How flat should a floor be for large tiles?

Answered by Brendan ~ August 25, 2010 ~ Comments

My wife and I tried to use a cement self leveling underlay on the concrete slab in our kitchen to prep for tiling 18'' tiles. Unfortunately, we did it in 2 batches because we misread the coverage and ended up with hills and valleys. In the worst spots the tile moves about 1/8''-3/16'' when pushed down on certain corners. Would it be safe to go ahead and lay the tiles? Should we try to flatten the floor more? Also, do I need to use concrete sealer before tiling?

Ansel J. ~ Anaheim, California

Brendan Fowler

If you have a tile that is moving that much you may or may not be okay. Depending on where the tile lands on the hump after you have finalized your layout, it may make the problem area less severe. You can try running a deeper notch trowel when applying the thin set, so the tiles set slightly higher above the floor. If it is a hump created by your leveling compound it may be sandable. I have had similar situations where I have used palm sander with coarse grit sand paper to knock down irregularities in floor leveling compound. It is a good idea to try and get your floor as flat as possible because it is going to make your tile job go much smoother if you do. Sealing your concrete is not essential, but if you are concerned about water wicking up from below, a seal coat won't hurt.

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