Rosemary or most of the box woods would be a good choice; but be cautious about the weight! of the plants --pots + dirt + water = HEAVY!! for pure decor, I'd opt for artificial -- there are absolutely gorgeous ones available on-line -- not 'cheap', but require no care, and won't compromise the integrity of your roof!
Thanx 4 the prompt response
I agree with landshark, plants in dirt are just plain heavy, if possible go artificial.
First find out what growing zone you live in. That will guide you to the plants you can use.
How to Pick the Best Roof Garden Plants - TerraCast Products