When there is a significant amount of movement on the substrate of a tiled floor or wall it may lead to grout cracking. When this happens it will leave what looks like a crack in the grout where it has come away from the tile. Unless your grout was installed very recently this is always due to movement in the tile. If your grout is new it may be caused by incorrectly mixed grout. With grout that has been doing this for a while it may lead to whole chunks of grout coming loose and leaving large voids in your grout lines.

While it may be tempting to simply mix up some more grout and fill these voids you need to know that it will not last. If you do this it will fill the grout lines just like new but over time will lead to the same problem. Grout over grout is not a permanent solution.

Why it won’t last

The first reason is that when you go over the top of grout that is already cured with new grout, there is no adhesion to the old grout. It will instead simply create a layer of grout on top of the old grout. These layers have no way of  sticking to each other. When you grout tile the grout actually sticks to the sides of the tile rather than whatever substrate is beneath it. While it will stick to the substrate at the bottom of the grout lines to an extent, it is not a permanent bond. Attempting to grout over the top of old grout is simply stacking two seperate layers of grout. It will always remain two seperate layers.

The second reason is no matter how much of the old grout you may take out to install new, if you do not fix the actual reason the grout failed in the first place, eventually the same thing will happen again. As with any problem you may encounter with a tile installation, you must figure out the initial cause and fix it to prevent repeated problems. This is an involved process which I will cover in a different article. For now I’ll stick with the solution for the grout problem.

What to do

Rather than simply filling the grout line where the grout is missing, you must remove the old grout at least 2/3 the depth of the tile so the new grout has a feasible surface to grab onto. The easiest way to do this is with a grout saw. Ideally you would remove the old grout all the way to the substrate before regrouting it, but 2/3 will be sufficient if it is difficult to remove.

You need to remove any of the old grout that seems loose or has come loose from the sides of the tile. You also need to make sure the spot where the new grout butts against the old is a 90 degree angle, or close to it. In other words from the top of the grout line straight down. You do not want a slope. Eventually a slope must be feathered to a very thin edge. That will be the first place it will fail again, any very thin layer. A 90 degree angle prevents that.

It is also better to make the transition from old grout to new in the middle of a grout line rather than at one of the corners of the tile. The grout line at the corner of the tile has six different spots it can fail, the middle of a grout line has only one. It lessens the chance of failure.

After you remove the sufficient amount of the old grout just mix some new grout and fill the lines. You must make sure you force the new grout into the grout lines very well. You want to make certain there are no voids and the lines are full.

The above method will work to temporarily fix your cracking grout. As I pointed out above, you must find the initial reason for the grout failure before a permanent fix will last. It’s difficult to say how long this fix will work, it may be two weeks, it may last a year. That depends on the severity of the problem that caused it. It also depends on the application (wall or floor) and how much use it gets. A floor in your main entryway will not last as long as a shower that is never used.

You should also take into consideration the age of the grout. If you are repairing grout that has been installed for five years, the new grout, even if it is the same brand and color, will not likely match perfectly. The best solution, of course, would be to fix the cause of the cracking and regrout the entire floor. Depending on what you’re fixing and why this method may solve your problem.

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  • Rahim

    Okay but of a pickle this one. Having researched high and low regarding deep under tile shower grout lines voids I cannot find answers for a good fix or perhaps patch- up method is not recommended for this type situation. Shower floor had seepage through the corners and first grout line from the wall 200mm in (tap spindle side of wall). Brick veneer home 29 years old. Suspected waterproof membrane may be damaged. As removing old grout we found two random void spots with depths of 40mm deep. Could be cement foundation issues, mortar bedding issues or something more problematic. Could these voids be filled in with something or would it be beat to redo whole floor?

  • Glenn L Tischaefer

    We had tile and grout done 10 days ago. The grout didn’t fill the area completely. It is too low. Our contractor is going to regroup over the top. Will that grout stick?

    • Joan Dunlap

      We had our floor grouted in our bathroom a month ago. We haven’t used the bathroom because we are still waiting for our cabinets. The grout is fresh but the guy that installed it didn’t fill it to the top like I prefer. Can I mix up more grout and go over the top so it fills it in more?

      • Roger

        Hi Joan,

        Not unless it’s acrylic (premixed) grout or epoxy grout. Regular cementitious grout achieves full cure after 28 days. Anything you go over the top of it with will end up flaking off. If it HAS to be fixed the existing grout needs to be removed to about 2/3 the depth of the tile, then it can be regrouted (grout sticks to the sides of the tile, not what’s beneath it).

  • Mary ODonohoe

    Had tile done a year ago just a few places he missed putting enough grout to reach top of tile

  • Matthew Daulerio

    Thanks for the info. I assume you would recommend having a professonal replace the grout.

    • Roger

      Yes. That is always the best option as they have the experience and tools to do it correctly.

  • Sara

    We installed a new kitchen backsplash in May or June of this year. It was the first time we did anything like this so the grout job isn’t the best (bumpy in some areas). I also want a different color instead of the white. Since the grout is fairly new is it ok to clean it, allow to dry, then go over it with the new color in the same brand?

    • Roger

      Hi Sara,

      Yes, but you need to clean it out to a minimum 2/3 of the depth of the tile so the new grout will bond.

      Grout bonds to the SIDES of the tile, not the stuff under it.

    • Brett Garon

      No. Clean it and get a color sealer to your liking and use that. Grout on grout won’t work.

  • Charles

    My grout wasn’t filled evenly to the tile when installed. The floor was installed about 7 months ago and it is driving me crazy looking at them. Do I need to use a grout saw and then regrout?

    • Roger

      Hi Charles,

      Unfortunately yes, you do. Sorry.

  • Corrin

    I have a few small areas about an inch where the grout is cracked. It is a very shallow crack and does not appear to be all the way through the grout line. Do I need to completely removed the grout or can I patch with silicone or grout? It’s a bathroom floor

    • Roger

      Hi Corrin,

      With something that small you should be able to just go over it with more grout.

  • Cassandra

    About 3 weeks ago, the person I hired to install tile and grout to my shower obviously didn’t know how to do the job. There are serious grout line that are not completely filled with grout and when you touch the gout white power comes off on your fingers. I am not good with mixture, but need to get this fixed. What can I do to correct this? The shower has not been used for fear of creating additional problems. Thanks

    • Roger

      Hi Cassandra,

      Unfortunately it sounds like the grout was not mixed correctly. If it’s powdering out like that it will likely need to be replaced. Do you know what brand of grout was used?

  • Chris

    We just had fine grout put in on our floor last night. The floor underneath is cement. We woke up this morni g to cracks between all the tiles. Can we wet it and put new grout on top? How do we fix this?

    • Roger

      Hi Chris,

      The most common cause of that is using unsanded grout in grout lines which are too large for it. It should only be used in grout lines of 1/8″ or less, and 1/8″ is pushing it in my opinion. But yes, you can wet it (lightly) and go over it again with more grout, even if it is unsanded. Once you get it filled properly and it’s allowed to cure the unsanded will work fine, it’s just a LOT more work to fill, let it cure (shrink), fill again, etc. Once filled and cured the stability is the same as sanded grout.

  • Amanda Hawkins

    I just grouted last night. This morning I am seeing places where I didn’t put enough grout. What do I do in this case?

    • Roger

      Hi Amanda,

      Wipe it down with a sponge so the existing grout is damp, then go over it with more grout to fill in where you need to.

  • Peter

    My grout is cracked from floor to ceiling in both corners of my shower. I don’t think the person I hired should have put grout where the tile comes together at a 90 degree angle. What is the best way to fix this? Remove the old grout and caulk the corners? He also left a big gap between tiles. Is this the correct way to put a 90 degree corner together?
    Thank You,
    Pete

    • Roger

      Hi Peter,

      It should not be grout, it should be caulk (good) or silicone (better). They can compensate for movement, grout can’t. This will explain it better. You’ll need to scrape out the grout and replace it.

  • Sonia

    Have you heard of Spectralock 1 by Laticrete? I want to regroup my shower floor (it tends to get mildew every week) Using regular unsanded grout seems a process and I have been reading about their epoxy “like” grout that is easy to use no drying time easy cleaning and no mold or mildew!! Also what epoxy brand would you suggest

    • Roger

      Hi Sonia,

      Spectralock 1 is new, I mean BRAND NEW. Like, I can’t even get it yet. So yeah, I’ve heard of it, but I wouldn’t recommend you use it for your shower floor (if you can even get it yet). Regular spectralock is what I would recommend.

  • robert boyko

    i use a officechair instead of a wheelchair but my grout lines are deep enough to where my wheels get cuaght in the grout lines and i have to ush myslf harder when i run into them which is quite often is there a compound like i was thinking durham’s rock hard water putty

    • Roger

      Hi Robert,

      If your wheels are getting ‘caught’ then your grout lines are HUGE. There is no product to put over existing grout to make it flush with your floor. A hard plastic mat for beneath your office chair is unfortunately your best alternative.