Your article lists Marine Grade plywood as an unacceptable grade for direct installation of tile. It is clear from the article that you think this is due to a chemical treatment or coating to make it into “Marine Grade.” This is untrue. Marine Grade plywood is described in some detail in “PS 1-07, APA Form No. H860, available for free download from our website at www.apawood.org. Marine grade is an upgraded Exterior plywood that is composed of all Douglas fir or larch veneers. There is NO treatment involved. I’m sure Marine Grade is almost never used in any type of flooring application due to the expense. I thought, however, that you should be aware that Marine Grade is not treated, is an Exterior Grade and therefore is as suitable for direct application of ceramic tile as any other untreated “EGP plywood,” as described in the TCNA Handbook. Feel free to call or email me to discuss.
Senior Engineer, Technical Services Division,
APA - The Engineered Wood Association