I've done raised beds on asphalt, so don't worry about it. Excess water will leak out from the bottom edges. You can put landscape fabric in the bottom, but it really isn't necessary on top of soil.

{{gwi:57633}}

Here is a link that might be useful: What's Growing On?

Thanks for the advice...I didn't think it would matter, even if it did not drain at all (as in your case). Water+gravity: I'm sure it'd find a way out. haha

Mine are on heavy clay soil...no drainage problems at all. The excess water just seeps out around the bottom of the wood box.

Happy gardening!