How to make sure a tile does not bust on a high point in the floor

Question

I am tiling a bathroom for the first time. It is an upstairs bathroom so I pulled out the vinyl and luan. The subfloor met the standards that I found else where on this site. I laid hardibacker board with total flex thinset and then screwed it all down. I then taped and used thinset on the joints. I started laying my tiles yesterday and everything was going fine until I reached an uneven spot. It is at the sink area on one of the joints. I have sanded that area down as much as possible, but the tile is still able to make a rocking motion when I lay it down without any thinset. I already cracked one with thinset on it when I tapped the block with my rubber mallet. So here is my question, how do I make sure the tile does not crack? Do I build up the thinset in the low areas? Wouldn't that make the tiles higher than the surrounding tiles?

Edit- this is the case for 1 tile only

Thanks, Tiffany

Answer 1

Your floor must be flatter period. You must fix that issue before continuing. This has nothing to do with thinset or anything else. Thinset is not used to build up the subfloor, it is used to bind to it and create flexibility.

Answer 2

I don't believe there's a problem here, though you haven't indicated how much movement occurs. Almost any tile will rock a tiny bit in one direction or another. If you've mixed your mortar correctly you'll have at least 1/8" of forgiveness for slightly uneven joints or shallow floor humps.

Re-read the mortar instructions and make sure you've mixed it properly. Set the tile with firm pressure (not hits) and slight twisting. If you're still unsure, lift the tile and verify that you get good, uniform contact and support from your mortar. Scrape the tile and the floor clean and re-trowel.

Answer 3

if we are safe in assuming you are correct in making sure the runout on the floor is within standards used for the tiles, subfloor and mortar you are using. then unfortunately, you are using too small of a notch on too big of a tile.

i say assume because you may be incorrect in either your measurements or which standards to use. ANSI A108.02 4.3.7 specifies a lot of different scenarios, but generally its considered to be maximum variation of 1/4" in 10 ft, with no localized variation greater than 1/16" in 24". these are pretty strict, but if your floor is good, then the issue is your mortar bed.

you probably need notches of 1/2 x 1/2 or 3/4 x 1/2 for your floor tiles. this is pretty common for 300mm x 600mm tile. you can either pull up what you have and just redo it again with everything done correctly, or you can further sand, grind, scrape, etc as you go and hope for the best. you may find other tiles that require the same treatment, and maybe even some that can't be remedied no matter how much base you remove. my suggestion would be just pull it up now while the mortar is still green, but its your place, not mine.

Answer 4

Maybe one of the floor joists is sagging or one joist was installed with a high crown (suposing that you have 2x10's, not trusses). If you are able to access the floor from underneath, you might be able to level it with some shims and or 2x4s. First, make sure your subfloor is nailed down, not screwed. Then use a couple of pry bars to distribute force evenly (don't break the plywood) when lifting the floor and insert shims to suppport the low spot. If you have 2x10's you should add a 2x4 scab. Of course you will need to recheck other areas to make sure the tiles are all level.