If you're thinking about putting a green roof on your garden office, then one of your earliest ports of call should be the excellent Greenroofs.com. Above is a good example of what can be done, the Ancaya Green Roof Garden Shed where you can find plenty of details of its construction (essentially a 112 sq.ft roof with a slope of 58.3%) by Living Roofs. Here's what they say:
"The green roof not only decreases the quantity of rainwater runoff from the garden shed roof, it also provides habitat for insects, birds, and butterflies, is an aesthetically pleasing addition to the urban backyard, and maintains cool temperatures inside the building creating a retreat from the summer heat. "Spotted by Shedworking Scout Bill Kratz.