I’ve heard a lot about green roofs. What are they, and how do I go about planting one?

MK, Somerset

A green roof is a roof covered with any kind of growing plants, from a layer of grass or hardy plants such as sedum to a roof garden with trees and flowering shrubs. Once rare, they are now becoming mainstream, as they are both low-maintenance and ecofriendly. They also encourage wildlife diversity, and the extra insulation they provide should cut heating bills.

The most common type is an “extensive” green roof. It consists of a waterproof underlay; a drainage layer of grit, stone or aggregate; a filter, which allows water to pass through; a thin layer of soil; and a plant layer of sedum mats or wild flowers. The other style is an “intensive” roof: a flat roof garden with plants in a deep soil layer.

You can’t just put a green roof on any building. You need planning approval and a structural survey to make sure your home can take the extra weight, and the slope of the roof has to be gentler than 35 degrees. The guttering can collect excess rainwater for use in the garden.

An installed extensive turf roof costs £50-£100 per square metre; an intensive roof about £135 per square metre. Suppliers include Bauder (01473 257671, www.bauder.co.uk), Blackdown Horticultural Consultants (01460 234582, www.greenroof.co.uk) and Urban Roof Gardens (0800 652 8848, www.urbanroofgardens.co.uk).

Catherine Monk is editor of Build It magazine, www.self-build.co.uk

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