Well, I'm no expert at all, and have never lived in one, but I've known a few huts with turf roofs, if thats the sort of thing you're thinking of - along these lines. I guess the main advantage is insulation and appearance. The number one disadvantage I would say is weight, they are bloody heavy, so if you're building something to take a turf roof it means you need to engineer it much more strongly than you otherwise would. Combine that with a hippy disinclination for triangulation and structural bracing (which I'm sure you wouldn't share), and the results can be a little alarming. You also end up with a big bald patch in your field where you dug up a load of turf I don't know if sedum roofs are lighter weight? The ones I've known have had rubber pondliner as the waterproof membrane, and it is prone to get punctured & leak.