It is tempting to think that you can collect excess water from any surface eg roof, patio, roadway etc. However, this is not generally advisable, and this article will explain why.
One of the first principles of rainwater harvesting is that in order to have clean, clear water in the rainwater storage tank, it is vital to collect from a suitable surface, and filter the water adequately. Rainwater filters will remove particles, but cannot clean water that is already discoloured.
1) Roof Collection
Most roof surfaces are suitable for rainwater collection – whether pitched or flat, providing it is clean and allows water to readily flow off. Pitched roofs of tile or slate are ideal; other man-made materials are also very good. However, it is not advisable to collect from a ‘green’ or ‘sedum’ roof if the water is to be used in the house. A ‘green roof’ is a roof with a plant covering – it is designed to retain water for longer, rather than allow it to run off into the gutters. That’s a great idea for stormwater attenuation, but the problems for rainwater harvesting are two-fold:
a) By its nature, less water is collected in the storage tank (the volume can be reduced by up to 50%)
b) The water that is collected will be discoloured by passing through the vegetation. It is not practical to filter out this colour, and therefore there is a risk that the porcelain of the WCs may be stained, and the washing may not be as white as you would like!
If you DO want to collect from a green roof, you could use the water just for garden irrigation – plants don’t mind what colour their drinking water is!
2) Collection From Paved Area e.g. Patios, Driveways
It is also not advisable to collect from hard surfaces such as driveways, paving etc. because of the risk of contaminating the water. It is rarely possible to protect these areas, so there is potential for the water to be polluted with animal faeces, pesticides, or petrochemical contaminants if the area is used by vehicles. These are not desirable and will affect the quality of the water that is collected. Again, it is not generally practical to filter out these contaminants.
3) Collection From Open Gullies, ACO Drains etc.
For similar reasons, it is advisable not to have any open drain points (eg back-inlet gullies) connected to the system. Where downpipes are located, these should connect directly into the below-ground drainage pipes, and not discharge into an open gully. The risk here is that excess garden pesticides, painter’s slops etc may inadvertently be poured into the gully, and thus enter the rainwater tank.
In summary, the ideal surface for collecting rainwater from is generally a pitched roof, made of tiles or slates, or commercially, steel, glass etc. Basically, any smooth, clean surface. Solar panels are fine to collect from! One final point to consider: if the building has overhanging trees, it may be advisable to fit leaf guards to the guttering to reduce the amount of debris collecting in the gutters.
Find out more by chatting to one of our team on 01452 772000, or by sending an email to . More information about rainwater harvesting systems, including design considerations, capacity and cost, can be found in our guide Everything You Need To Know About Choosing A Rainwater Harvesting System. Access a free download today by clicking here.