First of all, never put self leveling cement under the cement board. When the cement board gets nailed or screwed down the SLC will turn to crunchy peanut brittle and become an obstacle to a successful tile installation. The SLC should be poured on top of the cement board.
As pointed out OSB is a questionable substrate and it presence can be tolerated if a 5/8" sheet of BC grade ply is put over it. Technically you could tile directly to the BC ply but it is not advisable if you are a novice to installing tile, that is where the 1/4' cement board comes in, it offers a much better and a safer bond of the thinset.
The ridges on the OSB seams should be ground down with a belt sander to minimize the variation between boards. The cement board must have a layer of thinset under it, use a 1/4" square notch trowel, and then anchor the boards with galvanized roofing nails or backerboard screws. The thinset is under the board to fill in the voids much like you talked about with the OSB, it is not for bonding, it is strictly a filler.
After the SLC is applied and dries you can set the tile using a good grade modified thinset. The thinset is best applied using the method outlined by the National Tile Contractors Association in a video they sell called "Trowel and Error". Go to www.tile-assn.com and look for it there. Its 10 minutes for $10 and well worth it. |
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