I am replacing a kitchen floor with tile. When I ripped up the existing floor there was plywood in great shape. Do I really need to lay cement board down beofre tiling? What could happen if I dont?
I have neighbors that tried to cut a few corners and not lay the cement board. Floor looked fantastic after hours and hours of them carefully laying it and grouting. Floor look like hell in a year. Chipped cracked, loose and shifted tiles. They had to rip it all up and do it right...The second time due to how much they outlayed on the first tiles they put down the cement board and cheep fugly tiles. Never had an issue with them.
The reason you need the cement board is because it is considered a rigid or static material, just like the tile or marble you intend to install. You want to remove all flex from the floor. Otherwise, any weight applied to it, like walking, will cause the slightest flex under your feet. You can remove the flex with either thick enough plywood, or concrete board. The single layer under your old floor isn't enough. The tile will not bend because it is rigid and static, and so is the grout between the tile, but the grout is a very small thin strip, easy to break if slightly bent. The tile will actually act like a lever when the floor beneath it flexes, the grout joints between the tile will give up and crack, and eventually break out from between the tiles.
Had to scroll all the way to the bottom to find someone (Miss T) who bothered to consider what you were tiling over. If you are on a slab, no need for cement board. If you are on a raised foundation you need to make the substrate rigid enough to resist flexing. There are several ways to do this-another layer of ply (stagger seams with respect to the existing layer), Hardibacker, Wonderboard, etc. I generally use 2 1/2 inch screws to secure the existing substrate to the joists first regardless of what I'm installing over it. I occasionally sister 2 x 4s to the existing joists if the floor is particularly bouncy, or add cross bracing. I've never had a call back on any of my tile work (going back 25 plus years) Thinset doesn't stick to wood? install mesh? Where are these people coming from. Mesh is NEVER used on a floor and it wasn't used 50-100 years ago either. Only on walls and counter tops and thinset adheres to plywood just fine.
