What should I put on my roof-garden, when the roof is not very strong?

Keep in mind that plants with shallow roots require less ground, thus less weight. You could use the long rectagular planting boxes instead of the round ones. The weight in the long boxes is more spread out and use less ground. There are a variety of herbs that are pretty and aromatic (purple basil is a beauty!) You could probably plant any kind of annual, so take your pic! Don't try any perennials though unless you're certain their roots won't grow too big.