Building a green roof

Posted: Friday 18 November 2011
by Kate Bradbury

Since I bought my shed, I've been itching to put a green roof on it, but I've always been put off by how difficult the task looks.

A green roofSince I bought my shed, I've been itching to put a green roof on it, but I’ve always been put off by how difficult the task looks. Luckily, a friend asked if she could do the job for me, for a book she's writing. So last weekend, she turned up with some wood, landscape fabric and pond liner, and transformed the dead space on my shed into a wildlife oasis.

High up above our cities, green roofs provide a refuge for endangered black redstarts, wild bees, butterflies and other insects, giving them a taste of what life was like before we built on the land. Green roofs on our sheds and bird tables don't provide such 'wild' habitats, but they do offer an additional source of pollen and nectar, as well as nesting opportunities.

There are other advantages too. London alone, is losing equivalent to two-and-a-half Hyde Parks of land each year. Not only is this reducing wildlife habitats, but it also puts pressure on sewage systems, as there is less land to absorb rainfall. This causes drains to back up, leading to flooding. If every building had a green roof - be it an office block or garden bin store - much of this water would be absorbed before it even hit the ground.

My shed has a tiny, steep roof, so we needed to make sure any soil that was added wouldn't just slide off. We also had to consider the extra weight the roof would place on the shed. First, the shed was reinforced with sturdy wooden posts, screwed to the inside walls. Then a piece of landscape fabric was laid on the roof, followed by the pond liner, which was placed over the fabric and glued to the sides. A piece of water-retentive matting was then placed on top. A frame was built and laid on the roof, then fixed to the wall behind the shed. We fixed horizontal batons inside the frame to keep the soil in place. Finally we added loam-based compost (made lighter by adding perlite and polystyrene chips), and planted it up with shade-tolerant wildflowers.

The day proved that building a green roof is a little more complicated than making a raised bed, as you need to consider drainage issues and the additional weight put on the shed. But it can be done without buying expensive kits. And, while the plants won't put on much growth before spring, I think the roof looks great. The shed has not buckled under the weight of rain-sodden compost, nor the trampling of pigeons. It won't be visited by black redstarts, but I can't wait to see the first bee, visiting a flower that would otherwise not have been there.

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Gardeners' World Web User 18/11/2011 at 17:16

Looks superb, Kate, and remember, before roof tiles the rain was kept out of our dwellings by sempervivums and sedums growing in the sod roofs, and what glorious colours you can get in these.

Gardeners' World Web User 19/11/2011 at 11:41

I'm sorry to say that's not the roof happymarion! Mine looks a little less polished than that, and the plug plants are a lot smaller. The one above makes a better photo though!

Gardeners' World Web User 19/11/2011 at 11:50

Get yourself a little bit of tufa, Kate and put alpines in it and you will soon have a lovely. light feature on your roof. Alpines grow very slowly in tufa so low maintenance too. Wildlife friendly can be so beautiful too.

Gardeners' World Web User 19/11/2011 at 11:58

That's a lovely idea, happymarion, but I've already got the roof built now. The wildflowers will put on plenty of growth in spring. Kate

Gardeners' World Web User 19/11/2011 at 15:18

I would love to do something like that on my summerhouse roof. It is quite large though 5m x 4m but theres only a slight run off as its nearly a flat roof. What happens to the felt?

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