Tile is rigid, and a rigid floor only lasts if what sits under it is flat and stiff. Most cracked tiles we lift were not bad tiles. They were laid over a hump, a dip or a joist that moved.
How flat is flat enough
The standard for large format tile is no more than 1/8 inch of variation over ten feet. A 24 x 48 inch porcelain tile will rock over a hump you can barely see, and a tile that rocks will crack or come loose.
Finding the highs and lows
Before any tile goes down we run a long straightedge across the whole room and mark every high and low spot. Highs get ground down. Lows get a self levelling compound, poured and left to cure overnight.
The prep you never see is the reason the floor is still flat years later.
Stiffness matters as much
Over wood framing we check the joist span and add plywood or cement board where the floor flexes. Over concrete we bridge cracks with a membrane, so a moving slab cannot break the tile above it.
Large format tile: 1/8 inch over ten feet
Standard tile: 1/4 inch over ten feet
Levelling compound: one night to cure
This prep often takes as long as the tiling itself, and it goes in the written price so it never arrives as a surprise.
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Every post is written by the crew that does the work.