The December 2017 issue of Modern Railways included a piece on the progress being made with the new buildings for Crossrail.
The 118-kilometre … 73 miles in old money … railway line under development in England, runs through parts of London and the home counties of Berkshire, Buckinghamshire and Essex. The central section and a large portion of the line, between Paddington in central London and Abbey Wood in the south-east, are due to open in December 2018; when it will be named the Elizabeth line in honour of Queen Elizabeth II. Referring specifically to the new station under construction at Whitechapel, the author writes, “The main structural steel skeleton of the new concourse, which sweeps up and over the existing Overground and Underground tracks, is in place. Work continues to fit glazed panels and lay the environmentally-friendly green sedum roof by early 2018.”
Now when I think of roofing materials, I think of thatch, shingles, slates, stone slabs, tiles, asphalt rolls, polyester or glass. I do not think of sedum. Yet sedum, I discover, is the lightest form of green living roof. It can be used as a covering for any flat roof and is, apparently, simplicity itself to install. One simply ensures that the roof is protected by an assuredly waterproof membrane, plants the sedum and sits back to enjoy the benefits with some of the other life forms with which we share our environment.
Sedum is a large genus of flowering plants in the family Crassulaceae, members of which are commonly known as stonecrops. The varieties which are suitable for growing as roofs are low-growing creeping herbs with water-storing leaves. They thrive in many parts of the world and are perfectly happy in the United Kingdom. I learn, from Wikipedia, that Ford’s Dearborn Truck Plant in the American state of Michigan has a living, sedum roof covering 42,000 m2 … that’s 450,000 square feet in old money; that the Rolls-Royce Cars plant in Goodwood, England, has a 22,500 m2 roof complex covered in sedum … that’s 242,000 square feet; and that Nintendo of America’s roof is covered in some 7,000 m2 … 75,000 square feet …of sedum. So the use of sedum as a roof covering is well established in industrial and commercial buildings.
Sedum absorbs rainwater like a sponge, insulating the areas beneath to help keep them cool in the summer and help keep them warm in the winter. The cooling effect is achieved because the evaporation of the water held by the sedum in hot weather draws warmth from the surrounding area; and the opposite effect occurs during cold weather, when the presence of water in the sedum helps to slow down the escape of warmth from the areas protected by the roof. Sedum also prevents the sun’s ultra-violet rays from damaging the underlying membrane, reducing maintenance costs. These benefits are much more obvious on large commercial buildings and are of lesser significance to domestic properties. But there are householders who have installed sedum roofs and are reported to be very impressed by them. The cynic will say, “Well, they would be, wouldn’t they? We all tend to report favourably on the benefits of something which we believe in.” But I’ll accept that Ford, Rolls-Royce and Nintendo … not to mention Transport for London, the sponsors of Crossrail … are convinced that, for large buildings at least, sedum roofs bring cost benefits.
There’s also some environmental benefit, for a living roof will attract and help to sustain insect life, which will, in turn, help to feed insect-eating birds; and the air-purifying benefits of growing plants are universally acknowledged. A living roof? I like the idea.