Is there any reason I shouldn't put decking on a flat asphalt roof?

[treacletart]

We have a flat roof outside our big sash landing window and above our kitchen. We already have a window box out there, but I'd really like to make the most of it as we have no garden. Turning it into a proper roof terrace isn't an option due to lack of cash and also planning permission regs. It would certainly look much nicer if we decked it and put some more plants out there. I know we'd have to be careful about weight - (eg.no railway sleepers!) would decking help spread the weight or just promote puddles and terrible damage to the asphalt?