Porch/Deck Waterproofing

[andyfnp]

I recently converted a 20' X 24' deck into a covered porch. Roof is gabled with a guttered 16" eave on each side. We redecked with 3/4" Advantek with outdoor carpet or tile covering in mind for later. Once it was decked, the underside of the new porch, which previously served as a carport, was a natural for enclosure as a finished garage. The contractor was positive that the Advantek, now being under the roof, would be completely watertight with careful caulking of the joints and 2-3 coats of latex Dry-Lock. This was all completed, including finishing of the garage underneath. Unfortunately, there were several leaks through the Advantek into the garage underneath. Two more coats of Dry-Lock has not helped. One contractor proposed a rubberized membrane like for a flat roof to go on the porch before carpeting or tiling. Idealy, I've got a watertight masonry product or a rubberized liquid in mind. Anyone have any other suggestions?

Andy