Roofs New drainage legislation favours
living roofs on high-density housing Matthew Hoddinott, Cityroofs UK Ltd, explores with wider implications
NO EXTRA LAND-TAKE
Unlike most other SUDS techniques, green roofs do not require any additional land-take, making them ideal for medium and high rise, high-density housing. Of course, there will be urban situations where a complete SUDS management train is not viable due to space constraints on some SUDS techniques such as swales, ponds and wetlands. Here, green roofs can still be used in isolation to reduce, attenuate and improve the quality of water runoff passing into conventional drainage systems.
But there is more to living roofs than just planting, with planning policy and other demands for usable space pointing to both soft and hard landscaping features, while delivering SUDS and amenity benefits as well. For example, while the Greater London
National planning policies - such as PPS25 in England and PAN61 in Scotland - around the UK have promoted and prioritised sustainable drainage systems (SUDS) on new developments for some years. This is still the case, despite the government’s current aim of driving planning towards localism. But the 2010 Flood and Water Management Act goes further with far-reaching changes making SUDS effectively mandatory for all new buildings.
MANDATORY SUDS
The Act is taking effect now the National Standards for SUDS are in place - probably within a few months. Then, construction work cannot start until drainage systems have been approved by local ‘Approving Bodies’ - generally county councils or unitary authorities - and meet the new SUDS national standards. At the same time, rights to connect surface water drainage systems to public sewers will be limited to approved SUDS schemes only. So, SUDS will soon become the mainstream drainage technology.
As a particularly useful SUDS technique, living roofs will play a major role in satisfying these requirements - particularly on medium and high rise housing in more dense, urban environments where land is not available for other SUDS devices. The three cornerstones of SUDS are:
QUANTITY - to minimise and slow down the rate of surface water runoff
QUALITY - to remove pollutants from the water
AMENITY - to improve the local environment for both people and wildlife.
And planted living roofs play a major role in all three. As the head of the SUDS ‘management train’, they provide invaluable rainwater source control retaining anything between 40% and 100% of the rainwater, then delaying runoff for the remainder. Water is held in the substrate and drainage layers with uptake and release by plants through transpiration and photosynthe- sis, or evaporation due to wind and sunshine temperature. The plants also absorb airborne pollutants while the supporting substrate acts as a filter retaining and treating pollutants, so improving the water quality.
Authority (GLA) ‘Policy on Living Roofs and Walls’ requires: “
...major developments to incorporate living roofs and walls where feasible...”. Its Preferred Standard also proposes that : “At least 25 percent of the total roof space in any one development should be accessible to residents and/or workers.”
ACCESSIBLE SPACE
For high-density urban housing, living roofs offer a real opportunity to meet demands for useable external space. These demands are recognised by the Code for Sustainable Homes with a credit for areas where occupants can sit out, as well as by BREEAM for ‘Multi-Residential’ buildings calling for outside space easily accessible to: “all potential users of the building, regardless of age, disability or gender...”. So, in addition to planted green roofs, there is a convincing case to include areas of firm, level, well-drained and non-slip surfaces in line with Building Regulations Part M - but these must have sustainable drainage as well. And they will need safe perimeter enclosure to meet Part K, as well as seating, lighting and other facilities for a complete rooftop streetscape.
This growing complexity of requirements demands a broader, holistic approach to both design and installation of living roofs. Cityroofs has adopted this ‘complete elevated landscapes’ approach and works closely with designers and developers on comprehensive solutions. These include all the elements needed for rooftops, terraces, podium decks, promenades, balconies and living facades. For example, dedicated paving systems designed specifically for rooftops are now available for pedestrian use, play areas and even car parking. They offer an impressive choice of attractive and safe rooftop surfaces using stone, composites, concrete and other materials. In particular, the latest concrete paving systems can be supplied with numerous colours, designs and finishes - including exposed aggregate, polished, shot-blasted and easy-clean - to suit contemporary architecture.
ROOFTOP PAVING
But we also need to manage rainwater runoff from elevated hard landscaping. The latest generation of rooftop paving - whether for pedestrian or vehicle usage - utilises self-locking profiles for stability with integral spacers forming designed-in gaps between paving units. This allows water to pass straight off the paving - making the surface safer to walk on - into the void below where it can be managed. Here, innovative absorbent materials, such as Cityroofs Aquadyne, can be used to store and attenuate rainwater runoff below the paving, before discharge, as part of the SUDS management train.
These absorbent slabs can also be used at both the substrate and growing medium for planted roofs and walls as well. Manufactured entirely from recycled plastic materials using low-energy technology Cityroofs Aquadyne has high permeability and flow rates - the same as gravel and ten times that of sand - and no negative environmental impacts through leaching. It can act as both a water attenuation board and a capillary growing medium for living roofs and walls with no need for soil. Water simply passes through the structure, keeping it moist but never saturated.
www.cityroofs.co.uk
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