Are you planning on having zombies fall from the sky? How likely is it that this situation will ever occur? As long as they stay on the ground you wont run into any issues.
Ok, here you go. Screenshot from my game.
<img src="http://2ch.so/gd/src/1345914803529.png" border="0" />
See that platform over the main character? That's the answer to your question.
Also, I've already solve this problem. Let me know if somebody need solution.