Rainwater Harvesting provides a sustainable approach to water conservation in the UK, where this natural resource remains largely unregulated. By capturing rainwater from appropriate surfaces, property owners can access a supplemental water source. While this water is not “wholesome” (ie does not meet the regulatory standards for mains water) or “potable” (ie is not safe for drinking), it can serve numerous purposes when collected and managed in accordance with the relevant British Standard. Crucially, systems must ensure complete segregation of harvested rainwater from potable supplies to prevent any cross-contamination.


The British Standard BS EN 16941-1:2018 specifies the requirements for the design, installation and maintenance of rainwater harvesting systems.  Compliance with these standards is mandatory to ensure the safe and effective deployment of rainwater harvesting systems.  The Standard, for example, provides the methodology for calculating the correct size of the system’s water storage tank, which takes into account factors such as local rainfall statistics, size of the collection area and building occupancy.


On this basis, and subject to a system suppliers risk assessment, harvested rainwater that is not treated to make it wholesome poses no undue risk to health if used within homes and commercial buildings for toilet-flushing, clothes washing machines, and for supplying outside taps.  It is also suitable for specialist activities such as irrigation, fire-fighting and some industrial processes.


Although not discussed further below, harvested rainwater can also be put through further filtration processes that will make it safe to use as potable water; this can be useful for off-mains applications where rain is the only available source of water.  Under these circumstances, the water to be used for potable purposes falls under Private Water Supply Regulations.


Full domestic rainwater harvesting systems provide a supply of non-potable water to dwellings to ease pressures on mains supplies, either on the basis of single systems per dwelling, or by using larger systems designed to serve multiple dwellings.

The non-potable water supplied is used to provide the services that do not need potable or wholesome water, such as toilet-flushing, clothes washing machines and garden taps.  To ensure that the mains wholesome water supply and the harvested non-potable supply do not come into contact with each other, each supply has its own separate distribution pipework which must not be interconnected with the mains supply except, where necessary for top-up purposes, via a conformal air-gap.


There are two main types of rainwater harvesting, each with their individual characteristics around the central theme of using harvested rainwater to supply non-potable services in the home.

“Direct Pressure” systems deliver the water when required under pressure from a pump that is usually in the harvested water storage tank to the service required when that service is actuated, such as when a toilet is flushed.  As the service depends upon there being water available in the storage tank at all times, if in long dry spells the level of water in the tank runs low, a limited quantity of mains-water is introduced into the tank via a Type-AA air-gap that ensures harvested rainwater never comes into contact with the mains supply.

“Header Tank” systems operate slightly differently in that the harvested water is fed to services under gravity from a header-tank in the loft.  It is therefore the header tank that is kept supplied via a pump in the harvested rainwater storage tank, or is topped-up from the mains via a Type-AB air-gap when necessary during long dry spells.



These will vary in detail depending on the system design; the information provided below is therefore generic rather than specific.  All systems are likely to include, however:

Drainage Pipework to channel rainwater from the collection surface (ie usually a roof) to a storage tank (usually sited underground), and from the tank’s overflow connection to join the site’s sustainable drainage system.

The Storage Tank with a pre or in-tank filter that removes any solid matter collected from the roof to ensure that the harvested rainwater is aesthetically clear.  The size of the tank depends upon factors such as the area of the collection surface, average rainfall at the location and the likely consumption of non-potable water in the property concerned.  Broadly, this means that the contents of the tank when full equates to approximately 20-days usage, and results in the mains-water savings shown opposite.

A Service Duct that connects the storage tank to the property through which passes an electrical supply to the pump, a delivery-pipe from the pump back into the property and, on direct pressure systems, a means to introduce top-up water back into the tank via a Type-AA air-gap should the need arise.  A “pull-string” is also usually left in the duct to facilitate any subsequent post-installation maintenance that may be required.

An Electrical Pump to deliver water back into the house when demanded by one or more of the non-potable services such as a toilet, clothes-washing machine or an outside tap.  These come in two main types, ie “self-actuating” pumps that sense the demand and switch on/off themselves, and pumps actuated by a separate external controller that senses the demand and switches the pump on/off.

A Management System to control pumps that are not self-actuating, and to provide various other functions such as activating the mains-water top-up when needed by direct-pressure systems, and providing the consumer with information on the status of the system.


More details about the system components and their installation can be found the UKRMA training manual for system installers and maintenance technicians can be downloaded here.


The generic characteristics of domestic RWH systems described for stand-alone systems dedicated to a single property, apply equally when using suitably scaled components and designs to multi-plot applications.

On new housing developments with multiple plots, for example, there is an opportunity to install one or more larger (commercial-scale) systems to serve multiple plots, or a mixture of stand-alone systems on the larger plots and communal systems for smaller plots.

This latter combination is particularly effective in maximizing the use of the available harvested rainwater, achieved by routing any overflows from the stand-alone systems to the storage tank(s) of the communal system(s), thus helping to share the water generated by the larger properties with the properties that have smaller roofs and thus less harvesting potential.


Compared to full systems, systems whose use is restricted to outside uses such as garden irrigation or car-wash applications are simpler in their operation and need fewer components.  Typically, for domestic use such systems require no more than a pre or in-tank filter, an underground  or wall-mounted storage tank, a “smart” self-actuating pump, and or a stand or wall-mounted tap.

Such systems are often connected to a single roof-slope, thus avoiding the complexity of routing several downpipes to the storage tank, which might be sited above or below ground.  These simpler requirements make the retrofitting of irrigation systems a practical and cost-effective option in both domestic and commercial applications.

With an eye to future climate-change impacts, systems designed to supply water only outside the building are likely to become increasingly important to keen gardeners, and commercial operators with a need for secure irrigation water throughout the summer.

The introduction of a tiered water pricing system, rising when consumption rises above core hygiene needs, might also serve to encourage uptake of irrigation systems. Importantly, rainwater collected for re-use in this way is not subject to hosepipe bans or any other restrictions on mains-water usage that may become more regular events in the future.



The storage tanks for single-plot domestic systems broadly vary in size in the range 3000 to 6500-litres depending upon factors such as the size and occupancy of the property concerned.  Larger tanks, however, are required for domestic systems serving multiple plots, or for many commercial applications. These larger systems work on the same principles as those outlined above, but will be bespoke designed to match site-specific requirements.

Otherwise, the most significant differences between domestic and commercial RWH systems, arises through their mains-water saving characteristics, and differences in the ease with which they can be retrofitted.

Only around 50% of the water consumption in dwellings can be non-wholesome, the remainder needing to be wholesome for cooking, dishwashing and personal hygiene purposes.  Bearing in mind the likely need for occasional mains top-up, this constrains the mains-water saving potential of domestic systems homes built to current Building Regulations to around 40%.

In many/most commercial applications, such as office blocks, sports stadia, exhibition centres and leisure facilities etc, the predominant use of water is often that associated with toilet flushing and/or grounds maintenance.  This means any commercial premises that have the potential to harvest all the non-potable water they can use are able to makes mains-water savings closer to 80%.

Alongside this, unlike domestic systems, commercial systems can often be more easily retrofitted, particularly in relation to the installation of the distribution pipework.  This is because in many commercial buildings the existing mains-water distribution pipework is more readily accessible, either being installed in service ducts or visible at roof level in warehouse type buildings.