The simplest way to collect and store rainwater at home is to connect a covered water butt to a roof downpipe using a rainwater diverter. Put the container on a firm, level base, screen out leaves, provide a safe overflow route and treat the stored water as non-drinking water.
A basic system can supply water for established plants, garden tools and limited toilet flushing during a mains outage. It does not need a pump or connection to household plumbing. Larger tanks can collect more, but they also need more space, stronger foundations and a proper plan for overflow and maintenance.
What you need to collect rainwater
- A suitable roof: a house, garage, shed or greenhouse roof with guttering and an accessible downpipe.
- A water butt or storage tank: preferably opaque, weather-resistant and purpose-made, with a secure lid and tap.
- A stable stand or base: strong enough to support the container when full and high enough to place a watering can beneath the tap.
- A downpipe diverter: matched to the shape and size of the existing downpipe.
- A leaf screen or filter: to reduce the amount of moss, leaves and other organic material entering storage.
- An overflow route: either through a correctly positioned diverter or a separate overflow connection directed to an appropriate drain or soakaway.
Avoid improvising with a container that previously held fuel, oil, pesticides, cleaning chemicals or another hazardous substance. Residues can remain after washing. A purpose-made water butt is easier to cover, connect, drain and maintain safely.
Choose a suitable roof and downpipe
Choose a roof that is reasonably clean and drains through sound guttering. Check for accumulated moss, leaves, animal droppings and obvious contamination before connecting the tank. Trim back overhanging branches where practical and keep gutters clear so that organic debris does not continually enter the stored water.
Do not assume that runoff from every roof finish is suitable for every use. Coatings, preservatives, metal surfaces and recent roof treatments can affect the water. Check the roofing and treatment manufacturer’s advice if there is any uncertainty, particularly before using the water around food-growing areas or sensitive plants.
Select a downpipe that is close to a practical storage position. The container should not block a path, gate, air brick, inspection cover or escape route. In a rented home, obtain written permission before cutting a downpipe or fixing equipment to the building. Flat owners may need consent from the freeholder or building manager because roofs and rainwater systems are often communal.
Estimate how much rainwater you can collect
One millimetre of rainfall on one square metre of roof produces one litre of water before losses. The first useful estimate is:
Roof plan area in square metres × rainfall in millimetres = theoretical collection in litres
A 20 square metre shed roof receiving 25 mm of rain therefore receives a theoretical 500 litres. The actual amount entering storage will be lower because of splashing, gutter losses, filters, diverter performance and overflow after the container becomes full.
Use the horizontal plan area that drains to the chosen downpipe, not the larger sloping surface area of the tiles. The Met Office location-specific climate averages provide monthly and annual rainfall figures for assessing likely supply.
Annual yield can be misleading when choosing storage. A butt may overflow repeatedly in wet winter weather, then empty quickly during a warm dry spell when garden demand is highest. Start with a manageable container, observe how often it fills and empties, and add capacity only where the household will use it. The separate guide to whether rainwater harvesting is worth it for a UK home covers the financial and practical case for larger systems.
Choose and position the storage container
A good domestic water butt has an opaque body, a secure close-fitting lid, a screened inlet, a low-level tap and a connection for a diverter or overflow. Opaque walls and a shaded position help limit light and warming, while the lid reduces debris, insects, algae and the risk of a child or animal falling into the water.
- Check the filled weight. One litre of water weighs approximately one kilogram, so a 250 litre container holds about 250 kilograms of water before adding the container itself.
- Prepare a level base. Use a purpose-made stand or a sound, level masonry base on stable ground. Do not rely on an uneven or loose stack that could shift.
- Raise the tap. Leave enough clearance for a watering can or bucket without making the arrangement top-heavy.
- Keep access clear. Allow room to clean the filter, remove the lid, operate the tap and inspect every connection.
- Protect the building. Position the container and overflow so that leakage cannot soak walls, foundations, timberwork or electrical equipment.
Very large above-ground tanks need a base designed for their full load. Underground tanks add excavation, groundwater, access and structural considerations and should be specified and installed by a competent supplier.
How to connect a water butt to a downpipe
- Read the kit instructions. Diverter designs and drilling positions vary, so use the measurements supplied with the product.
- Place the butt in its final position. Level it on the prepared base and confirm that the tap, lid and filter remain accessible.
- Mark the connection height. A diverter normally needs to align with the inlet or maximum water level so that excess rain continues down the original pipe when the butt is full.
- Fit the diverter. Isolate the work area and cut or drill only as directed. Do not work from an unsafe ladder position or modify rainwater goods containing hazardous materials.
- Connect the hose. Keep it supported and free from sharp bends that could trap debris or restrict flow.
- Check the overflow path. Confirm that excess water returns to the intended drainage route rather than running against the building or across a path.
- Inspect during rain. Look for leaks, overflowing gutters, a blocked filter and water escaping around the lid or base.
Do not change where the roof drains without understanding the existing arrangement. Sending overflow to the wrong drain, an undersized soakaway or the ground beside the house can create damp or local flooding. Ask a drainage professional if the existing route is unclear.
How to link several water butts
Linked containers are a practical way to add storage without excavating for a large tank. Use a purpose-made connector kit or siphon and keep all containers on stable, level bases.
- High-level connections allow the first container to fill before water passes to the next. A leak in the second container will not normally drain the first below the connector level.
- Low-level connections allow containers to fill and empty together, but every fitting remains under water pressure and a low leak can drain the connected group.
- Each lid and inlet should remain secure and screened, even when only the first container receives water directly from the downpipe.
- The final overflow must cope when every container is full. Extra storage does not remove the need for a safe discharge route.
Do not add tanks simply because there is room. Stored water stays more useful when it is turned over regularly, and every extra connector creates another possible leak. Use experience from one wet and one dry period to judge whether more capacity would actually be used.
Keep stored rainwater clean and usable
Stored rainwater is not sterile. It may contain dust, animal droppings, plant material, insects and micro-organisms. The aim of simple maintenance is to limit contamination and stagnation, not to turn a water butt into a drinking-water system.
- Clear gutters, leaf screens and diverter filters whenever debris accumulates, particularly during autumn leaf fall.
- Keep the lid closed and check that inlet screens, seals, hoses and overflow fittings remain secure.
- Use the water regularly rather than leaving a warm container untouched for long periods.
- Disconnect and drain hoses after use. If spraying water, use a coarse setting rather than producing a fine mist that could be inhaled.
- If the water smells unpleasant, empty and clean the container instead of adding garden chemicals or improvised disinfectants.
- Clean an ordinary water butt annually, or sooner if sludge, odour or contamination becomes apparent. The Royal Horticultural Society’s stored-water guidance recommends warm water and a little washing-up liquid.
- Follow the manufacturer’s instructions for pumps, cartridge filters, frost protection and any treatment equipment.
For a larger system, a first-flush device can divert the initial roof runoff that may carry more contamination after a dry period. Screens, filters and first-flush devices reduce debris but do not by themselves make the water safe to drink. The Drinking Water Inspectorate’s protection guidance explains the importance of clean gutters, screened storage, tank inspection and properly maintained treatment where rainwater forms part of a private supply.
Use stored rainwater safely
| Use | Suitable? | Conditions |
|---|---|---|
| Watering established garden plants | Generally yes | Keep the system clean and apply with a watering can or coarse spray |
| Watering seedlings | Best avoided | Stored water may contain plant pathogens associated with damping-off |
| Cleaning garden tools | Generally yes | Wash hands afterwards and avoid creating a fine aerosol |
| Flushing a toilet by bucket | Useful during some outages | Follow local advice and use care when lifting and pouring |
| Toilet or washing-machine plumbing | Specialist system only | Requires correct design, separation, labelling and applicable approvals |
| Drinking, cooking or brushing teeth | No | Ordinary collected rainwater is non-potable |
| Preparing food or infant formula | No | Use a safe drinking-water supply |
| Paddling or swimming pools | No | Untreated stored rainwater can contain harmful micro-organisms |
The UK Government’s water-outage guidance says water-butt water can be used for plants or toilet flushing but should not be drunk. Rainwater storage therefore complements rather than replaces a safe reserve. See how much emergency drinking water to store and what to do during a water supply outage.
Prepare the system for winter and severe weather
Check the container and pipework before freezing weather. Follow the manufacturer’s instructions on draining taps, pumps, filters and exposed hoses. Water expands when it freezes, so a rigid full fitting can split even if the main container appears undamaged.
After a storm, inspect the roof, gutter, diverter, lid, base and overflow before using the water. Disconnect collection if the roof has been damaged, contaminated by smoke or chemicals, or covered by floodwater. Do not assume that filtering or boiling will remove chemical contamination.
When a simple water butt is not enough
A system supplying toilets, a washing machine or pressurised outside taps is not simply a larger water butt. It normally includes dedicated pipework, filters, a pump, controls, storage overflow and sometimes mains top-up. It should be designed to the current edition of BS EN 16941-1 and installed by someone competent in alternative water systems.
Rainwater pipework must remain separate from drinking-water pipework. Water Regs UK explains that rainwater must never be connected directly to the mains supply. Any mains top-up needs compliant physical separation, and non-potable pipes and outlets require clear identification.
Contact the water supplier before work begins to establish notification and approval requirements in the relevant UK nation. WaterSafe lists rainwater harvesting among the systems that may require notification. Underground tanks or altered drainage may also involve building, planning or environmental requirements specific to the property.
Rainwater collection checklist
- Choose a reasonably clean roof and accessible downpipe.
- Check that the roof material and treatments suit the intended use.
- Use an opaque, covered container with a screened inlet and secure lid.
- Install it on a firm, level base that can carry its full weight.
- Match the diverter to the downpipe and follow its fitting instructions.
- Provide an overflow route that will not cause damp or flooding.
- Keep gutters, screens and filters clear.
- Use the stored water regularly and clean the butt annually.
- Treat ordinary stored rainwater as non-potable.
- Use a competent specialist for any system connected to household plumbing.
Rainwater collection and storage resources
- The Royal Horticultural Society guide to collecting and storing water covers water-butt installation, cleaning, plant use and precautions around stored water.
- The Met Office climate averages tool provides local monthly and annual rainfall data for estimating collection potential.
- The UK Government water-outage guidance explains appropriate water-butt uses and the need to keep safe drinking water separately.
- The Drinking Water Inspectorate’s supply-protection guidance covers contamination controls, screens, covers, filters and tank maintenance for more advanced systems.
- Water Regs UK guidance on alternative sources explains why rainwater and mains drinking water require physical separation.
- WaterSafe’s notification guide helps householders identify plumbing work that must be discussed with the water supplier.
- The UK Water Reuse Association’s rainwater harvesting overview describes larger domestic systems, typical components and non-potable applications.
- Looking after water in your home provides UK-wide guidance on water-reuse separation, labelling, filters, gutters and uses that require wholesome water.
