Q. My home is approximately 1 year old. And the main floor has porcelain tile. Over the past few months we noticed the grout changing color (darker) in only some spots throughout the home. The spots are about 1-1/" in length. Since we are still under the builders warranty, they came out and scratched out the grout let it dry for a couple of weeks then regrouted. The stain came back there and in other spots throughout the home. They also checked for leaks and there are none. However, they used a moisture meter and where these dark spots are the moisture content was higher than the other areas. There is no plumbing run under the foundation (everything is overhead). We did have our floor acid washed before we moved in since the grout was discolored from construction debris. I am not sure it maybe the grout is possibly discoloring from the acid. (Maybe the acid is eating away at the grout?) My neighbor is experiencing the same issue.
A. In a home that’s only about a year old it must be frustrating for you to see this problem and have no clear answers.
This issue is very unlikely to be a plumbing leak, since there are no plumbing lines under the slab and the builder has already ruled out leaks. Plus, the discoloration appears in short, scattered spots rather than following any pipe layout.
This issue is probably not simple cosmetic grout discoloration, since the problem returned after the grout was removed and replaced.
The acid wash performed before move-in is also not the cause. While acid washing can make grout more porous and reveal moisture-related problems more easily, it does not introduce moisture into the slab and would not explain higher moisture readings in specific locations.
The most likely cause is moisture vapor transmission from the concrete slab. Concrete naturally retains moisture, and if the slab was not fully dry when the tile was installed, or if the vapor barrier beneath the slab is compromised, moisture vapor can migrate upward.
Moisture does not rise evenly. It travels through small cracks and variations in the slab. Because porcelain tile is relatively non-porous, the moisture makes its way out through some of the grout lines. The reason regrouting didn't work was that the new grout is simply absorbing the same moisture again.
What you do next is critical, especially while you are still under the builder’s warranty. Start by documenting everything. Take clear photos of the darkened grout as it appears and reappears, keep records of moisture meter readings, and retain all communication with the builder regarding repairs. If possible, obtain a brief written statement from your neighbor confirming they are experiencing the same issue, as this strongly supports that the problem is systemic rather than isolated. You should also formally request proper slab moisture testing in writing as part of a warranty evaluation.
Avoid accepting another regrouting, as it does not address the underlying cause and has already failed once. Any permanent repair must involve moisture mitigation at the slab level, not just cosmetic surface work.
Finally, ask the builder to confirm whether a vapor barrier was installed under the slab. Request documentation. Doing so signals to the builder that you understand the issue and expect a long-term solution.
ABOUT THE AUTHOR Fred Hueston is the Chief Technical Director at SurfaceCarePROS.com and Director at StoneForensics.com. He is also the author of Stone and Tile Restoration: The Manual, a comprehensive online manual for stone and tile restoration contractors. [Learn more about Fred.]
Article ID: 842
Created: January 1, 2026
Last Updated: January 11, 2026
Author: Fred Hueston
Online URL: https://surfacecarepros.com/kb/article/why-is-grout-darkening-in-isolated-spots-despite-no-plumbing-leaks-842.html