Green roofs have been used extensively on the continent for some time and are now becoming more popular in the UK.
Creating a green roof
Mosses and lichens will grow naturally on most roofs, but a green roof is intentionally designed to support vegetation.
The type and extent of green roof you can have depends on the strength of the structure beneath. Before creating a green roof, ensure the proposed site is waterproof and structurally sound. It is important to seek professional advice from a structural engineer or roofing contractor.
There are three types of green roof, determined by the degree of substrate and vegetation they support. The advice here refers to the ‘extensive’ type, which is most applicable to a domestic situation.
Advantages of a green roof
Green roofs benefit wildlife and ease the load on drainage systems by reducing water run-off. They provide additional insulation and prolong the life of the roof by protecting its surface from the elements.
City dwellers also have the added bonus of looking upon surrounding roof tops and seeing greenery.
Small-scale, or extensive, green roofs use a very thin layer of nutrient poor soils, which produces a patchy vegetation cover, susceptible to drought stress. This is beneficial to some specialist plants and insects.
Extensive green roofs are suited to sheds, garages and small extensions. Flat roofs are better suited to greening as pitched roofs require more preparatory work to establish green cover.
Depending on the strength of the structure, living roofs can also be created on small extensions, outhouses, balconies and terraces. Sheds can collapse easily and you should only consider using a comparatively new and well-preserved shed, stood on a good base. The amount of substrate used should be minimal and while unlikely, compression of the shed walls may cause glass to break.
- Ensure the proposed site is suitably waterproofed and structurally sound. Seek professional advice from a structural engineer or roofing contractor first.
- Before placing soils on your roof, cover the area with a series of layers to enhance the waterproofing of the existing surface and protect it from penetration by the roots of any plants.
- Add layers of sheets above this to allow drainage but to allow the spoils to retain. just enough moisture to support plants.
- Add the soils. These should be a mixture of crushed brick and/or limestone chippings mixed with sub-soil.
- Seed or plant the roof. Mosses are appropriate for the lightest roofs, and based on a scale of increased structural stability, sedums and with deeper soils of 5 to 10cm in depth fine grasses can used, along with small bulbs, small wild meadow flowers and alpines. Vetches, trefoils and clovers are attractive to butterflies, bees and other insects.
- Small logs or boulders add additional structure and dimension, as well as provide further habitat for insects, lichens and fungi.
- In nutrient poor soil, plants tend to be smaller, grow slowly and require less cutting. Nutrients accumulate over time and woody tree and shrub seeds become established. With good drainage, this build up can be slowed as the nutrients are leached from the soil through the various layers of matting beneath.
- If you only want sedum or moss to grow on your roof, weed the area a few times a year to prevent other plants colonising.
- Cut wildflower roofs late in the year and leave the clippings over winter to allow birds access to seeds. Remove excess thatch in late winter/early spring.