Answer: First of all, you should verify that the roof is strong enough to hold the excess weight of pots and wet soil as they are incredibly heavy. Assuming that it is safe, the roof will reflect heat and intensify it so you will need to look at plants that are extremely heat tolerant. Herbs would probably be a good bet. Some easy ones to try would be thyme (there are many different varieties of thyme), sage, lavender and fennel. If you can provide a bit of shade you might also try mint, parsley, chives, basil and dill. While these last do well in full sun in the garden, I am afraid it may be too hot for them on a sun baked roof. You will also have to pay attention to watering, even witht he more Mediterranean types of herbs. Use larger pots and consider using a water holding polymer to try to keep them from drying out too quickly. Also, if it is windy up there you will need to devise some sort of windbreak. For more information about growing all kinds of plants in containers -- the variety of what you can grow is just amazing -- you might want to look at a book or two on container gardening. One I particularly like is "Container Gardening for Dummies" by Bill Marken, ISBN 0764550578, Dummy Press.
Have fun with your garden!
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