Industry standards call for a minimum of 5/8" ply. Most pros don't like to use the minimum, so 3/4" is better. This is for ceramic...natural stone REQUIRES a second layer. The industry standards do not call for gluing the second layer. While it can be done, it requires a full spread of a wood glue (not something like construction adhesive from a tube) and proper fastener use and layout. BTW, all ply used for this must have NO 'D' faces, and have exposure I or exterior rated glue.