What can the water be used for?

Water harvested from rainfall is classed as ‘non-potable’ or non-drinking water but may still be used for other purposes - flushing the toilet, running the clothes washing machine and watering the garden. Rainwater is soft and limescale-free, helping to prevent appliances scaling up and reducing the amount of detergent needed. 

In the UK, the average household uses around 350 litres of water a day, according to the Energy Saving Trust. Flushing the toilet can account for up to 30 per cent of total water usage. This means about 100 litres of drinking quality water is literally flushed down the toilet every day in the typical home. Rainwater harvesting can help reduce mains-piped water usage, and if you are on a meter, cut household bills.

Recycled rainwater is not suitable for drinking, bathing or food preparation – anything that involves direct human contact. While It can be boiled and treated with UV filtration to make it drinking quality, this is only economically viable where occupants have no other water source.

How much water is saved?

The amount of water saved will depend on the size of your catchment area – usually the roof of a building – and rainfall in your area. Typically, domestic rainwater harvesting will reduce mains-water consumption by up to 50%, rising to 80% for commercial users, such as hotels and offices, says the Rainwater Harvesting Association.  The other environmental benefit is that it reduces storm water run-off and flooding.

How does it work?

Rainwater recycling involves collecting water from a roof or any other surface, such as garden lawn or permeable drive. The rainwater then passes through a filter to remove leaves and any other debris and is stored in a holding tank usually placed either underground, beside a building or in a loft.

The most basic example is the humble water butt. More sophisticated systems can capture, filter and store rainwater which is gravity-fed or pumped directly to appliances, such as a washing machine or toilets.  Under UK plumbing regulations, the harvested rainwater must pass through a separate pipe system from a building’s drinking or bathing water piping system. This adds to the cost of installation.

What happens in a dry spell?

When there is insufficient water in the storage tank, such as during a drought or lengthy dry spell, rainwater harvesting systems automatically switch the supply to the mains-supplied water, so the toilet still flushes.  This is known as the mains back-up.

Is my home suitable for rainwater harvesting?

Most average sized rooftops can be used to collect water with the right gutters and down pipes. You may need to have your guttering and piping altered so that all the water from the roof feeds into the tank. In addition, you will need space in your garden or loft for a collection tank.  Suppliers usually advise below ground tanks. This means the collected rainwater isn’t affected by light and stays very cool – so bacteria can’t grow.

If you don’t want the hassle of digging up your garden, above ground storage tanks are available. If you opt for a tank in the loft, you may need to strengthen support structures. Even a 1,000-litre tank presents a serious weight challenge when full.

Rainwater harvesting is simpler to install in new build homes but can be retrofitted to existing buildings. If you live in a listed property or conservation area, contact your local planning authority for advice.

What size storage do I need?

This will vary with the size of your roof or catchment area and the likely consumption of non-potable water in the building. Rainwater harvesting suppliers often have handy calculators to find your ideal tank size. Options include a ‘shallow dig’ underground tank. These have a large footprint but are only 1-1.5m in depth, making them easier to install.

Is planning permission required?

Planning consent is not usually required as rainwater recycling is covered by ‘permitted development’ rights. However, it’s always a good idea to check with your local planning authority before installing a system. If you are building a new home, a rainwater harvesting system could score points in a planning application as it’s eco-friendly.

Since 2008, all new homes in the UK have had to be rated against the Code for Sustainable Homes. Effective rainwater harvesting reduces main-supplied water consumption, so will result in a higher rating.

How much does it cost?

The simplest rainwater harvesting is the water butt which can hooked up to your drainpipes to collect run-off and save water for the garden. Water butts cost £40 for 227-litre or £60 for a 350-litre from Homebase, for example (prices correct September 2019). More high-tech systems include water pump, filters and mains backup. According to the UK Rainwater Harvesting Association, the cost of an average domestic system is between £2,000 and £3,000, excluding installatio

So, is it cost-effective? Rainwater harvesting can slash your water bill by half. In the UK, the average water and sewerage bill is about £354 in 2019/20. For a system costing £3,000, this means it would take 17 years to break even. So, it only make financial sense if it’s your not planning to move before then. That said, having a system installed is likely to make your home more attractive to potential buyers, especially with climate change experts warning water shortages will become more common in future.

Some water supply companies are offering financial incentives for developers to install the technology in new builds. Unlike Germany, there aren’t currently any grants for UK homeowners wishing to install a water harvesting system. However, the Environment Agency has lots of advice on saving water, such as low-flush toilets, water efficient taps and showers.