Planting a Green Roof

12-Oct-2017

Improving the environment for people and wildlife.

Green roofs are a way of improving the environment by covering the roof with a carpet of plants. Also known as living roofs they are now widespread in Germany and Switzerland, and are becoming more popular in Britain, especially in London and Sheffield where local authorities have particularly encouraged them.

A few schools have incorporated green roofs into new buildings, such as Sharrow School in Sheffield which now boasts a Local Nature Reserve on top of its buildings. Others, such as King Alfred School in Golders Green, London have added green roofs to existing structures. Major projects like these need significant input from professionals such as structural engineers, but smaller projects are more feasible on a DIY basis.

Why install a green roof?
 

Green roofs bring a range of benefits and though these will vary with the type of building and make-up of the green roof they can include the following:

  • Protecting the surface of the roof from the weather, especially UV light and frost, and thus

  • prolonging the life of the roofing felt or other waterproof layer

  • Insulating the roof, so inside is cooler in summer and warmer in winter

  • Cooling the surrounding air, through evaporation of moisture from the soil layer and plants

  • Absorbing rainwater and allowing it to drain away slowly, so reducing rapid run off which can lead to flooding

  • Providing sound insulation

  • Absorbing air-borne particles and improving air quality

  • Increasing biodiversity by providing a natural habitat which is likely to be different from the surrounding area and can provide food and shelter for different wildlife

  • Helping buildings in green spaces to blend in with their surroundings

Where could a green roof go?

Flat or gently sloping roofs are most suitable, and it makes sense to start with a small area. Suitable structures may include sheds and garages, porches and covered walkways, tool stores and bin shelters, hen houses and rabbit hutches, hedgehog homes and insect hotels. If you can get involved at the planning stage then a green roof can also make a great addition to an outdoor or temporary classroom.
 
What’s involved?

  1. Make sure the structure is strong enough to take the additional weight of the green roof. Even for a lightweight living roof this can be up to 90 kg (14 st) per square metre (square yard) when fully saturated. An alternative solution is to support the living roof on a wooden frame that overhangs the building and can be supported separately by stout timber posts set in concrete.

  2. Ensure the existing roof covering is in good order and fully waterproof.

  3. Fix edging around the roof to hold everything in place. Leave a gap along the bottom edge of the slope to allow water to run off – ideally into a gutter – or just have a gap at one end and let the water run away down a chain.

  4. Prepare the under layers. On top of the roof place a layer of root-proof material such as pond lining made from butyl rubber. You could use heavy-duty plastic, but would then need a soft layer below, such as old blanket or capillary matting, to protect it from any roughness on the roof surface. On top of this goes a filter sheet that allows water to drain away but retains the growing medium. Next comes a moisture blanket to act as a water reservoir. You can buy specially designed materials from specialist companies, but you can also improvise, for example by using weed-suppressing materials for the filter sheet, and capillary matting for the moisture blanket. Finally you need a drainage layer to stop water collecting in puddles and waterlogging the plants.

  5. Add the growing medium. Garden soil is too heavy, and likely to be full of weed seeds, and potting compost has too many nutrients. Suitable mixtures comprise around 80 percent free-draining, inorganic material such as clay aggregate, crushed brick or limestone chippings mixed with about 20 percent bulky organic material such as leaf mould or well-rotted garden compost. You could also add other materials such as bark in small areas to give some variation.

  6. Add the plants. The simplest type of living roof to install and maintain is one based on a mat of different species of sedum. These small, fleshy plants are very drought tolerant and can survive on a very shallow layer of growing medium. There are several native species including biting stonecrop (Sedum acre) and English stonecrop (S. anglicum). You can raise them from seed, buy a few plants and take lots of cuttings, or buy them already established on a mat that comes rolled up like turf. Using a more diverse choice of plants can make the roof more attractive and encourage a wider range of wildlife, but will need a deeper, heavier layer of growing medium.

  7. Increase diversity by adding other materials such as logs or large stones to give additional habitats for minibeasts. Areas of bare gravel will also attract different species from planted areas.

  8. Once finished, this type of roof needs little maintenance, just checking over say once a term to remove weeds and patch up any areas that have become bare. 

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