If you’re thinking of greening a roof, you can be reassured that it’s easier than you may think. We managed to install one in a couple of hours for a very happy customer last week. But before we got to that stage there are a few steps you need to think about:
1) Check that the structure is strong enough to support the roof. It’s not just the materials you’ll be putting up there – the living material and substrate may seem light enough – it’s actually about the water they will absorb when it rains. Simple lightweight green roofs weigh between 60-150 kg/m2 (13.0-30.0 lb/sq.ft) and so the shed (if that’s what you’re putting the green roof on) needs to tolerate this load plus water saturation.
2) Make sure that there is an angle to the roof. Even if it’s a pretty flat roof, it needs to slope very slightly in order for the water to drain, otherwise you’ll get a brown stagnant roof instead of a green one. If the angle is over 10 degrees you may need to place barriers across the pitch to prevent a landslide. Any roof angle up to 43 degrees can be greened.
3) Make sure it’s fully waterproofed. We made sure ours was lined with a heavy duty waterproof liner that can resist rooting, and took care to ensure any cutting was done well away from the roof, so as not to compromise its waterproof nature. Or your structure may start to leak…
4) Think about drainage. As we said earlier, rainwater collecting in the substrate will need to go somewhere eventually. If guttering is too ugly for you, why not use a more imaginitive way for water to drain away – rain chains have been used for centuries in other cultures like Japan and south America, and work really well whilst being much more decorative. More info here. Excess water can be drained into a flower bed, or into a reservoir to be re-used as irrigation.
5) The roof will need a rim to hold the materials in safely and stop them sliding or dropping off.
6) Now you can start to put things on the roof. First put down a membrane which will help to retain some of the water, and which the substrate can sit on without damaging the liner. Next a thick layer of substrate. This is what the plants will root into. For a sedum or wildflower roof you can just use a light substrate that is made of crushed brick – surprisingly the plants need very little nutrients, so this works really well. If you’re growing something else like wild strawberries, you may need something a little more nutritious. This will also be higher maintenance.
7) What are you going to plant on it? There are a range of plants that will do well on a green roof but the two main categories are sedums and wildflowers. Both can be bought very simply in mat form, but wildflowers need a deeper substrate. Simply roll the mats onto the substrate and cut to fit. If your mats have arrived well watered, then there’s no need to do anything else. Simply relax and enjoy your new roof. If they’re dry, then give them a good water to get them settled in and then it can be left to do its thing.
8) If your green roof is large, you may need to consider a 300mm shingle edge as a fire protection.
Good luck and let us know how you get on if you decide to install a green roof – alternatively we can do it for you!