Retro fit - Thinking it through

Rainwater harvesting can be applied to most buildings, regardless of size and purpose. However, due to certain practical issues, it is a technology that is generally best suited to new-build properties, as the system can be easily designed in at the start of the project. Having said that, it is some times desirable or necessary to incorporate it into an existing building. Retro-fit rainwater harvesting systems are not generally readily available as most suppliers only target the new-build market. In response, we aim to provide advice and assistance for those who wish to collect water from their existing property.

There are certain potential problems with any existing property from the point of view of collecting rainwater, so careful consideration is advised:

  • it is conventional to install an underground storage tank.  To retro fit, this could be very disruptive and expensive on an existing site and may even entail re-routing of some services.
  •  it will probably also necessitate re-laying some of the stormwater drainage. Fitting a tank and filter unit into an existing drainage scheme will involve changing the levels of some of the pipework.
  •  the drainage system supplying a rainwater harvesting scheme should only carry water from roof surfaces. If the existing drainage also takes water from trafficked areas, or worse still, from ‘grey’ water sources such as baths, basins etc. (not uncommon, particularly on older properties), then the drainage will require alterations.
  •  some changes to the rainwater downpipes and / or the internal plumbing will be required.
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to learn about the important points to consider if you are contemplating the retro-fitting of a rainwater harvesting system.
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Above Ground Storage - External

It is therefore easier and often preferable to store the water above ground for retro fit systems. External tanks are usually used, but consider where the most appropriate place is, bearing in mind that the tank must be near to the building and close to at least one of the rainwater downpipes.

Slimline tanks are available where space is limited and two or more can be linked together where larger capacity is required.


The best method of collecting the water in this situation is to use a downpipe filter. The filter simply replaces a small section of downpipe and then filters and diverts the water as it passes through the pipe. For larger commercial properties with 110mm or 160mm pipes, the Vortex filters can be used and fixed to the wall using a special mounting bracket.

Although the tank is external, the pump and other electrical items must be indoors, away from rain and frost. This includes the mains water top-up unit, which must be mounted higher than the top of the tank to allow a gravity flow of water.

The illustrations here show a simple arrangement with a separate pump and mains water ‘top-up’ unit. An alternative is to use the WRAS-approved Sigma unit. This combines the pump and mains water backup together with our Smart Zeta controller in one compact unit.

Retro-fit external
Above Ground Storage - Internal
Retro-fit internal

It is also possible to locate tanks within the building if required, such as inside a cellar or attached garage. However size is generally limited by the width of doorways etc.

Alternatively we can supply sectional tanks which can then be assembled in situ, but this tends to be an expensive option, so generally only suitable for larger commercial projects.

Do call us to discuss your requirements; we can help design the right system for your application, but please read our useful guide first to gain the information we will need.