Living on the ceiling
Green roofs are among the biggest developments in urban greening. The movement, popular for the past 40 years among environmentalists in Germany and Scandinavia, is still gaining momentum in the UK, but for the most part it’s going largely unnoticed. The reason for this slow uptake of a simple but brilliant idea is puzzling. It may be sour grapes because we, as a nation potty for anything to do with gardening, didn’t think of it first, or maybe it’s a misguided fear of roots invading our lofts. But a combination of new waterproofing methods and the realisation that it might save us money is causing a buzz.

Commonly planted with sedum (a low-growing succulent), green roofs are largely low-maintenance, need little if any irrigation, offer the building protection and benefit wildlife. They are quite different from roof gardens. The typical roof garden requires greater planting depths for a wider range of plants. That, combined with the extra weight of landscaping materials, calls for more attention to the load-bearing structure and more maintenance.

Installing a green roof is an expensive option initially, but there are significant energy savings, as the insulation helps to keep buildings cooler in summer, reducing air-conditioning costs for offices. And, by shielding the roof from UV rays, it can almost double the life expectancy of the roof itself. In cities, more green roofs could help reduce sweltering summer temperatures. The extra layer of substrate helps delay storm water run-off, in turn reducing the threat of flooding. And it helps create a habitat for birds and insects.

Raising the building’s footprint
It’s no wonder the subject has caught the imagination of architects and garden designers. The aim is quite literally to take the land that a building occupies and stick it on the roof. Obviously there are technicalities to consider, especially on a house where increased load might require the services of a structural engineer, but in many cases green roofs can be installed without modifications. Specifications are needed for drainage and protection for the waterproof membrane but raising a building’s footprint is not nearly as complex as it sounds. Rubber liners are now used on private and commercial roofs with a distinct advantage over felt in that they can be made as one completely sealed covering. They expand and contract without damage, making it perfect for flat roofs notorious for leaks, and a sedum layer protecting it from sunlight can potentially double its life expectancy to 50 years.

Roll on sedum
Commonly known as stonecrop, sedum has naturalised throughout Europe and, often found self-sown on tiled roofs and the tops of walls, is an obvious candidate for thin, free-draining substrates where drought tolerance is tested to the extreme. It can endure almost a month without water and with weeds unable to compete in such conditions its survival is just about guaranteed, apart from with prolonged freezing temperatures on the thinnest substrates.

Most common are the yellow flowering S. acre and white S. album whose green foliage will turn purple-bronze in dry weather. S. rupestre, a taller plant with yellow flowers, provides an interesting contrast where variety is needed. They need little if any maintenance and are commercially grown on synthetic or biodegradable matting sold in rolls by the square metre. On a tight budget you can plant patches that in time will spread.

Useable roof spaces
This approach may ultimately transform the way we treat useable roof spaces. It addresses the middle ground between high-maintenance and low-maintenance systems, which may not have the visual impact required where the roof might be seen or even used. On a smaller scale, green roofs might be considered for flat roof extensions, garages and the garden shed. If a shed can play a role in biodiversity, just think of the implications if these methods are applied to the rooftops of our cities.