Planted roofs are still a rarity in the UK, unlike in Germany, where planning permission for developments on greenfield sites is granted only if the building’s roof and landscaping can provide greenery equivalent to the square meterage lost under concrete.

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.