The UK market for eco-roofs is a fraction of the size of Germany’s, but supplier Erisco-Bauder believes it is catching up fast. In the past decade, the company has designed about 400 roofs; so far this year it has already specified 200.
As Erisco-Bauder systems designer Robert Woolston explains, there are two main types of green roof. To date, most in the UK have been “intensive”, using a natural soil base to plant grass, bushes flowers and trees. A roof of this type requires as much attention as a ground-level garden. However, the “extensive” type used in Walsall has recently grown to more than half of Erisco-Bauder’s output. “Earlier this year, we used this type of roof in the BBC’s Dreamhouse programme, which raised a lot of interest,” says Woolston.
In the extensive roof type, the seeds of a flowering, moisture-retaining alpine plant called sedum are planted in a special layer of moisture-retaining granules. The resulting meadow-like green carpet can simply be left to grow, requiring very little watering and maintenance. The drawback is that it takes 12-18 months for the sedum to establish itself. But for smaller projects, such as a house or extension, impatient architects can opt to cut pre-planted “turf” cultivated by Erisco-Bauder for an instant green roof.
Woolston says clients opting for green roofs can be motivated by aesthetics, eco-friendliness or both. Apart from producing extra oxygen, the main eco-advantage of a green roof is that it counteracts dust and pollution: the relatively high humidity above the planting attracts dust particles and draws them into the substrate.
As for weight, there is a substantial difference between the two roof types. Extensive roof systems weigh 100 kg/m2 and the the natural soil alternative 170 kg/m2.
On cost, Woolston says the slightly higher-than-average installation cost of one of Erisco-Bauder’s systems is countered by the protection that the plants offer the membrane, giving the entire roof system a longer-than-average lifespan.