As green roofs move into the mainstream, Simon Blackham of SIG Design and Technology describes the secrets of a successful specification
Once something of an exotic novelty, green roofs are heading towards the mainstream as their thermal and acoustic qualities as well as their bio-diversity benefits become more widely understood. green roofs are heading towards the mainstream as their thermal and acoustic qualities as well as their bio-diversity benefits become more widely understood.
The mass of a green roof increases its thermal performance by acting as an extra insulant. It can also help with rainwater attenuation by slowing the flow of water off roofs in the case of very heavy rain, at the same time dampening the sound of a downfall, something especially useful in education buildings. Green roofs also work well in combination with solar pv as their evapo-perspiration helps to stop the panels from exceeding the optimum performance temperature of 25°C. Biodiverse roofs –also known as brown roofs – can bring additional advantages in the planning process.
Along with aesthetic appeal, these factors have helped increase the popularity of green roofs as clients look to do something more interesting than the instant greening offered by sedum. Architects however can be apprehensive about specifying them. But don’t be scared. No two green roofs are the same but specification is straightforward if you follow a few basic rules.
Make the decision early
Green roofs should be included in the original design concept for the building so there can be early engagement with the supplier and manufacturer. This is key to cutting costs. Also, if there is just one supplier for the whole job – from insulation to membrane to green roof – it is covered by a single guarantee which keeps liability simple for the client.
Choose the right roof for your purpose
There are four main types (see right) of roof to suit different aesthetic and biodiversity priorities, which each have their own weight and cost implications. These range from the lightweight Extensive Modular type for an instant green effect through to the far heavier Intensive option. This is suitable for more ambitious planting and the only option that requires regular irrigation and maintenance.
Make sure that whichever option you choose, the roof meets both FLL penetration standards – for rhizome as well as root.
On top of a warm roof construction, a typical green roof build-up has three layers. A drainage/protection layer generally consists of a laminated, composite water reservoir core with a moisture-retentive protective fleece on the underside and a geotextile filter mat on the upper side. Above this is the factory blended growing medium layer with a plant layer on top. Plants can be grown by seed, plug planted or supplied as a pre-grown blanket.
Accommodate the weight
The weight of a green roof varies considerably – from around 64.5 kg/m2 to 200 kg/m2 depending on the type. Once a choice has been made, the structural engineer should speak to the supplier to get the figures for both dry weight (parched) and maximum saturation in order to provide sufficient structural support for the loading in the design. The height of the green roof system will affect the height of the upstand needed. It’s much easier to get this right in the beginning than correct it later.
Don’t forget the pitch
A common mistake is not taking account of the battens needed for a pitched green roof to prevent the green roof layer slipping off. This detailing needs to be thought about along with the roof design.
Get the phasing right
The rest of the roof should be finished before work starts on the green roof, to avoid other trades walking on it and damaging it. Make sure there are walkways for maintenance – sedum in particular doesn’t like being walked on. Once the green roof is on, you’re less likely to have a problem with the actual roof than with a more traditional, exposed roof because the waterproofing membrane is tucked away and protected, so less likely to suffer surface damage and UV degradation.
Use the best installers
Green roofs should only be installed by qualified contractors. SIG recommends its DATAC (Design and Technology Accredited Contractor) network of expert installers.
Green roof types
Build-up height: 80-90mm
Suitable for smaller projects requiring instant greening. Supplied with a pre-grown sedum layer fully established in shallow trays that clip together. The growing medium supports hardy succulents, herbs and grasses with limited growth that are stress-tolerant and require no maintenance.
Build-up height: 70-120mm
More economic for larger areas than the modular type. These are supplied in separate elements such as drainage layer, growing medium and plant layer which are installed on site. Planting can be pre-grown, plug-planted or seeded, in which case it can take two months with irrigation to get established.
Build-up height: 150-1500mm
Weight: 200 kg/m2
This roof garden option is far heavier and therefore needs more structural support. It also requires irrigation but is capable of supporting lawns, shrubs and even some small trees. Usually this is specified for structures that are built into a hillside and have a particular desire to blend into the landscape.
Build-up height: 70-200mm
Weight: 90-225 kg/m2
Popular for recreating or enhancing a pre-development habitat in order to encourage a particular plant or wildlife such as bats, bees, butterflies and birds. May include plug-planted sedums and hardy native species according to the habitat being created. Low maintenance.
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