Should I install a rainwater harvesting system?

There are lots of different considerations to bear in mind when you’re considering whether or not to invest in a rainwater harvesting system. This FAQ from Owls Hall Environmental will help you to answer some of the most common questions. If you want to understand more about how rainwater harvesting works as a process, read this guide first.

How clean is the water from a rain harvester?

If the water is taken only from a normal roof (i.e. not a green roof), and the system has adequate filtration, then the water is very clean. However it is not drinkable or bathing quality, this means you cannot use the water for: dishwashers, showering/bathing and drinking without significant further treatment like UV treatment which kills bacteria. It is unlikely you will be able to get enough water from your roof to replace all your water needs; therefore, it is generally not worth trying to clean the water for drinking purposes. Sticking to the common non-potable water usages is the most effective use of rainwater. These are as follows:

  • Toilet flushing
  • Clothes washing
  • Garden watering
  • Car washing.

Using water for the above will save you 40-50% of your water consumption and doesn’t require complicated filters to do so (you do however require good filtration).

Can surface water go into a rainwater harvesting system?

No, not without significant further treatment.

Surface water is water from the ground (driveways, paths and patios). This should be kept out of the rainwater harvesting system which should only collect water from the roof. Surface water can contain oil, mud, animal waste and other contaminants which, once inside the rainwater tank, will be hard to remove. Filters within rainwater systems are designed to remove leaves and therefore are perfectly suited to taking water from the roof.

Can I install a rainwater harvesting system in an existing house?

It’s possible to install a rainwater harvesting system in an existing house. However, if you want to use the water within the house for toilet flushing and clothes washing, a new dedicated plumbing circuit would have to be installed. This is only feasible if you are doing a complete renovation of an existing property. If you are not renovating, then a garden watering system will likely be the most suitable system to install. A garden rainwater harvesting system can easily be installed, you can opt for a simple system that has the taps in the lid of the tank or, if you have the ability to lay pipework and put taps on the wall, you can send the water to multiple locations (a tap by the driveway for car watering for example).

Feel free to contact us to discuss your plans, we will be happy to give you impartial advice on what system is best for your needs.

Can I harvest water from a green roof?

It is possible to harvest water from a green roof. However, the water quality can be too low in quality to use for anything other than watering plants. There is also a possibility of it developing a smell and therefore we would advise against it. Furthermore, a green roof will soak up a lot of rainfall, which would normally be sent to the storage tank. This makes systems ineffective. If you have a portion of your roof as a green roof, we would advise you to consider bypassing that portion of the roof, only collecting from the normal tiled roof.

Can rainwater be collected from any roof surface?

No, normal roof tiles are perfectly fine, and whilst one could collect water from a green roof, I would urge caution against this because water collected can become coloured and potentially smell. Also because the green roof is designed to absorb a lot of water, you will not be able to collect much from it. If you are planning on installing a rainwater harvesting system and have a green roof, consider omitting the green roof and only using water from other roof areas.

Surface water should also not be put into the rainwater harvesting system as contaminants will enter the system over time causing problems with water quality.

Can I have an above ground rainwater harvesting system?

Yes, although if you are planning to use water for flushing toilets and washing clothes, you will get better quality water if the water is stored underground. If you do plan on having an above ground tank for your rain harvesting system, investing in good filtration will help to ensure no leaves get into the tank which will keep water quality high.


For more information, explore Owls Hall’s full range of rainwater harvesting systems, including options suitable for use in your home and garden.

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Last Update: December 5, 2019  

March 8, 2018   1516    Rainwater Harvesting FAQ  
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