Bring your shed roof to life!
Planting rooftops could double the amount of green space in urban areas. Adrian Thomas explores why it’s worth the effort
Planting up rooftops could double the amount of green space in our urban areas. Here’s why it’s worth the effort
Have you seen those timelapse satellite shots of the Earth’s surface, showing the tide of concrete sweeping across parts of the planet? As our cities and towns rapidly expand, the spread of grey is shocking, and enough to make us want to put some of the green back.
We can all play our part in this, by filling our gardens with as much plantlife as possible and restricting the area of paving and decking we put down. But that still leaves huge areas of lifeless roof – unless we choose to top our buildings with a thin but life-giving layer of plants.
The benefits are massive: green roofs not only help to redress the loss of habitat to development, but also intercept rainwater and insulate the building from extremes of heat and cold. They can help a building blend into its landscape, trap dust and pollution particles, and reduce the overheating of our urban areas. ➤
In fact, the green roof revolution is under way as we speak, with London playing a leading role. Pioneers such as Dusty Gedge and John Little are helping green up the capital from the top down. Many tower blocks, businesses, retail outlets, schools and universities are getting a living crown, but overall we still lag far behind some of our European neighbours.
That’s where gardeners can come to the rescue. While houses weren’t typically built to take the weight of a green roof, our garages and sheds offer plentiful opportunities. But where do you start? Although making a green roof can seem daunting, with a bit of know-how and some basic DIY skills, it’s much more achievable than most people might think. Here, then, is your essential guide to making a living roof. ✿
1 Look at your roof
A key consideration is whether your chosen roof can hold the weight of all the soil and plants you need to put up there. There’s no simple equation for working out how much weight a roof can hold, but as a rule of thumb, a sturdy garden shed or single-width garage roof needs a soil depth of about 5-15cm (2-6in), which weighs about 60150kg (9-24 stone) per square metre. If you’re unsure, commission a structural engineer to check it out.
Technically, a green roof can be added to any angle of roof, but the steeper the slope, the more challenging it becomes to stop all the soil slipping down. A slope of up to about 15 degrees shouldn’t present too much of a problem, and on slightly steeper roofs it’s possible to create a wooden grid to hold the soil in place.
At the other extreme of slopiness, a flat roof has a different set of challenges because rainwater doesn’t flow off it, so you’ll need to build in more drainage.
2 Build the frame
Build a frame on top of the roof using wooden planks or specialist aluminium edging – like a low raised bed. The planks can be joined with screws, but using metal brackets at the corners makes the frame more robust.
Unlike a raised bed, your planks must have plenty of drainage holes at roof level to allow excess rainwater to drain out into the gutters. Drill 1cm (½in) holes every 5cm (2in) along the base of the wooden boards. Then add the following layers inside your frame:
● A waterproof sheet. Place a butyl pond liner over the roofing felt, but make sure it doesn’t curve up the inside of your edging or you’ll create a pond!
● A root-proof sheet. Usually a man-made fleece to stop plant roots penetrating the roof itself. The waterproofing layer may be adequate on smaller roofs.
● Pea gravel (10mm) or drainage membrane. The latter is a plastic sheet with little cup-shaped depressions, each with a small drainage hole. These temporarily store a little water after rain, helping to keep plants hydrated.
● A filter sheet (optional). This stops soil washing down into the drainage layer.
● Gritty soil. A 70:30 mix of crushed bricks or concrete and un-enriched compost.
● Border gravel. Lay along the lower edge, or around the entire roof to prevent drainage holes becoming blocked.
3 Get planting!
There are three main ways to plant a green roof.
● Seeding. This is the cheapest method and is just a matter of scattering your chosen seed mix across the bed at a rate of about 4g per square metre. Sow in spring or autumn and water in carefully in hot and/or dry weather to help the seeds get established.
● Plug planting. Although this costs more, you can arrange the plants as you want them, and they have a better chance of establishing. You need 20-30 plants per square metre (2-3 plants per square foot).
● Pre-planted matting or wildflower turf. This is by far the quickest method, with the plants arriving ready-established on a textile mat or blanket.
You just roll it out onto the roof as you would turf, but this does come at a higher price (about £24/ 1 sqm from online specialists).