The green house effect: Anna Pavord explores the possibility of planting on her shed roof

Does this count as a green roof, I wondered, looking at the corrugated asbestos that tops my hut? The hut is where I work; it's a wooden building with a room big enough to hold all my books, a bathroom, an overflow bedroom and a kitchen. It faces south, and both the south and the north face of the roof are mottled with orange lichen and spotted with firm humps of bright green moss. It's slightly greener on the north face than the south and the richest seams of moss run along the lines where the asbestos panels overlap. But will any green credentials the moss might give me be outweighed by the fact that they are sitting on asbestos? Asbestos is all right though, provided you leave it alone. It's when you try to remove it that it becomes dangerous, releasing its fibres.

Parallel to my hut there's another wooden building, a workshop and store, which has a corrugated tin toof. Nothing grows on that. It's smoother, more slippery, not so porous as the asbestos. It is more difficult for growing things to get a foothold. I'm cheered to learn from Ron Porley and Nick Hodgetts' book Mosses and Liverworts (Collins 25) that corrugated asbestos roofs - they have a special section on them - "provide a highly calcareous habitat, mimicking natural limestone, and may support a variety of species" including my green pincushions, Syntrichia ruralis. Hooray! I have a habitat, not a hazard.

Thatch is good for mosses too, though not so good as it once was. Thatched roofs tend to be replaced wholesale now, not patched up bit by bit, as they used to be. Mosses get chucked out just as they are really beginning to settle in. Fire retardants don't help. Nor does the use of chicken wire on thatch to stop birds burrowing into it. Mosses hate the zinc that gradually leaches out from galvanised wire. Only the orangey-red moss Bryum pallescens can tolerate it. It's the one you sometimes see colonising the ground under motorway crash barriers.

But I don't think that Nigel Dunnett, the Sheffield-based King of the Green Roof, would be impressed by our mosses. His concept of a green roof is a much more complicated construct, with proper load-bearing beams underneath and what the green roofers call "substrates" on top supporting a complex, self-sustaining colony of plants to trap rainwater, provide a habitat for insects and invertebrates and a pleasant view for those fortunate enough to look down on it.

A green roof, though, does not need such serious underpinning as a full-blown roof garden, where you want to grow trees and shrubs and pick your own veg. For that, you need a load bearing of at least 200kg per square metre. Covering the roof of a garden shed with a mat of sedums is a less complicated affair. The loading goes down to 70-80kg per square metre because you only need 5-10cm of substrate under your plants. The sedum mats are sold like strips of carpet, a mixture of low-growing types such as S. album, S. kamtschaticum, the vigorous evergreen S. spurium and S. rupestre (now naturalised in Britain). The substrate won't be plain earth - it's too heavy - but can be made up from recycled materials such as crushed breezeblocks, bricks and tiles, expanded clay granules or lightweight materials such as perlite and vermiculite. Typically, there'll also be compost in the mix, but not usually more than 10-20 per cent of the whole.

The simplest thing to do is to buy the whole concept off the shelf; you can do that through Taylors Garden Buildings, Ashwellthorpe Industrial Estate, Norwich NR16 1ER. They make wooden sheds on site at Norwich, with roofs ready to take mats of sedum, which they can supply and fix. A typical sized shed (8ft x 6ft) costs 387; you'll pay another 277.34 to get the green roof in place. Delivery is extra. A bigger shed (12ft x 10ft) costs 1,217 plus 550 for the roof. For more information call 01508 489260 or visit www.taylorsgarden buildings.co.uk. If you just want the sedum matting, then look for Enviromat at www.enviromat.co.uk or call 01842 828266.

Nigel Dunnett customised his own shed, supporting the green roof on 4in x 4in posts sunk into the ground a few feet from its corners. For extra strength, the posts were braced to the shed structure. He draped a heavy duty pond liner over the sloping roof (it must be root-proof as well as waterproof) and rested a grid framework of wood on top of the liner, to make 10 separate compartments, each the same size. separate council tax for it.