How does a rainwater harvesting system work?

According to the Energy Saving Trust, the average UK roof collects around 85,000 litres of water a year. But instead of letting this water flow away into the drain and into local sewer network, what if it could be repurposed and used for things like flushing toilets, washing clothes and watering the garden?

That’s exactly what a rainwater harvesting system offers, helping to reduce your reliance on mains water, potentially lowering your bills and saving money, all the while helping the environment. Here we go into more detail about how a rainwater harvesting system works.

The amount of water you can store depends on the size of your roof and the catchment zone. This refers to the total area size of the roof (which is similar in size to the ground floor of your home). You will also need to find out the expected annual rainfall per m2 in your area. Visit the Met Office website to check the average rainfall in your region.

Once you have these two figures, you multiply the average rainfall by the area of the roof which will tell you the total amount of water (in m3) that your roof will collect. You should estimate that you will lose around 20% of that number due to overflow and evaporation. To add this into your calculation, times your total collection number by 0.8 to see how much usable water per annum you will have.

For example, if your ground floor measures 8m x 12m (96m2) and your local area was expected to receive an average rainfall of 745mm per annum, the total amount of water would be 71.52m. To reach the final harvest figure you multiply 71.52 by 0.8 to reach 57.2m3 of usable water.

Due to the shape of most roofs, you may only find that one side of the property will allow for straightforward rainwater harvesting. Additional pipework could be connected to collect more, although this can be rather expensive.

In general terms there is very little maintenance required to keep your tank in good condition. Mostly, you will need to ensure you have good filtration in place to prevent leaves and other forms of debris from getting inside the tank via the guttering. The filter should be washed a few times a year and this is a straightforward task.

Some modern rainwater harvesting systems self-clean and do most of the work for you, while others will refuse the first flush of water that arrives during a downpour as this usually contains most of the debris that can get inside a tank.

There are several different variations of rainwater harvesting systems which are split into two types; a direct system that pumps filtered water directly to appliances and gravity fed systems that rely on a header tank. Most harvesting systems include a storage tank (which is connected to the drains and downpipes), pre-tank filter, electric pump and a management system. 

The tank size is based on the type of roof you have, its size and the average local rainfall (using the calculation we described above). Another important factor to consider for the tank size is the amount of non-potable water used by the household. According to the Energy Saving Trust, the average home uses about 349 litres a day and some water providers offer water usage calculators online that can help you get an estimation on the amount of water you use.

Supply levels in the tank are kept in check by the management system, using mains water to top it up when required, while also sending water to the toilet, washing machine or the outside tap (for use in the garden).

If you are using a direct-pumped system, it is likely you will have a float switch inside the storage tank to manage the mains backup. When the water level falls beyond a certain level the float detects this and will allow a limited amount of mains water to refill the system. It won’t completely refill the tank as it will allow enough space to accept rainwater for the next downpour.

You can retrofit a rainwater harvesting system onto most buildings, although installation is made easier when added to an extension or a new build property, as the piping is much more straightforward to fit.

Some houses that retrofit choose to install an underground tank, which can be larger or of a slim profile. Keeping the tank in a cool and dark space maintains the water quality and offers protection against potential damage. However, above ground water harvesting tanks that can sit quote comfortably in the garden close by to the roof downpipes and guttering are also commonplace and can easily fit in with your outdoor aesthetic without looking like an eyesore.

In most cases you will not need to seek planning permission to install a rainwater harvesting system, although you should always check with your local authority. You will have to ensure that the tank is installed to building regulation standard, making sure that the tank site and piping are suitably secure and safe. The backup system must also meet water regulations to ensure contamination is avoided.

Climate change is increasing the number of extreme weather events that occur in the UK, making traditional warm and cold seasons more unstable and unpredictable. Harvesting rainwater is an eco-conscious move which can not only save you money (important to note as energy bill prices continue to rise) but by reducing stormwater run-off you are always helping the environment.

While discussions about a potential water crisis have yet to really make a mark in mainstream arenas, the UK government are gravely concerned about our current infrastructure and the effects it could have on the population in decades to come. By investing in a rainwater harvesting system, you can reduce the amount of ground water used by your home and do your bit to make the world a little greener.