Crack in Grout

Tiled a small bathroom a few months ago and now have a crack in the grout running across the floor. How can I fix and what would prevent this from recurring?

 

The substrate may have flexed or shifted. To repair, dig out the grout and replace.

HTH

 

You have movement.
If this is on concrete, you have a crack ing the foundation, running across the area.

If it is wood. You didn't use a CBU underlayment. Going over plywood has its failures.
If you did use CBU. You didn't Lay it in a mortar bed, or you didn't leave a gap between panels of CBU, to be filled with thinset for strength. Also the mesh tape wasn't used.

If you did use CBU and installed it by the manufacturers specifications to the letter. Then the subfloor is too weak and flexes. Hinging right where your grout is cracking & crumbling.




Here is an idea that may save it, from being a total failure.

Grind all the grout out of that line. Use sanded colored caulking, that matches your grout, in that line.