Happy when it rains

Not only can a rainwater harvesting system be a first line of defence against flooding, it can also help you achieve long term sustainability in our unpredictable climate. Jae Lester of RainWater Harvesting explains

In recent years the UK has suffered times with too little or too much rain. A cycle of near-drought and flood conditions.

Sometimes we need to look to the past to find solutions for the future. Rainwater has been collected for domestic use since time immemorial, and can still be used today in our modern, sustainable homes. Although the principle remains the same, obviously the technology has changed.

Water is collected off the roof via standard guttering and downpipes; the water is filtered and stored in an underground tank. When required, it is pumped back into the home to be used for flushing toilets, washing machines and outside use, which greatly reduces the demand on mains supply. There is no need to flush toilets with processed drinking quality water bought from miles away. The other question in our quest for sustainability is – should we use mains water for non-drinking purposes?

So how does a rainwater harvesting system work? Installing a system is suited to new builds or major renovations as there will be a rainwater feed to toilets and other non-potable appliances. Taps, basins, showers, and baths will remain on mains supply.


Tanks can be made with a number of materials but the majority are constructed of high-grade plastic. They are usually buried underground where there is no light and only a small temperature variance to keep the water cool and fresh.

They can also be large (and some would say not great) visual addition to the garden landscape.

Tank sizes are calculated on location, collectable roof area, number of people/toilets, and application for the water. There are a number of tank size calculators available on the web to enable you to find a suitable size.

Most systems are designed to require minimum maintenance and have self- cleaning filters. As the water flows into the tank there is a mesh that allows water to flow into the tank but leaves and debris are directed into a soakaway. The filters are designed for easy access so once or twice a year it is worth removing and flushing with clean water.

Within the tank, there is a pump that activates when a drop in pressure is identified. For example, when a toilet is flushed there will be no water in the cistern so the pressure will be low and water will flow to refill.


Tank sizes are traditionally calculated on a 21-day supply, as it’s assumed it will rain within that timescale. However, should the water run low, the toilets still need to flush. Therefore if the water is going to be used within the home and not just for the garden, it is important to specify a mains back up system in case of severe drought conditions.

There are two main types of systems – direct and gravity fed. In a direct feed system, the pump is directly connected to the appliance and so runs every time a toilet is flushed. If rainwater becomes critically low a small amount of mains water would be introduced into the underground tank. In a gravity-fed system, the water is pumped up into a header tank located in the loft space and appliances are serviced from there. The latter method only requires the pump to be operated when the header tank needs replenishing. This can make running costs as low as 1p per person per day.


The advantage of collecting rainwater is that it is a relatively simple process and it also has a further benefit regarding Sustainable Urban Drainage (SuDS). During heavy rainstorms, water is collected within the tank first preventing localised flooding. New emerging systems allow a volume of the tank to be used for non-potable applications, but have an extra capacity specifically when there is excess rain. This extra volume is calculated specific to the site and meets the criteria for severe rain events. Water is released back into the storm drain or soak away at a controlled rate so reducing the risk of downstream flooding. This can be as low as 0.05 l/sec per plot, and so will not overwhelm the network.


In a new build, there are a number of trades to complete the build, and a small amount of time by some of these will be required to install the rainwater system. Apart from the electrical element however, most of the work can be project managed by any competent person.

A groundworker is required to dig the hole and site the tank under the manufacturer’s instructions. Some tanks are ‘shallow dig’ so only require a depth of 1-1.5 metres, making it easy to install in areas where there is a relatively high water table and ease of calculation of levels. The laying and connection of pipework to and from the house. There must be a ’fall’ from the downpipe so the rainwater flows towards the harvesting tank. It is very important that the piping from the roof to the tank is ‘closed’ and there are no open gulleys. This will help keep the water as clean as possible.

A plumber is required to install the pipework to the appliances and connect both the rainwater and mains supply, and an electrician is needed to connect the pump and sensors to activate the flow of water. Modern systems are designed to make installation relatively simple however, and therefore should not add significant cost to a building project.

Jae Lester is a director at RainWater Harvesting