What is the cause of my grout staining, and how can I remove the stain?

QUESTION

About a year ago we had a light colored porcelain tile installed on our pool deck. We had a rolled chattahoochee rock deck which we had sealed multiple times with a marine epoxy sealer. This deck was removed with a chipper in preparation for the next layer which was a mortar mix used to pitch the deck along with the tile to move water away from the house. The pitch seems to be perfect because we do not have standing water at all after a rain.

Shortly after the install we noticed some of the grout lines had turned dark. I emphasize that the stains are only on some of the tiles and only on some of the sides. Most, but not all, of the staining seems to be in the same general area.

Since we here in Miami get a lot of rain and construction on our home had been continuing with dust, etc. I thought maybe we had some mold and mildew. I tried a few products to remove mold and mildew but had no luck, so we had the tile acid washed and the grout re-sealed. I know they advise not to use acid wash but we had successfully done that inside the house with what was supposed to be the same tile in a different color. We needed to do that because that tile inside our home had a haze left from the grout which I could not remove because the tile has a small textured surface. Anyway, the stains on our deck are still there and seem to grow during the time when it is dry and has not rained for a couple of days.

I do not know if the grout is sanded or unsanded but it is an off-white. We are now in the rainy season so the rain is keeping the staining down, but it occurred to me today that it might be rust. I tried using hydrogen peroxide on a small area, but I got the same result I always get. A good deal of brown stain comes off on the cloth I use but the grout is still stained and the tile next to the grout also has surface staining. I have notice that the surface stains can be removed with Barkeepers Friend.

One of my other thoughts was that the grout is deteriorating for some reason. It does seem to be cracking in some tiles that do not have staining. Again I point out that although most of the staining is in one general area, there are some tiles in other places that are isolated that also have this staining. At this point I am thinking that our only recourse is to cut out as much of the grout as possible and re-grout and re-seal. I would appreciate any suggestions or advice you might give us.
Thank you so much in advance.

ANSWER

ANSWER - You didn't mention what kind of tile you had.  Natural stone of various types are known to contain ferrous oxide that can lead to staining in the tile or the adjacent grout.

Since the staining is isolated to certain areas, it could be that there is some underlying metal or other contaminate that is causing the staining.  Maybe there are some residuals from the original epoxy chattahooochee rock deck that is causing the staining.

Sometimes the staining could be due to an area being subjected to excessive moisture.  Although the slope of the tile surface may be adequate, the underlying slope under the tile could have low spots where water collects and causes or contributes to the staining.

If it were me I would remove the stained grout and look for underlying evidence of what caused the staining.  I would leave the removed grout joints open to dry and to observe what may or may not occur over several days.  I would regrout and see if the condition reoccurs.  If it does reoccur then it practical, I would remove all of the tiles in that area to look for evidence that caused the staining.

Keep in mind that when you remove the grout and regrout with the same grout, at first the grout may not match well and will stand out.   Over time it normally will blend in.

7 thoughts on “What is the cause of my grout staining, and how can I remove the stain?

  1. Vadim Kashtelyan says:

    Hi,
    I came across your website and hope you can help me. My wife and I moved into our new house a year ago and are having issues with our shower floor. I think the shower floor was installed in 2007, but I don’t know what materials were used. The shower floor is stone. Over the year we have noticed the grout become increasing darker and develop what looked like mold spots. Parts of the grout have also chipped off during the year. I used a mold/milder remover from Home Depot and it cleared up the grout very well. My question now is what is the correct order of things to fix/repair the existing grout?

    Can I re-grout the whole shower floor or should I just grout the parts that have chipped away and color match the grout as best as I can? Do I need to seal the stones before applying new grout?

    Which products would you recommend?

    • Donato Pompo says:

      I saw the photos you emailed. The river rock floor installation didn’t look good in terms of the workmanship by the installer. The spacing of the rocks were irregular leaving wide grout areas.

      The dark areas of the grout could be an indication that the drain weep holes beneath the stone flooring were not protected and could be plugged or that the type of drain used doesn’t have weep holes to allow moisture below the stone surface to drain. So the darkness could be indication of moisture. I doubt that is microbial growth because microbes/mold need organic food with the moisture to develop. Plus cementitious grout has a high pH and mold doesn’t thrive under those conditions. If that is the case you have to remove the area around the drain and correct the problem. If the darkness is moisture it might dry out and go away.

      It is possible that the darkness is some sort of staining. You can experiment with poultices to try to remove it.

      If I were you I would remove entire shower floor and make sure the drain weep holes are open and protected and reinstall the floor in a proper way. It will be much more functional and look a lot better.

  2. Donato Pompo says:

    l had additional grout added to existing. The shower was used about 6 times. I wiped he tile with a damp cloth from a bucket of warm water and Dawn Detergent. I made sure the cloth was just damp enough to remove any soap on the tile. the grouter arrived 4 hours later and regrouted on top of existing grout. Some of the grout is Bright White, as it should be, some now is medium gray. The company is saying that it’s because i wiped the tile and it’s my fault. Is it? They say it has to cure. It looks the exact same as the minute he had finished. I pointed the darkness out to him immediately. Thank you for your time in responding and advising.
    Could there be another reason? Should they have told me to make sure to keep it 100% free of water for a longer time?

  3. Joseph Ressa says:

    We are purchasing a home built in 2019 and 2 of the bathrooms have dark grout near the toilets. Our inspector checked the moisture levels and the readings were 40+% but the sellers, sellers agent and my agent state nothing could be wrong with a 3 year old house and the tile is just stained. Am I being paranoid or is there a real concern here?

    • Donato Pompo says:

      Moisture meters measures the electrical impedances within various materials. The electrical impedance varies depending on the type of material. Wood is normally measured in wood from 5% to 30% scale. Although drywall, ceramic tile and other materials are measured on a calibrated relative scale from 0 to 100. So when you say 40% that doesn’t make sense. If it was a relative reading of 40, that is only moderate moisture.

      So unless the grout is wet or gets wet from flushing the toilet then you likely don’t have a problem . If it gets wet then it is likely the wax ring where the toilet connects to the drain that is leaking.

      It is not uncommon to see grout near the toilet to be darker as some men/boys are not very a good shots….

  4. Nicole says:

    Hello! We recently had a shower completed and after the very first shower, we noticed a single grout joint stayed dark and took days to dry out. After now ~3 months of use, the issue has spread to affect 1/3 of the floor grout, plus grout on lower wall tiles and the shower curb. Important to note that 3 different types of grout are affected by this problem, as well as sealed and unsealed grout. Some spots do dry out with a hairdryer, others remain wet. My contractor and 3 second opinions have been stumped because the completed job looks well done overall (aside from grout being used at the changes of planes, which I have requested be changed to TCNA standards). We have ruled out preslope, weep holes, the grout itself, and sealer as potential causes.

    I started to think perhaps the root of the problem is mortar-related as that is a common denominator among the 3 grouts and 3 surfaces affected, then found a couple websites stating that insufficient mortar can cause water to sit in the voids, leading to deer/dark grout. I brought up with my contractor, who basically brushed off my concern and is adamant the discoloration cannot be due to excessive moisture because it would drain down due to gravity.

    However, I have a picture I took during the reno showing that the wall tile has no where close to 95% mortar coverage. Appears to be spot bonding. Unfortunately I didn’t know at the time I was looking at a problem.

    I’m not sure where to go from here. I wish I could attach photos to this comment so you could see if I am onto something. Btw, I did use a licensed contractor and did my due-diligence when selecting a company for our remodel. Any insight or advice would be tremendously appreciated!

    • Donato Pompo says:

      To really understand your concerns and to fully understand all of the potential variables that could be involved would require a detailed forensic investigation with laboratory follow up testing to narrow down the cause of the issue. This likely would be expensive and possibly not practical for most to pursue.

      Based on my understanding of your situation the way that you described the single grout joint turning dark, what comes to mind is that perhaps the the tile area was sealed and unintentionally the one grout joint was missed. If that were the case then the unsealed grout joint would absorb and retain moisture that causes that grout joint to turn dark. Perhaps the other floor tile grout joints used a sealer and it is wearing off in the areas where the grout is darker. The absorbing grout is the grout turning darker.

      You can always apply an epoxy grout colorant over the existing grout that will change the color of the grout to what you want and it will seal the grout. it.

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