So the house I bought had a guy who wishes he was good at doing renovation work but really just sucked. And cut a lot of corners. He didn't put any cement board down under any of the tiled floors.. ie second floor wash room, hallway full bathroom and master bathroom.

End result: my floors are audibly cracking from the thin set cracked underneath the tile and grout is falling out from between the tiles. There are areas with no grout at all because of the floor movement.

Are there any ways to do anything about this, other than just regrouting the entire floor? Or pulling up all tile, installing cement board and redoing all the floors?

  • I suspect it’s not just missing cement board. It’s probably bad grout, bad mortar, bad install (not back buttering the tile, uneven mortar bed, etc.)
    – Lee Sam
    Commented Jan 17, 2020 at 0:48
  • That said, I've done three floor tile tasks just adding 3/4" OSB to the existing subfloor, screwing it every 6" and applying two coats of paint to keep the OSB from slurping water from the mortar, and to reduce the chance of water damage later. One is a laundry room, so wet happens. The other two are hearths with steel stoves on them. No cracks yet after 15 years. Commented Jan 17, 2020 at 23:00

2 Answers 2


My experience has shown me if the mortar is crumbling and tile cracking the base is not thick enough to start with. Even a perfect mix cannot stand up to flex!

In existing structures my normal plan is to remove existing flooring. Next I put down backer board first with thinset under the backer board and screw it down. Next I put the tile down again with thinset, the additional level of backer board with thinset on each side has not failed be in 40 years,

First I must say I check the Base , if I walk on it and it feels solid I STILL add the backer in most cases. I am 6’5” and a big guy and at 250 as a skinny guy if I see or feel any flex I add structure and that is the reason I have not had a tile failure similar to yours.

I can say most DIY tile jobs that fail because the base ( sub floor two thin, no backer) mixing is easy , if two wet it is not as strong and ends up two thin. but not usually a problem, under mixing with dry clumps is really to spot as big chunks flake with no base problems.

I would be looking at the base on a first floor and especially on a second floor with the problems described.


There’s no easy fix. I’d remove the existing tile flooring and check the size of the supporting joists, then 1) check thickness of the subfloor and underlayment, if any...2) then install cement board and 3) install new tile as per the Ceramic Tile Council of North America (CTNA) guidelines.


First, the supporting floor joists must be strong enough to support a tile floor. They cannot deflect or the tile will crack, etc. verify that the joists have a maximum of 1/360 th deflection...there are tables to calculate the deflection based on load, spacing and span.

If the structure is adequate, then 1) check the thickness of the subfloor and underlayment. If your joists are 16” on center, you’ll need a minimum of 5/8” plywood (or equivalent) and a minimum of 1/2” underlayment.

2) Cement board is recommended to be set in 1/4” thinset. (Get a trowel with 1/4” grooves.) Fasten the cement board to the joists using the recommended size and spacing of screws.

3) Install the tiles on the cement board following their precise instructions, including curing times.

As you can see, this not an easy task. Make sure you follow the TCNA guidelines exactly, including making sure the floor joists are strong enough and you have the proper thickness of subfloor and underlayment.

Typically, cracked tile is due to movement (deflection) in the structure.

  • Good info. I'm not really sure what's under the tile and carpet at this point other than the fact I know it's a hardwood floor. The previous owners simply carpeted over the hardwood in the bedrooms and hallways. A strange little fact is that there is a noticeable height difference between the tile floors and the carpeted floors. I will find out next weekend when I demo the master bathroom but it seems they cut out all the hard wood floor and tiled over whatever subfloor is under it without adding cement board. Commented Jan 17, 2020 at 15:40

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