How to collect rainwater?
Collecting rainwater is easier than most people think. With basic DIY skills and a few readily available components, you can start harvesting rainwater from your roof in under an hour. This guide covers everything from the simplest water butt installation to advanced rainwater harvesting systems.
Method 1: Installing a Water Butt
The simplest way to collect rainwater is with a water butt connected to a downpipe diverter. Start by choosing a location close to a downpipe where the water butt will sit on a level, firm surface — preferably raised on bricks or a stand to provide gravity pressure for filling watering cans. Select a water butt with a capacity appropriate for your garden size; 200 litres is typically sufficient for a medium garden. Install a downpipe diverter by cutting a section out of your existing downpipe and fitting the diverter according to the manufacturer's instructions. Connect the diverter to your water butt using the supplied hose. Fit a lid to prevent debris, algae growth, and mosquito access. Position an overflow pipe to direct excess water away from your property's foundations. This basic system can be installed in 30–60 minutes and will provide a steady supply of soft, chemical-free water for your garden throughout the growing season.
Method 2: Above-Ground Rainwater Harvesting System
For larger water requirements, an above-ground system with multiple interconnected water butts or a larger tank provides greater storage capacity. Choose a tank made from UV-stabilised polyethylene that is opaque to prevent algae growth. Position the tank as close to the downpipes as possible to minimise pipe runs. Connect multiple downpipes to the same tank using a network of pipes and diverters. Install a submersible pump if you want pressurised water for hose pipe use. Fit a float valve or mains water backup if you need guaranteed supply during dry periods. Consider a basic filtration system to keep the water clean for garden use. An above-ground system can store 1,000 to 5,000 litres and is suitable for larger gardens, allotments, and smallholdings.
Method 3: Underground Rainwater Harvesting System
For whole-house supply, an underground rainwater harvesting system is the gold standard. These systems require excavation for tank installation and should be designed and installed by professionals. The tank — typically 2,000 to 10,000 litres — is buried beneath a driveway or lawn, with only the access cover visible. A submersible pump delivers water under pressure to toilets, washing machines, and outdoor taps. Advanced filtration ensures water quality for indoor non-potable uses. A mains water backup system automatically switches to mains supply if the tank runs dry. An electronic control system monitors water levels, pump operation, and system status. Underground systems are invisible, silent, and capable of meeting 50% or more of a typical household's total water demand.
Essential Tips for Successful Rainwater Collection
Whatever system you choose, following these best practices will ensure success. Always use an opaque tank to prevent algae growth. Install a leaf filter on every downpipe connected to your system. Clean gutters regularly to minimise debris entering the system. Position tanks on a firm, level base capable of supporting the weight of a full tank — water weighs 1kg per litre. In winter, protect above-ground tanks and pipework from frost by insulating exposed pipes and, if necessary, draining the system. Check and clean filters according to the manufacturer's schedule. Use collected rainwater promptly — stored water should not be left stagnant for extended periods. Label pipes clearly if your system connects to internal plumbing to comply with water regulations.
Legal and Regulatory Considerations
As covered elsewhere in this guide, collecting rainwater is entirely legal in the UK. However, if your system connects to internal plumbing, you must comply with the Water Supply (Water Fittings) Regulations 1999 and Building Regulations Part G and Part H. These regulations require backflow prevention devices to protect mains water quality, proper pipe labelling, and professional installation by a competent person. Your local building control authority can provide guidance on compliance requirements for your specific installation.