water Regs UK's page on alternative water supplies is direct about it. It says "Alternative water supplies should never be directly connected to the mains drinking water", and that "The only legal and safe way to combine mains and another source of water is to use an arrangement called a break tank". It adds that this is typically done using a Type AA or Type AB air gap.

An air gap means the mains water pours into the tank from above, through open air, so there is no pipe through which rainwater could be drawn back into the mains. A valve on its own is not the same thing.

The same page says that where mains water and other sources combine, "it is essential to notify the relevant water undertaker" so that adequate backflow protection is in place.

The Water Supply (Water Fittings) Regulations 1999, Schedule 2 also say, in paragraph 14, that any fitting carrying rainwater or recycled water "shall be clearly identified so as to be easily distinguished from any supply pipe or distributing pipe", and that a supply pipe must not be connected so that it can carry such water.

What this means for a householder:

• Do not connect a rainwater tank, pump or butt to your household pipes yourself.
• Do not fit a valve between the mains and a rainwater pipe and treat that as the backup.
• Do talk to your water company before installing a system, and use an installer who knows the water fittings rules. A WaterSafe-registered plumber is a good place to start. Our guide to hiring a plumber covers how to check.
• Do keep labels on the pipes and outlets, so a future owner or plumber knows which is which.

These are the rules for England and Wales. Scotland and Northern Ireland have their own water fittings rules, so ask your supplier there.

Building regulations and drainage

A buried tank and changes to how roof water drains can also touch the Building Regulations. In England, Approved Document H covers drainage, and Approved Document G covers water efficiency and sanitation. We have not summarised either here. Your installer or local building control can tell you what applies to your project. Wales, Scotland and Northern Ireland have their own building regulations.

Is it worth it?

That depends on your roof area, local rainfall, how much water the house uses, whether you are on a water meter, and how easy it is to install a tank. Retrofitting a buried tank and separate pipework into an existing home is a much bigger job than building it into a new house or a major renovation.

Running costs to think about:

• the pump uses electricity;
• filters need cleaning;
• the system needs servicing to the maker's schedule;
• if the tank empties, you are back on mains water through the backup.

Ask any installer to explain the expected saving in plain terms for your house, and ask what maintenance the system needs. We do not give payback figures because they vary so much.

We have not listed products for this guide. Domestic systems are usually supplied and installed together, and the installation is what matters most.

Can rainwater be used for drinking?

Not from a domestic harvesting system of the kind described here. It is not treated to drinking water standards. Use it for WC flushing, suitable washing machines and the garden.

Can I plumb a water butt into my toilet?

No. Connecting rainwater to household pipes needs a proper system with an air gap arrangement on the mains backup, separate marked pipework, and notice to the water company. Water Regs UK says alternative supplies should never be directly connected to the mains.

What happens when the rainwater tank runs dry?

Most systems top up with mains water so the toilets keep working. The top-up must go through an air gap or break tank, not a direct connection.