I used 5/8's inch plywood. But I glassed it on the bottom as well as the top. Then I added some drainage and ditched the foam that was under the floor. The first subfloor lasted since 79 and it wasn't even glassed all the way across. This floor is glassed all the way across and underneath, it should out last me.
~Brian
http://i116.photobucket.com/albums/o...8679/001-5.jpg