Q: I have a small bathroom floor 5’x 8′ with good plywood. On the subfloor screwed down every 6″ there is a wide dip or valley in the floor about 1/8″ deep by 2 1/2′ wide. Can I do anything to level the floor for ceramic tiles?
A: When I run across this kind of a leveling problem, the first thing I look at is the underside of the subfloor. If you can access it from a crawl space or basement, that is where you should place the shims.
If you have no access, for example, a second-story bathroom, it’s best to use a self-leveling product. This product is mixed with water and raked over the surface. In a short period of time, the leveling agent will find its own level.
Self-leveling products are a little on the expensive side, but are worth it when you consider the time they save. One sack of mix does not go very far, so buy a few extra and return what you don’t use.
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When your leveling is finished, install either your Hardibacker backer or wonder board. Always follow instructions on installing these subfloors. For a bathroom floor, you may want to install a felt roofing material as a moisture barrier between the subfloor and Hardibacker backer. This will help protect your home from those overfills in the tub.
You also might consider the more traditional chicken wire method of tile installation, which many professional tile setters consider the best. Most countertops are still installed this way and this method would also take care of your leveling concerns.
First, you will install roofing felt over your subfloor, then put down a little mud bed. Next embed chicken wire into the mud, apply more mud, and level the area. After you let it set, you can install your tile.