Flooring Tile - cement board thickness over OSB for bathroom ceramic tile ?

I have 3/4" OSB subfloor. Going to put down cement board. How light can I go with the thickness so that I don't raise the floor too much ? I've seen it in different thicknesses.

thanks for the advice.

One half inch. This will be the thickness needed to protect the ceramic tile or grout from cracking.
fred

What I say needs to be taken with a grain of salt. Since the CBU does not, and does not claim to, provide any structural strength, 1/4 inch will do. I think that we have a tendency to think that if 1/4" is good, 1/2" would be twice as good.

Your structure is what is basically important here first. Not less than 2" X 10" members 16" on center. Then the TCA recommends a minimum of 1-1/8" subfloor thickness before the tile so 1/4" cement board DOES NOT qualify.

Heighth of course is your main issue but it shouldn't be. A little extra heighth can usually be delt with using the right transition method, please don't compromise your structural readiness because of heighth issues.:)

My Home Depot Tiling 1-2-3 how-to book says that OSB shouldn't be used for tile underlay.

I quote: ANYTHING LESS THAN SOLID WOOD OR EXTERIOR-GRADE PLYWOOD IS UNACCEPTABLE.

Any diifering viewpoints?

Alan,

Not OSB alone, but that's why we're saying to use 1/2 inch cement backer board on top of the osb.
fred

Cement board over OSB gives me chills but unfortunately I do it all the time. With no repercussions I might add...b-r-r-r-r-r,:) but your 123 Book is correct, exterior ply would be better. That's one of the smarter things I've heard coming from HD.

Cement backerboard units do not add structural strength to a floor. It will only perform as well as the subfloor beneath. There are a variety of cementious backer boards available that range in thickness from 1/4" to 1/2". Because all subfloors fail to be perfectly flat, these backer boards will provide a more stable surface for ceramic tile. The different thicknesses allow tiled areas to match up well with adjacent floor coverings. But, as Bud indicates, transitional pieces deal with differences in floor heights.

O.C. joist spacing for Ceramic Tile according to the Tile Institute is 16" max. for 5/8" plywood subfloor and ½" underlayment and 24" max. for ¾" plywood subfloor and ¾" underlayment.

Deflection in a floor system occurs along the joists, in the sheathing between joists and in the movement of one joist relative to another. The induced strain in the substrate creates shear stresses in the bonding material and compressive and tensile forces in the grout and tile. Thus, movement tends to result in grout failure and possibly cracked tiles. To muddy the waters further, there is an excellent technical note at
http://www.thetiledoctor.com/ctioa/reports/fr81.html

The typical subfloor info, although some I found was vague and conflicting, tends to be that you need at least 1 1/4" to 1 1/2" total subfloor thickness in order to support the weight of the ceramic. The top layer should be a type of backer board or concrete board. Plywood is reported to be acceptable, but ceramic should never be installed over any type of particle board. I have also read that only exterior grade plywood is acceptable if installing over plywood because interior-grade plywood and particleboard are not considered a strong enough floor underlayment. Of course, the floor should have very little movement, if any. Concrete is the best surface for installing tile, but new concrete should cure for at least 30 days, be structurally sound, and in good repair. For DIYers, cement backerboard is recommended because you don't have to deal with a mortar bed. Backerboard has a solid concrete core and is faced on both sides with fiberglass. It can't be damaged by water which makes it ideal for bathroom and kitchen installations.

Most websites recommend cement underlayment board (CBU) as an underlayment material as it tends to reduce deflection and provides a clean, smooth, nonabsorbent surface on which to install tile.

I am still at a loss to determine if the increased thickness of CBU offers greater compressive strength or reduces deflection (flex).