Lofty ambitions: why green roofs are the future of urban gardening

Whether you’re familiar with the concept or not, green roofs are becoming increasingly common, as people realize the advantages of having a garden on top of, not next to, their houses. For the uninitiated, a green roof is most simply described as being a living layer of grasses, shrugs, and mosses, planted over a waterproof membrane on your roof. A fascinating combination of engineering and ecology, green roofs can be tailored to suit almost any building, and can be installed on new buildings or retrofitted to existing ones. A green roof provides a mini-habitat for birds, bugs and butterflies- you’ll have your own little ecosystem on your roof.

February 28, 2012, The Ecologist

Run out of space in your garden? Look to higher ground and you’ll find an eco-friendly solution

You may have seen them installed on adventurous new builds on Grand Designs, or caught a glimpse of one in Canary Wharf, Lambeth or Sheffield – or you might have never heard of them. Whether you’re familiar with the concept of not, across the UK, green roofs are becoming increasingly common, as people realise the advantages of having a garden on top of, not next to, their houses. For the uninitiated, a green roof is most simply described as being a living layer of grasses, shrubs, and mosses, planted over a waterproof membrane on your roof. A fascinating combination of engineering and ecology, green roofs can be tailored to suit almost any building, and can be installed on new buildings or retrofitted to existing ones. The big question is: why would anyone want one?

Sedum and plantation expert Jim Blundell lists several potential reasons. ‘Firstly, a green roof provides a mini-habitat for birds, bugs, and butterflies – you’ll have your own little ecosystem on your roof. Also in urban areas, you often don’t have space for plants or trees, so a green roof is a good opportunity to get some green space back into the concrete jungle.’ What’s more, green roofs dramatically reduce storm-water run off, improve air quality, lessen the impact of the urban heat island effect, increase biodiversity, and increase a roof’s lifespan. In short, they’re utterly brilliant for the planet. Dusty Gedge, wildlife consultant and green roof expert, extols the aesthetic advantages too. ‘People should view [them] as a sustainable measure, but also as a real life enhancement. The pleasure I get from it is huge.’

The most important practical advantage of a green roof, however, is its energy saving potential. While green roofs do provide some degree of insulation to a building during the colder months, the major advantage lies in their ability to keep buildings cool during summer, as Blundell explains. ‘If you’ve got a concrete or a metal roof, it heats up quickly and transfers that heat to the inside of the building. With a living roof, the plants absorb that heat and reflect some of it back as well, so it keeps the building a lot cooler.’

If the idea of a living roof sounds appealing, there are some practical aspects to consider. The term “green roof” covers a wide variety of installations, as Gedge points out. ‘Buying a green roof is not like buying a table, you can’t just go down to IKEA and pick up a flat pack. You’ve got to do some research first, and find out what type is best for you.’ Broadly speaking, green roofs come in two varieties: “extensive” and “intensive”. Extensive roofs consist of a thin stratum of soil and rock wool, supporting small, hardy sedum plants and mosses, and thus are lightweight and relatively low-cost. Blundell explains that, unless there’s a long drought, extensive roofs don’t require much in the way of TLC. ‘It is a living area so it will require a haircut maybe once a year. But sedum can survive with no water for up to three months; it’s a tough little plant and maintenance is minimal.’

“Intensive” roofs, on the other hand, feature a thicker layer of soil that can host a much wider variety of flora, including lawn grasses, large plants and even small trees. The thicker substrate, however, has a higher weight and cost, and requires more maintenance. The depth of a green roof can be varied according to taste and requirements, and green roofs are flexible enough to be adapted to many different situations.  There are, of course, some places where a green roof would not be suitable, as Blundell explains. ‘The extra weight isn’t normally an issue, although on a traditional steep-pitched roof, a green roof might end up sliding off. 11 degrees is about the maximum angle. I’d always get a structural engineer out first to have a look at your property, give you ideas and give you the thumbs up. You don’t want your roof collapsing, obviously.’

So what are the downsides? Gedge warns that some basic quick-fix green roof products on the market are not up to scratch. ‘People should try to think of it as a living thing. Don’t just buy a flat-pack green roof in a box, they’re too light and tend to dry out in hot weather. You need a minimum of 120kg in saturated weight, or you’re going to be very sad whenever there’s a drought.’ Following the Green Roof Building Code, says Sorrill, will ensure a high-quality, durable installation. ‘People do worry about leaking and roots penetrating your roof, but if you install it properly and waterproof the substrate, that’s really just not an issue.’| Blundell agrees. ‘In my view, there are no downsides! If your structure can support a green roof, it really is the way to go. I’d much rather have a green roof providing its own little habitat and doing its bit for the environment than a slab of felt or concrete.’ In fact, the only downside to a green roof might be the price. ‘It is more expensive,’ Gedge admits. ‘But you simply can’t price the value of having a green visual space which you can enjoy. Every day, my life is a joy because of my green roof – it’s the best thing I’ve ever done.’