With green roofs, timing really can be a matter of life and death for the major building component. Getting this wrong is the most common pitfall for green roof installations, especially when the contractor is new to the process. It is crucial that the roof is ready with necessary access for the installation team when the green roof is delivered.
When installing a green roof you need a lot of planning and experience is invaluable at assessing how the foibles of the site and build conditions might affect the installation programme. Certainly planning was the main challenge for foreman, Aaron Scanbrett of GR Regan Roofing; installer of an extensive sedum roof at a recently completed café pavilion on a large country estate in East Sussex. For this 422m2 application the green roof took a week to install on the single storey building plus a further two weeks of watering.
‘I’d never laid a green roof before but found the system really straightforward to use after the green roof supplier had showed us what to do,’ says Scanbrett. ‘The only tricky bit was the timing.’
The multi-layer green roof system was installed over FDT’s Rhenofol single ply waterproof membrane. The system consists of a moisture retention layer overlapped by 100mm followed by a drainage and reservoir board and a filter layer. The growing medium was laid above this and raked before the sedum blanket was rolled on and watered.
Finally, pond-washed pebbles were laid around the perimeter of the roof and around the gutter to form a firebreak. Guy Regan of GR Regan Roofing says getting these and the other green roof materials up on to the roof was particularly labour-intensive, but the end result will be worth it. ‘Green roofs are one of the best roofing systems as they’ll last indefinitely – as long as the waterproofing is laid properly – because the roof isn’t exposed to the elements,’ he says.
Timing isn’t the only watchpoint, architects should avoid green roof systems that use just drainage boards rather than drainage and reservoir boards. The latter reduces the chance of the plants failing. A drainage and reservoir board will allow excess water off but also holds a chamber of water to evaporate back through he says. Architects and specifiers need to be vigilant and should only specify the best components.
‘The GRO and FLL Guidelines are in place to help specifiers quiz the performance data of the green roof system.
Reproduced by kind permission of RIBA Journal
