Hello Allyson,
Yes, I have seen Clematis scrambling about horizontally rather than climbing vertically so it could be an option for your flat roof. I do not think that the lack of opportunity to climb would effect the vigour.
You could also consider the following to grow in pots and trail about on the flat roof:
You could also consider Nasturtiums (Tropaeolum) although these are annual and would need replacing each year. But remember you will be able to use the leaves and flowers in salads.
Hope I have been of some help,