Green roofs on sheds are becoming extremely popular. Although I wasn’t the first to have a go I like to think that featuring the idea on television a couple of years ago had something to do with today’s green shed revolution. I designed a small garden that had to have decent storage in it as part of the brief, but I didn’t want to lose the valuable growing space where the shed stood, or create a view of a dull asphalt roof when looking down from the first and second-floor windows. Building a green roof on the shed was the answer, with the bonus of encouraging wildlife into the plot. If planted with a well-chosen palette that can cope with extremely dry conditions it will not need watering except while establishing itself or during the worst of droughts. Although you need to be a little handy, this is a weekend project that is satisfying and should last for many years.

I built on to a new shed but still put a sheet of 20mm ply covered with heavy-duty plastic sheeting over the existing roof to spread the weight. Then a simple but strong framework must be constructed around the outside using 150mm x 25mm (6in x 1in) gravel boards screwed together. I then subdivided the frame into six more segments (two rows of three), which means that when you fill them with compost they will individually hold in the compost rather than it all slipping down because of the slope on the roof. You could also set two extra posts into the ground at the front edge of the shed to help to hold the weight or, if you don’t think your shed is strong enough, build the entire structure on posts straight into the ground and float it over the roof so that the shed is not bearing any extra weight.

Once you have what is, in effect, a large home-made container on the roof, it’s a case of filling it up with the right compost mix and planting it up. I used plenty of broken-up polystyrene pieces from old packaging as a drainage layer about 25mm thick in the bottom. It weighs next to nothing, doesn’t rot and keeps the bottom of the compost completely free-draining. The compost mix itself also needs to be lightweight and free-draining, so I used 40 per cent peat-free multipurpose compost, 30 per cent sieved garden compost and 30 per cent perlite, which is an organic lightweight medium made from naturally occurring volcanic rock. This improves the planting medium and keeps the soil open. If you don’t have any garden compost yourself then just make it all up with the peat-free bags.

As with all forms of gardening, picking the right plants for the right place is critical for happy, flowering plants. My selection for the green roof boiled down to what would survive in a windy spot with shallow dry soil. Short stocky plants under 15cm (6in) tall were key, as anything too tall would suffer from “wind rock”. I chose mainly grasses, alpines and succulents that would knit together to form a tough but colourful and textural tapestry with a natural look. I placed the grasses towards the back as they would break up the top line of the shed and then interspersed the Sedums, Sempervivums and alpines.

You could buy sedum matting by the roll, which is easy to lay and could survive on shallower soil, or would even do well enough for a year or two straight on to a moisture-retentive mat. The sedum mat is shorter and more even in look and is also lighter weight but it can dry out in hot summers. I wanted to create something more individual with a wider range of plants and in the autumn added spring bulbs, too. All in all this was one of the most satisfying gardening projects I’ve done.

My plant list

Sedum album, Sedum album ‘Coral carpet’, Sedum acre, Saxifragas for early spring colour, Sempervivums, Festuca ‘Elijah Blue’, Uncinia rubra, Anemone pulsatilla. Also small Dianthus such as Dianthus deltoides, Dianthus carthusianorum, Pulsatilla vulgaris ‘alba’ and Armeria ‘bees hybrids’

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