From a green perspective a grass roof has many benefits, including insulation, sound proofing, and roof durability. CAT Publications produce a DIY Tipsheet that gives;
'A detailed DIY guide, with materials list and pros and cons'. At only (GBP) £0.50 it is a bargain to boot!
So, if you've ever wondered how to tackle such a project on an existing building, or are planning a build from scratch, then this tip sheet should be able to answer most of your questions aswell as explain the methods involved.
CAT (Center for Alternative Technology) have many other tip or fact sheets aswell as educational booklets at very reasonable prices for sale on their site, so go and take a look, and see what they have to offer.
How about an alternative to grass for covering a roof? A UK based company does just that for garden buildings and sheds here.