Bathroom Tile Repair after Cracked Grout: a Case Study

This customer contacted us about repairing the grout on their bathroom tile floor: they had observed the grout separating and cracks forming around the tiles. Upon investigation, however, we realized more was needed than a simple regrout. We noticed that the tiles had actually lost their bond to the floor. When tapping on the tiles, we could hear the loose tiles move. This will often be a smacking sound as the tile hits the substrate, or a gritty sound as the sand in the joints crumbles and grinds against the tile. The video below shows the type of sound we are listening for:

Before repairing bathroom tile, it's important to figure out the underlying issue. Using a screwdriver to probe beneath a loose tile with cracked grout.
Testing this tile corner to see whether prying here will be a good strategy. You can see where the grout has separated from the tile in the upper left corner, creating a hairline crack.

Repairing grout around loose tiles will never fix the issue as a tile assembly can only absorb minimal movement. The extra movement in the tile as it is walked on will be enough to crack the tile or grout over and over. The only way to fix this problem is to remove the tiles and reinstall them to industry standards.

Using blue tape to mark and track the order in which the loose tiles are placed before removal.
Tiles marked out for removal

After tapping around the floor and getting a feel for where things have come loose and cracked the grout, we marked the tiles for removal. Then start to *very carefully* remove tile. As this was the original installation by the builder, we didn’t have any leftover tiles to use. Rather than letting any break, we had to be surgical in our approach to removal. This involved scraping out grout first, then slowly and gently wiggling to get the tiles to come up completely.

Observing the mortar coverage pattern after tile is removed. Most of the mortar is left on the plywood substrate.
You can see that there was minimal bond between the mortar and the tile.
Checking mortar coverage pattern on the back of a tile after removal. The vast majority of the mortar has lifted up with the tile, losing its bond with the plywood substrate.
The mortar was well-bonded to this tile, but poorly bonded to the floor.

We were able to remove our tiles without too much difficulty. While it was clear the mortar wasn’t troweled very well, the bigger issue seemed to be that the mortar just didn’t bond well to the tile and the plywood underlayment. The most likely reasons for this are: 1. using a lower-grade mortar than necessary (bonding to plywood requires a mortar with significantly more polymer modification than most – ANSI 118.11 or better). 2. Also possible that the plywood substrate nor the tiles were very clean. This is especially possible in new construction. 3. Perhaps the installer was nearing the end of the job and added extra water to the drying mortar to “make it last”. This can significantly inhibit the bond of the mortar and weaken the molecular structure.

Back of tile after grinding mortar down leaves swirl marks from the diamond cup wheel.
Mortar carefully ground off the back of the tile.
Plywood substrate, cleaned of residual mortar.
Mortar removed from the substrate. The remaining grey color is portland cement staining in the fibers of the wood.

The next steps to repair the bathroom tile were to clean the remaining mortar off both tiles, and plywood substrate. In order to reuse the existing tiles, we had to grind the backs of the tiles. This in particular was slow work as we needed to minimize heat and vibration to ensure none of the tiles cracked.

Using a sponge to wipe down the plywood
Sponge the surface, both to remove any bond-inhibiting dust, and to prevent the wood from pulling water from the mortar too quickly.
Pulled a tile after setting it to check wet mortar coverage. Stalactites and stalagmites indicate the ridges of mortar were fully collapsed and no air was entrapped behind the tile.
Checking for coverage. If we see trowel marks, it means we haven’t collapsed our ridges to remove air between the tile and the substrate.

Then, time to re-install the tile! Here, we used a highly-modified rapid-setting mortar for the installation suitable for installation of porcelain tile over plywood underlayment.

Tiles set in place. Bucket of mortar and trowels visible in the corner.
Tiles set and alignment double checked.

Once the mortar has cured, it’s time to grout. On new installations, we typically use newer-style grouts (epoxies or urethanes) but on bathroom tile repairs, we want to match the existing grout closely. Using a color-matched cementitious grout seemed to do the trick.

Tile repair in this bathroom is complete! Tiles grouted with grout color-matched to the surrounding tiles.
Tile/grout repair finished!