We all know about redirecting rainwater to a water butt, but it’s not all that difficult to scale up, or even install a larger system to completely replace the mains supply for a dwelling. If you’re going to go this far you need a really large storage tank, filters and a purification system. You don’t normally need planning permission or a license to harvest rainwater (except for in a few western states of the US). An increasing number of jurisdictions around the world offer tax incentives for installing various types of catchment.

Even the driest parts of the UK receive enough annual rainfall to reach the 600mm (24″) thought necessary to make rainwater feasible as a sole supply for a free-standing home. Regardless, in the UK rainwater harvesting systems are usually seen as a way of reducing mains water use rather than replacing it, with rainwater captured by houses being used to irrigate their gardens.

Still rainwater captured in bucket, image

Rainwater quality

Because it is captured before it hits the ground, rainwater has not picked up the mineral and other contaminants that ground and surface water can contain. This means it is very soft, so if you are able to use it for washing and laundry your use of soaps and detergents will be lower. No lime in the water also means there’s no lime building up in your plumbing, and no need for a water softener.

Stored water of any kind deteriorates because of the action of bacteria and algae. Because of this it is important to keep debris out of the rainwater harvesting system, starting with coarse screens at the top of downspouts. For a butt that is used for watering a flower bed this is often enough. For a large tank that is intended to supply a household with all its water needs, a suitable filtration system or settling compartment will be needed to screen water before it enters the tank to stop it from spoiling. For drinking quality water, a purification unit may be necessary (as for a borehole or well supply). Have the water tested at least annually for contamination.

Capturing rainwater

The rainwater capture system – just a technical way of saying ‘roof’ – is the one element you can’t expand easily. However you can, and should, capture rain off as many structures as possible. Sheds, garages, greenhouses and polytunnels are all potential rain harvesters. If you plan to drink it, the material of the roof is important for the water quality. The best roofing material for rainwater catchment is uncoated stainless steel or factory-enameled galvanized steel. Clay or slate tiles support the growth of more algae, bacteria and moss, which can all potentially contaminate water supplies.

At the edge of the roof, rainwater is collected by a gutter or gulley, which is inclined towards a downspout. Gulleys are used where two sloping roofs meet. There is no such thing as a ‘flat roof’ – such roofs are always constructed with a slight incline to allow water to run off. The collection of water at the roof edge is exactly the same. A key point of rainwater harvesting is the need to keep debris out of the system, so there should be a coarse prefilter at the top of each downspout. This doesn’t have to be fancy since it’s just to keep out larger debris such as moss and leaves. A piece of coarse screen or mesh will do fine, and will only need to be cleaned a few times each year.

Storing harvested water

The downspout connects to piping to take the water to a storage vessel. For sheds and outbuildings this usually means using a covered barrel or butt, or several of these linked together using a butt linking kit. If the downspout was previously discharging to a drain or soakaway you can continue to use this to cope with overflow by using a rainwater diverter valve. This automatically diverts the water back to the downspout once the butt is full.

For more substantial above-ground storage, second-hand IBCs (Intermediate Bulk Containers) are an inexpensive option. These hold between 600 and 2000 litres (132 to 400 gallons). They are usually attached to a wooden or plastic pallet and set in a wire cage to make them easy to transport, but bear in mind that they weigh around 60-80kg (130-180lb) when empty. They can also be ugly in a domestic setting, so you may need to consider screening or otherwise hiding them in some way. Make sure they have only been used for holding food grade products, and ask if they have already been washed out before you buy.

For storage serving a house, you may wish to go for a very large underground storage tank. This is easily the most expensive part of a rainwater harvesting system. Make sure that you buy one designed for underground use, otherwise it may be crushed by hydrostatic (ground water) pressure when the water table rises. If this tank is to be the house’s sole source of water you may need to site it where it can be refilled by tanker in an emergency.

Getting stored water to where you want to use it is all about height, or ‘header space’. The more header space you have, the greater the pressure. For butts and above-ground tanks such as IBCs it is enough to have a tap fitted near the bottom, but for underground tanks a submersible pump is needed.

Other water articles

Further information

Rainwater Collection for the Mechanically Challenged by Suzy Banks and Richard Heinichen

UKUK Rainwater Harvesting Association and the Water Regulations Advisory Scheme.

USAAmerican Rainwater Catchment Systems Association

Australia: Rainwater Harvesting Association of Australia

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