The main problem with growing on roofs is the weight, and there's the possible leakage problem. If you can get past those, I've got to ask, WHY GRASS? Are you going to mow it? Grass is kind of a pain to keep up when it's on the ground. It isn't really useful for much. Why do you want to put it on your roof?

Sue

Where exactly do you live? In my opinion unless you live in a big City with bad air or live in a place were its hot and your garage is overheating I'd say that grass isn't the permaculture answer to your problem, you need to maintain it and pull weeds or spray it with harmful chemicals, or just let it be a safe launching point for invasive plants, and its on a roof so thats one more obsticle to pulling weeds, Could you possibly install an irigation system and grow some moss up there? You will need to get an EDPM membrane to cover your roof and protect it from the moisture, and soil, other than that and the wieght on your roof its just like planting in shallow soil on the ground.

I second the moss... was just thinking of this actually as I slept in a camping shelter three nights ago that has a moss-covered roof. Didn't get as hot, looked nice, and fit in more with the nature around it.