Why is my Floor Grout Cracking?

by Roger

Your grout is cracking for one reason and one reason only: your tile is moving. That’s it. Okay, that’s not it – Unless your grout is non-sanded and was installed in the last 28 days – your tile is moving. That’s it. Yes, 28 days has significance, it is the amount of time it took my teenage son to clean his room. It is also the amount of time it takes for grout to fully cure.

So let’s figure out why your grout is cracking:

Your grout is newly installed – incorrectly

If you do have grout that was installed within the last 28 days then your grout is not actually cracking – it is shrinking. Either your grout lines are too large for non-sanded grout (smaller than 1/8″)  or it was incorrectly mixed. NO! You cannot simply mix up more and fill it in. Read this post about adding more grout to your grout lines.

If your grout is not fresh, well, you need to repair the reason your tile is moving. And stop using your pogo stick in the house. Diagnosing the reason your tile is moving is extremely varied. It could be anything from inadequate deflection in your flooring for the type of tile all the way up to and including the aforementioned pogo stick.

The most likely reasons your tile is moving:

Your tile does not have proper thinset coverage

The most common reason I run into is improper coverage. This simply means that there is not enough thinset beneath your tile to properly adhere it to your substrate and support it. If there are any unsupported areas beneath your tile along the edge or in the corner of the tile, walking on it will eventually work what little support it may have loose and the tile will move down and up every time you step on it. The tile moves, the grout does not. The grout loses the battle and starts to crack out. By ‘crack out’ I don’t mean like that ridiculous Intervention show on cable, I mean it will start to crack and come loose.

To fix this you need to remove and properly reinstall the tile. If it is only one tile it may be an isolated incident in your installation and you will be fine. If you have cracked grout all over your tile installation it was either improperly installed or . . .

Your subfloor is moving

If you have a wooden subfloor and your tile is directly installed to it – go pick out new tile. That is more than likely an improper installation. While tile can be installed directly to plywood it requires a VERY specific method. And I do mean very specific. More than likely it is simply installed improperly probably by someone that did not know any better. If it was properly installed over plywood, well, your grout wouldn’t be cracking. Start reading this paragraph all over. Or . . .

Your backerboard was improperly installed

If you have Hardiebacker, Durock, or any other type of cementious backerboard beneath your tile it should have been installed in the proper manner. It needs to have thinset beneath it, it needs to be screwed down (properly), it needs proper spacing, etc., etc. Read How to Install Backerboards for Floor Tile to see all the things that should have been done.

More than likely there is no thinset beneath your backerboard. Thinset is not used to adhere the backerboards in any way – it is used simply to fill voids beneath the backerboard. It is placed there specifically to prevent your tile from moving. Moving tile leads to cracking grout. But you knew that – or you should start reading this page all over. This would be another time to start shopping for new tile.

Other reasons your tile is moving

You do not have expansion space around the perimeter of your tiled room. No, the tile will not expand – but your walls do. If there is no space between your tile and walls it will force all the pressure into your tile. This will cause cracking grout and, eventually, ‘tenting’ of your tile.

You do not have expansion joints in your tile. For every application there are specific spans of tile which can be installed before a ‘soft joint’ is required. This is simply a grout joint filled with a matching caulk or silicone to allow for movement without cracking your grout or tenting your tile. Most of these measurements are over 25 feet. So in English: if your room is not 25 feet long or wide this is not the reason your grout is cracking.

If your tile is on concrete – directly on concrete – your slab may not have proper expansion joints or the tile installation did not honor those. If installing tile directly to concrete (and you should not) there needs to be a soft joint directly above, or two inches on either side, of the slabs expansion joints. If not your slab will move differently than your tile.

And there could be a host of other, less apparent or less common problems. However, if your grout is cracking it is probably for one of the reasons above. The method of repairing it depends entirely on why it is cracking. Most of it, as with most tile installation problems, is due to improper installation.

Or your pogo stick.

If you have any questions about the proper way to repair your tile or grout just leave a comment. I answer every one of them – really, look around the site. I’m just super cool like that.

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Mr. Elf,

This is my situation: I live in a condo in a house from the early 1900′s which has 2×8 floor joists that are 16 inches apart and span the distance of 13 feet. The subfloor itself is on two layers of 3/8 plywood (nailed together). There is underfloor heat beneath the subfloor (hot water). The floor was very out of level so my tiler primed the floor and put self level on it. He tiled on top of that, of course using thinset, and put sanded grout in between the tiles. He put matching caulk where the walls meet the floor. Now some of the grout has started cracking. My vanity is placed on top of the tile on his recommendation, it is not tiled in.

The same tiler also tiled my neighbors condo beneath me in a similar manner. He has the same problem, but it’s very mild compared to mine. However, since he lives on the first floor he had the benefit of having sistering joists (added later), instead of single ones that I’m stuck with.

So my question is: is the cracking of my tile caused to incorrectly tiling over the self-level, due to the poor joists (too much bounce), due to the heated subfloor or some other reason, and what can I do to correct this? I’ve read through this article many times, but I can’t trouble shoot the situation by myself.




Correcting a minor detail: the grout now has hairline cracks across 1/4 of the room, which is 8×9 ft in size. The tiles are not cracked. The installation was done a year ago. Some of the cracks appeared immediately (within a week), others took 2-3 months to appear. It seems that the situation is not getting worse, but I’m concerned that eventually my tiles will start cracking as well. I was hoping to keep this bathroom for decades to come.



Hi Steve,

It sounds as if your floor joist system is not supporting the tile. He should also have installed at least another layer of 1/2″ ply. It should be a minimum of 1 1/8″ ply even under slc. It’s hard to be certain from here, but I think it’s simply inadequate substrate preparation. The two 3/8″ layers of ply is actually weaker, although it’s 3/4″ thick, than one 1/2 layer of ply.



Hi Roger,
I had a new kitchen installed last February/March and the grout is cracking in several places across the kitchen floor. Is this most likely due to the thinset? I wanted to find out before contacting the people who installed it to make sure they were definitely liable!



Hi Alison,

It’s due to movement somewhere. I would need to know how your floor is constructed to even venture a guess as to what is causing it. I can say, however, that it is their fault.


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