This is the sort of project that is a little bit of a folly. Is it really going to make a big difference having your shed roof covered in plants? Probably not, not on a shed anyway. But, on a larger scale the idea has a lot of merit so I decided it was worth experimenting on a small and manageable scale first.
Green roofs are becoming an important tool in the mitigation of some of the problems produced by the world's larger cities - things like the urban heat island effect from hard surfaces like concrete and bitumen reflecting heat back up into the atmosphere, or harbour pollution from water runoff from roofs. If you take a birds-eye view of a city, all you see are the roofs: if you can turn these into non-reflective surfaces you'll reduce the temperature build-up around cities.
Water runoff, especially after heavy rain, means overloaded drainage systems flow directly into the harbour, taking all sorts of rubbish and pollutants with it. If this run-off can be slowed by plants which will hold and clean the water first, the harbours and waterways will be significantly healthier. Then there's the benefit to inner city wildlife. Clever planting attracts birds and insects to do their various biological jobs.
Grand ideas, then. I might not be saving the planet with my little shed-top garden but I love the idea of "greening" buildings.
However there are a few practical considerations, like weight.
If you're in any doubt whether your shed can take the weight of a green roof you need to consult an engineering professional. Soil is way too heavy to put on most roofs so it needs to be mixed with a lighter planting medium. Bunnings has perlite available which makes a huge reduction in weight. The roof will also need to be irrigated. Big commercial buildings with roof gardens actually capture and store water run-off. On a simpler scale, you could mimic this with a rain barrel and a piece of guttering. Just be aware that if you decide to do something like this to a bought shed, it won't be covered by the manufacturer's warranty.
If you're still keen, good for you. If not, you can always grow a grape instead, which is what I'm doing on my garage.
Cut 6mm thick h4 tanalised plywood to fit over the roof. Then glue and screw 100x20mm h3 tanalised pine framing around the edges.
Cover the roof in thick black polythene. It should be a grade suitable for use as a pond liner (ask at Bunnings). There is no need to glue or screw the polythene to the roof as it will be held in place by the weight of the plywood sections.
Place a 100x20mm baton every 500mm, horizontally across the plywood. This is to help hold the soil in place to stop it sliding off the roof.
Place the two plywood sections together over the polythene on the roof and screw together. Lift this on to the roof. Because of the weight of the soil the plywood will not need any further attachments to stay on the roof.
In a wheelbarrow, mix a small amount of potting mix with perlite (about five parts perlite to one part potting mix) to make the soil light enough to put on to the roof. Then load it on the roof, using a bucket.
Install irrigation pipe and drippers.
Select drought-tolerant plants and plant into the medium. I've gone with mondo grass, planting around 10 clumps per square metre, but other options include rosemary prostrata, thyme, rata vine, prostrate guava and star jasmine. If you use these creepers you can plant as few as four plants per square metre, but the sooner the garden establishes into a dense mat, the better, so don't be too mean. For a more coastal look you could try succulents and alpine grasses.