Green Roofs - A Guide

You'd be surprised what will grow on a green roof. These sunflowers were planted by our local squirrel, and were about a third of the size they'd typically grow to in a garden (but they were still too high for the squirrel when he attempted to harvest the seeds...).
Snowdrops and daffodils are other plants that aren't typically found on green roofs, but we've had success with both. There may not be much soil for them to spread their roots in, but there is just enough to plant some of the smaller varieties of bulbs such as Tete-a-tete.

As with everything we plant, we've tried to make sure there are plants that attract bees. We had a particularly abundant crop of chives during 2010 - and even more so in 2011 (the photo above is from 2009), which buzzed with bumblebees and solitary bees.

There are also some small pink geraniums, which the bees adore. However, those on the roof don't get deadheaded often enough, so go to seed more quickly without producing as many flowers (while the same variety in our raised beds are in flower for at least a month, because they are deadheaded almost every day) - green roofs are not ideal for plants that need a lot of attention, unless you can get up there easily.

If you haven't read it, it is worth reading our Introduction to Living Roofs, for a more on the rationale behind having a green roof, a potted history, and a look at some inspirational green roofs and walls.

Creating a Green Roof

There are many commercial systems you can buy to make a green roof. The simplest for a small, domestic roof would be to buy sedum matting (as shown below), but we created our own for a lot less than most commercial systems such as Enviromat or SIGSS.
The sedum roof above includes a habitat for solitary bees and other insects made out of bamboo. The roof below, at RHS Wisley, uses a much wider array of sedums.

Our shed is brick built, across the whole garden, and we had the builders seal the plywood roof with  bitumen waterproofing (pictured below). The roof must be strong enough to hold the weight of all the soil, plants, rainwater, and anyone gardening up there, so you might find that an existing shed may not be strong enough to build our sort of roof on, but should be strong enough for sedum matting. Note the raised lip at the front, to keep some water, and the soil, on the roof.

On top of the waterproofing, there is a rubber pond liner, to keep it really waterproof, and to stop plant roots from penetrating into the roof. Next comes a layer of fleece - this protects the pond liner but also wicks water from wetter areas to dryer areas. Both were obtained from the pond supplies section of a garden centre.
One of our urban foxes on our green roof shortly after it was first planted. Note the squares of turf at the back and the layer of fleece sticking out from the bottom corner. Photo taken late in the evening (f5 @ 1/5s, 800 ISO), so a little blurry.

The grey fleece quickly became covered with moss

Then we put two or three layers of turf. The bottom layer, and most of the other layers, were placed upside down (as we mainly used it as a good way of getting soil on the roof that wouldn't blow away. We also placed some of the top layer the right way up (mainly at the back/top) of the roof, where we knew it would dry out most and even dry grass would protect the soil more.

We obtained the turf from a garden project where the contractor had over ordered and allowed us to take as much as we wanted for a small payment. He had advertised on a local forum, but there is often turf available on Freecycle or other websites, often from people replacing their existing grass. As you'll be placing most of it upside down, the condition is not important, although turf that has wild flowers in it would be a useful addition to any green roof. As our turf was grown commercially, it had a layer of thin, green mesh to hold it together, which seems to have helped a little. Turf is very heavy, so we cut it into smaller squares for lifting onto the roof. This has meant that even where turf has been turned upside down, some bits of grass at the sides has grown up.

 A wild flower roof seen at the Hampton Court Flower Show 2010

We are still experimenting with plants. Lavender survived for a couple of years but the cold winter of early 2010 was too much for it and it all died. Green roofs are very exposed, none of the plants up there have deep roots so if it gets too cold or too hot and dry - they tend to suffer. This is why we don't worry about weeds. If it can grow and survive it stays.

The sedums are the exception, they have survived no matter what the weather. The only problem is they are pretty boring, which is why we want to experiment with more bulbs, they seem to be surviving. In early November we planted lots more bulbs for the following year.

There are a wide range of suitable plants that can be used on the roof, such as this list at or this at the Green Roof Project.

There are lots of commercial systems available for building green roofs. The GardenArk (above) is by LivingArks with green roof by Oxford Green Roofs, and was shown at the Hampton Court Flower Show 2009, complete with many eco-friendly features such as wool insulation, solar panels and a wind turbine.

The thinner and lighter a green roof is, the cheaper and simpler it tends to be, and the more limited planting opportunities you'll have. Low-profile, eco roofs, or extensive roofs, can be used on a roof with a slope of up to about 30 degrees (or steeper if you fit some method of stopping it slipping off...) - typically 60-200mm thick. Intensive roofs are more like a traditional roof garden, with lots more planting opportunities, but generally need a flat or nearly flat roof and are typically 150-400mm thick (weight can be calculated as about 1kg per metre squared for every 1mm thickness - you can buy lightweight soil to reduce the weight). Our's is best described as a semi-intensive green roof; although the soil is fairly thin, what we grow in it is more diverse than a typical extensive roof (which means it needs some maintenance and occasional watering).

There are also numerous modular living wall systems you can buy - but watering is more of an issue than on the roof (this system, above and below, was at the Hampton Court Flower Show 2010).

You can buy complete green roof buildings from the likes of Green Roof Shelters, which specialises in using recycled shipping containers and mainly recycled materials, and can build bike sheds for schools or provide site storage for an eco build - one of the people behind it is the green roof expert, Dusty Gedge.

A useful book is Planting Green Roofs and Living Walls by Nigel Dunnett (whose projects included a green roof bus shelter in Sheffield - a great idea but it was only a one week demonstration) and Noel Kingsbury (available at Amazon and other good book sellers). Nigel designed our favourite garden at Chelsea 2011, the RBC New Wild Garden, which had a wonderful green roof on a garden office made from an old shipping container, along with lots of other ideas for helping wildlife and reducing your environmental impact.

Potentially useful links (besides our Introduction to Living Roofs):

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