One of grout’s primary functions in a shower is to fill the area between the tiles to provide a water tight space. Any type of grout failure, including simple hairline cracks, can, and often does, lead to moisture seeping behind the tiles. When this occurs, it can either begin to damage the wallboard onto which the tile is attached or leak to the area below or both!!!
In addition to the damage water can do to the wood, wallboard and drywall on the ceiling below the bathroom, the existence of moisture behind the walls can lead to mold and mildew growth that can go unchecked.
In the situation shown, the water had damaged the wallboard causing the tiles to come loose and fall away from the board. The water also had saturated the wooden studs to such a degree that they crumbled to the touch. Additionally, over time, the moisture led to significant mold and mildew growth on the wallboard and studs that remained.
To address the issue, all the damaged wallboard was removed. The area behind the wall was treated for mold remediation and the damaged studs were replaced. At that point, new cement board was installed where the damaged board had been removed. Because some of the tiles were damaged, new complementary tile was used, separated by an accent row below the window.
In the end, the area was rebuilt; however, early recognition of the warning signs (cracked and missing grout) could have made the repair much easier and far less costly.