Is the 1 1/4" thick sub floor rule of thumb under a tile/stone floor necessary if the floor joist are on 12" centers
Instead of using 2 sheet of plywood and spending the time to screw and glue I'm thinking of using one layer of 1" plywood.
One failing I can think of is more deflection at the ends but this can be solved with some blocking.
I had the joist engineered for deflection (L/1000) rather than just the live/dead load. Deflection along the length of the joists is not a concern.
Don't want any deflection in the plywood between the joists.
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.