The green green roofs of home
Look up to find a route to better sustainability, says Ian Moores
Over recent months, most social housing providers have been grappling with the reality of achieving level 3 of the code for sustainable homes under the requirements of the latest Housing Corporation funding. With hundreds of live schemes seeking to achieve level 3, there are numerous solutions emerging across the country.
As part of that process, many designers have begun to see the benefit of using specific types of roofs to help hit code objectives. Looking to the future, the roof is likely to become a major element in true zero carbon building.
The roof is the obvious place to achieve substantial benefits when designing and specifying any new building because of its position relative to sun and rain. Historically, the roof’s main functions has been to keep out the weather and keep in the heat. Successive changes to building regulations have ratcheted up the thermal performance without considering the role of the roof in reaching overall sustainability goals.
In relation to the code, the roof can retain and collect rainwater, provide usable amenity space and help to increase biodiversity, adding substantially to the total credit score.
The mandatory standard for surface water run-off (Sur 1) requires that peak rate of run-off into watercourses is no greater for the developed site than it was pre-development. This figure would normally be calculated by a drainage engineer, using the criteria set out in the technical guidance.
The implication for most sites is that a sustainable urban drainage system or large-scale underground attenuation will be needed. The cost and scale of an average installation is considerable for developers, and the alternative of using the roof to assist in this task is becoming increasingly attractive. Green roofs decrease the total amount of run-off and slow the rate of run-off from the roof itself. Depending on the specific system, they can retain up to 75 per cent of rain water, releasing it back into the atmosphere through evaporation and transpiration.
Intensive and extensive
Part of the resistance to the widespread use of green roofs has been cost, but weighed against other attenuation methods it has begun to look far more economically viable.
There are various types of green roofs, but in simple terms, varying depths of soil or other growing mediums are used to support differing planting requirements. Intensive systems have deeper construction and can support larger plants, although they may require irrigation and higher levels of maintenance, while extensive systems require almost no maintenance apart from an annual weeding and feeding. Extensive roofs are usually planted with sedums and mosses. The build- up is usually laid on an insulated waterproof membrane that is then supported by a structural substrate.
A cost-effective extensive roof has an additional price of approximately £40 to £60 per square metre, in excess of standard flat roof construction. At present, the National House-Building Council buildmark requires that green roofs be supplied as a complete system by a specialist, to provide a single point of liability for installation, so it is not possible to simply add your own green build-up to standard flat roof construction.
Run-off from green roofs can also be used for rain water harvesting to reduce the potable water requirements and score credits in the indoor water-use section of the code (Wat 1). Intensive roofs can be developed with input from an ecologist to specifically increase biodiversity with potential to gain credits under section Eco 4.
With the appropriate edge protection, roofs can be used to add a private outdoor space, although for many an open space at high levels is still contentious for family housing. Credits can be gained under section Hea 3.
Developers will need to bear in mind that by creating roof areas that need to be maintained, consideration should be given to providing safe access under the construction (design and management) regulations.
Put to work
Because a green roof can be used to achieve credits throughout the code, it can have a real impact and be a very cost-effective way of moving towards the higher scores required for level 3 and above. The use of roofs for mounting renewable sources will also be part of the evolution of the roof to become a multi-functional working platform.
We expect to see more green roofs included in housing specification in future, and increased demand should mean overall cost is likely to fall.
Ian Moores is a principal of ARK Design & Architecture and a code for sustainable homes assessor