Groundworks & Foundations: Rainwater Management

ABOVE: The Water Efficient House - This diagram shows the ideal water and sewerage management solution for the water-efficient, off-mains house. It includes a BioDisc treatment plant, reed beds to provide secondary filtration and a rainwater harvesting system, all from Klargester. The obvious issue this brings up is the need for a large garden to fit it all in, although for many self-builds, particularly those off mains, space isn’t a big problem.

Rainwater has to be managed and disposed of to foul (sewer) drains. The conventional way to do this is to run drains collecting rainwater from roofs and hardstanding and direct them into a soakaway in the back garden. Surprisingly, it is not uncommon to have longer drain runs for rainwater than for foul drains.

Not all sites are suitable for soakaways and there may be other options, such as running into ditches or into highway storm drains. There is, however, growing interest in slowing down the flow rates in storm drains as they are already working near capacity and can easily get overwhelmed. Increasingly, planners are requiring rainwater to be directed into on-site holding tanks, known as SUDS. Ensure you check your planning conditions for rainwater disposal.

Rainwater Harvesting

Many self-builders choose to harvest rainwater. This is done by filtering it, then collecting it in large tanks and pumping it into the house for use in toilets and washing machines. There are several proprietary systems available — costs run from about £2,000 upwards. You won’t be able to use it to supply potable water — there are strict regulations surrounding this.

"We catch rainwater for use in the garden"
Philip and Margaret Nierop installed a rainwater-capture pond in their new house

Philip and Margaret built a seven-metrelong rectangular pond, into which rainwater is directed from their roof. An L-shaped piece of reinforced glazing was specifically installed for this purpose. “We wanted to include as many ecological elements as possible,” explains Philip. Their pond not only lets them water the large garden for free (apart from installation costs), it also serves as an attractive water feature... Read more about Margaret and Philip's Sustainable Self Build in Devon

Getting Services Connected

Supply of electricity, gas, water, drains and telecoms are laid on by public utility companies. You have to negotiate with them individually and this can be surprisingly challenging and stressful, not to mention expensive.

Contact the utilities early (you don’t even have to own the site) and get quotations for a new supply. Work out where the supplies will run, where they will enter the house and whether you can make savings by undertaking some of the work yourself. Some companies will supply ducting which you can lay and bury so that pipes or cables can be drawn through later.

Also co-ordinate services with the groundworks so that trenching work can be minimised and service penetrations into the house are planned ahead and built in, rather than being drilled through after the superstructure is complete.

ABOVE: The Envireau rainwater harvesting tank being installed; it allows you to replace much mains water with rainwater from your roof. kingspanwater.com

Further reading:

• Groundworks & Foundations: Excavations
• Groundworks & Foundations: Foundations
• Groundworks & Foundations: Ground Floors
• Groundworks & Foundations: Drainage

Return to 'The 12 Steps of Self-build: Groundworks & Foundations'