Safe and effective installation of rainwater harvesting systems

May 13, 2014
Posted by in Suppliers & Service Providers, Technology, Water Management | Tagged , , , , , , , , , , |

After the wettest winter since records began resulting in extensive flooding across the country, there is little doubt that we need to find better ways of managing water resources.

Climate change has dramatically altered our weather patterns, causing warmer, wetter winters and hotter, drier summers. This in turn has locked the country into drought/flood cycles, the impact of which has been considerably exacerbated by building on flood plains, over-abstraction from watercourses, and population growth. It’s a sobering thought that even after a winter like we’ve just experienced, some areas of the country are still expected to experience water stresses in the coming months.

In essence, as urban areas spread and the earth continues to warm up, the UK needs to have tailored, localised strategies both to reduce the impact of future droughts and to lower the risk of flooding.

The strategy needs to take account not just of removing the threat of floodwater, but also the need to store and reuse rainwater in-situ. In doing so, it will bring financial benefits to users, but also provide a much needed water source in drought.

Kingspan Water's gravity-fed Envireau rainwater harvesting system in a domestic setting

Kingspan Water’s gravity-fed Envireau rainwater harvesting system in a domestic setting


Part G of the Building Regulations now requires that water consumption is limited to 125 litres per person, per day. Capturing and recycling rainwater for non-potable uses such as flushing loos or cleaning clothes, can reduce the amount of water used within a domestic setting by more than 50%, and by up to 85% for a commercial building – which clearly also translates into lower water bills. The latter can also benefit from another government scheme – the Enhanced Capital Allowance; if it has been bought from a supplier within the scheme, the cost of the technology can be written off against tax during the year the investment was made (

However, an interest in rainwater harvesting goes beyond a drive for more sensible use of mains water. Rainwater harvesting systems reduce surface run-off during heavy rainfall and pass the recycled water indirectly via WCs, washing machines etc. to the foul system rather than the storm drains, so that it poses no contribution to flood risk.


The operating principles for UK rainwater harvesting equipment are broadly the same.

Rainwater is collected from roof of the building and filtered before entering either an above or below ground rainwater holding tank.
The water can then either be pumped either directly to WCs and vehicle jet washes, or to a high level header/break tank within the building which will then service the non-potable appliances as per normal.

Systems that serve WCs or washing machines are automatically backed up with mains water at no inconvenience to the homeowner.
Those that comply with British Standard BS 8515 will be designed to meet minimum specifications while also allaying any concerns

about the possibility of cross-contamination of mains drinking water with rainwater.


We would always recommend bringing in technical experts and engineers from the company supplying the system at the very beginning of the project planning to assess your best options. Unfortunately, all too often this happens when the plumbing is already part complete which can mean costly rebuilding work to reconfigure the system.

Having a technical team from the supplier on site also ensures effective co-ordination of the three trades required to install a system correctly: ground-workers, plumbers, and electricians.


Selection of the correct size of storage tank is critical if the property is to comply with both BS8515 and the Code for Sustainable Homes.

The geographical location, annual rainfall, the area and type of roof or collection surface, and intended applications (now and in the future) are all considerations to ensure full compliance.

For domestic applications, the calculations are simple and can be safely estimated by using general storage sizing tables. These tables (contained within the Standard) equate roof area with annual average rainfall depth to determine a suitable tank capacity. Note: to comply with the Code, you must use the Code’s intermediate method of calculation.

Sizing of the tank should also take account of the ability of rainwater harvesting systems to reduce surface-water run-off and limit the risk of flooding, under SUR1 of the Code – our technical team can provide appropriate advice.

Thanks to our guest blogger, Brian Pickworth, for this post.

Brian Pickworth, is Technical Manager at Kingspan Klargester, and Chair of the UK Rainwater Harvesting Association.  To find out more about rainwater harvesting, or to discuss any aspect of installation including BPEC training courses, email