I am removing a laminate tile floor. There looks to be 2 layers of 3/4" Plywood as the sub-floor. Do I remove 1 layer of Plywood and install 1/2" concrete board or leave the plywood and install 1/4" concrete board on top of the plywood before installing the porcelain tile floor? My concern is that if I leave the existing sub-floor material I am going to have a ½” rise in the floor from one room to another.
wirelath nailed over the plywood(overlapping the lath 2",use paper backed lath to avoid moisture of setting/grout materials)nail or staple(pneumatic,not a hand staple)in a 8"x8" grid.wire acts as a trampoline,the thinset gets into the mesh but won't twist when the plywood deflects.normal thin-set is fine in the application(unless a modified is needed for the tile).
Although cement backer board is an ideal substrate for tile, you could bond directly to the existing plywood if it is clean, flat and strong (little or no flex); this is acceptable for areas that see only incidental moisture. There are polymer fortified thin-set mortars specifically designed for direct application to sound plywood flooring that does not exceed maximum deflection specs. I have had very good results, proper application is key. Check it out: fortified thin-set mortar –
1/4" cement backer will conform to irregularities in the floor (high/low spots), so if the existing plywood is not smooth and flat I would probably remove the top plywood layer and use 1/2" backer board to give the smoothest flattest surface.