On Thu, 03 Jun 2010 03:41:18 -0400, VM wrote
Re Like plywood, but weatherproof:

Looking for something to use as a simple unattached 'floor' for on a
fire escape. Needs to be rain-proof of course. A big sheet of 1/8"
aluminum would probably do it, but I have a feeling that there are
cheaper ways. Anyone know of anything?

Exterior grade plywood would work for this.
http://www.lowes.com/SearchCatalogDisplay?storeId=10151&langId=-1&catalogId=10051&N=0&newSearch=true&Ntt=exterior+ plywood&x=0&y=0

--
Work is the curse of the drinking class.