As some of you may know (or simply don’t care about – but know anyway) I actually do reply to every question I’m asked on this site. It may take a while for me to sober up enough get enough time away from work to answer it – but I do. As such I am frequently asked the same or similar questions a lot. I’m going to start turning these questions and answers into blog posts to leave myself more beer drinking time to help people with the same problems. So here’s my first one. If you have a suggestion for future posts like these just let me know and I’ll be more than happy to cobble together some similarly ridiculous advice for your problem. I will not share any names or other information with anyone else because – well, it’s none of their damn business – so no homeowners were harmed during the writing of this post.

Here’s the question:

My husband and I have recently tiled our kitchen and laundry room. It is a 35 year old house that had kitchen carpet in both areas. We removed the carpet and old linoleum was underneath. We installed backerboard thinset extra like you suggested on this site, which i read just now to make sure. Now we have something like a fault line running through our grout. We did remove the grout down to the thinset and cleaned out as much as we could then regrouted. Of course it cracked again within 2 weeks of the repair. I do suspect a tile is moving but is there any sure way to know if it is just that tile or more and if so what is the best way to fix it. Also I am not sure why it would move with backerboard under it could it be the floor itself?

My response to this all-too-common question is a bit complicated since it is rarely one specific reason and could be a number of things.

Hi (Name removed to protect people who actually ask me for advice),

The best way to determine what is moving (something is moving) is to go by layers. One of them is moving in an unusual manner or has not been properly addressed while preparing the floor for tile. We’ll start with the tile itself.

You stated that it appears to be a ‘fault line’ which says to me that it runs the length of several tiles in a row and down one grout joint. Is this correct? If so it means that your actual tile layer is more than likely not the problem – it’s below that. If a tile (or tiles) were moving the grout would be cracked along at least two sides of an individual tile and would not create the line you’ve described. UNLESS (you knew that was coming, right?) that line is the one you snapped a chalk line on or drew on the floor as a reference and did not get thinset completely up to it on the edges of the tile. This would cause voids (conveniently enough) under one edge of every one of those tiles in a straight line. That’s doubtful, though, and would also cause cracking in the grout joints running away from that line.

Your backerboard. Unfortunately this is where I believe the problem may be. You stated that you installed thinset beneath the backerboard – correct? You did not state that you taped the seams of the backerboard with thinset and an alkali-resistant mesh tape. I would be willing to bet (my wife doesn’t let me do that, by the way, so it isn’t binding) that the fault line you’ve described is either directly over or within about three inches of a seam in your backerboards. If they are not taped but are thinsetted down they are fully supported beneath but still move independently of one another. Simple things like walking on one side of the backerboard seam more than the other (completely normal with room layouts) will cause more stress on one sheet than the one next to it. If these two are not tied together with mesh tape and thinset they will move independently and crack your grout. The only remedy for this is to pull out enough tile to tape those joints.

Screwing the backerboard into the joists – this will cause uneven movement in your substrate layers as the area directly above the joist will move differently than the rest of your substrate. The joist will expand and contract moving the layers directly above it more than any areas between the joists. The area above it has direct pressure pushing up on it while the areas between the joist have no pressure below them but, instead, are simply being pulled along for the ride – at different speeds.

The linoleum – I doubt that is the problem but even with thinset beneath your backerboards it can still cause movement depending on the type of linoleum. You stated that your house was 35 years old so it’s possible that it was ‘cushioned’ linoleum. If so there is no way short of tearing it out that will eliminate all movement beneath your backerboards.

Your subfloor – another possibility beneath the linoleum. Since you did not remove the linoleum you do not know what shape the subfloor is in (unless you can see it between the joists from below) nor how the joints in the subfloor are staggered. You also do not know whether or not the linoleum has full contact with the subfloor or if there are voids. Another strong possibility is that the linoleum is installed to ‘luan’ which is a very thin laminated plywood commonly used beneath linoleum. If this is the case it is very likely that there are voids between the luan and the subfloor allowing movement of everything above it.

A joist – You did not state whether the area of the fault line is one on which you walk quite a bit or one that sees very little foot traffic. If the latter it may be a joist directly beneath that fault line which is expanding and contracting an unusual amount. It’s very rare but I have seen it. If this is the case (you’ve eliminated the above possibilities) you can ‘sister’ that joist with another joist screwed directly to it on either one or both sides of it. This will help stabilize that joist and possibly eliminate the problem.

The most likely cause (with walls as well as floors) is not taping the backerboard seams. This would require tearing out at least one or two rows of tile along that line (depending on where the seam is). The next strongest possibility is the layer of luan beneath your linoleum with voids beneath it. A fault line of cracking grout is a very distinctive problem with only a handful of causes and it’s usually the backerboard seams – in floors as well as walls.

Unfortunately anything other than the seams will require tearing out the installation at least down to the problem layer.

There you have it – quite anti-climactic, yes? That’s why I attempt to inject extremely bad humor in my posts – I don’t want you falling asleep on me :D .  As I’ve stated before the only reason your grout would crack is due to movement (unless it is brand new, incorrectly mixed grout). The trick is to find what movement is causing the cracking. The best (only) way to prevent it is to make certain that your tile installation is properly constructed from the joists up or wall studs out. There will always be movement, the purpose of your preparation is to eliminate as much of that as possible and allowing for what is left.

If you have an idea or question which you feel may help out other people just let me know. You can simply leave in in the comments with something like ‘hey jackass, make this a post, willya…?’ or similar. Or you can shoot me an email at – I’ll answer you, I’m pretty friendly. No, really…

Besides, I need all the help I can get. :D

{ 30 comments… add one }

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  • Patsy C

    I’m putting down Mannington Adura tile with grout. Will it stain, like with wine for instance? I’m told it will be sealed and “easy” to clean and won’t crack. Is this true?

    • Roger

      Hi Patsy,

      That is a vinyl-based product. I don’t know. :D Mannington could answer that for you, give them a call on their tech line.

  • david

    Dear Floor Elf:

    I taped my Hardibacker board seams with alkali resistant mesh tape imbedded in old, expired modified thinset (Ultraflex). As I prepare to layout my tile a week later, I’ve noticed the thinset is easy to abraid with a putty knife. It just turns into dust with a bit of scraping. So out of curiosity, I wanted to see how easy it would be to pull up the tape in a seam. Pretty easy! After pulling up the tape, I moistened the thinset along the seam, which turned into a pasty mud, and scraped it off. I plan to retape that seam as I tile or before tiling (is there a requirement or preference for seams to be dry before laying tile?). Should I redo all the seams and re-tape with fresh thinset, or am I being excessive?

    Thanks in advance for any advice!

    • Roger

      Hi David,

      No need to redo it. Nothing will ever be pulling your tape up, it is made to tie in-plane movement between boards. It’s not a problem unless you can pull the tape off by yanking it across the board (sliding). You can set the tile before the seams cure.

  • Von

    After mixing a bag of grout with booster our grout was thicker than it should have been. We used it anyway even though the grout had to be crammed into the cracks, by hand, because it was so thick. My husband said he had seen it done this way and it works fine, its just a lot of extra work. By the time he was done with the shower/bathtub walls, it look like he was standing in a box of kitty litter. He swept the grout up,and used it on the floor. By then it was super dry , almost like sand. I checked it this morning and think that a lot of it will just vac right up but he will probably try to find a way to save it with water or something he won’t listen to me so I sure could use your opinion, he will listen to you. When he did the shower the grout still had some moisture to it, do you think it is Okay? Thanks.

    • Roger

      Hi Von,

      No, I don’t think it’s okay. If it was that thick then it means there was not enough liquid to fully encapsulate the cementitious particles in the grout, which means that it will not fully cure and/or never begin to cure. It’s like play-doh, it will remain pliable and never bond correctly nor cure fully.

  • NoMoreProjects!

    Hi Roger, what a great resource — great thanks for sharing your vast knowledge. Here’s a semi-related teaching opportunity for you:

    We’ve contracted with Big Box Retailer to tile our first floor. So far they seem to be doing a great job, but we are only 90% through the removal of the laminate wood floor. Most of the adhesive has been removed, and I’m seeing some cracking that worries me. They are all fairly fine (~ 1/2mm or so), horizontal only, but they extend about 63 linear feet in our 850 sq. ft. tile job. Our house is 10 years old, in southern California (to our knowledge, no noticeable earthquake damage), and the slab is not on backfill. Some of the cracking did transfer up through the adhesive, so it happened at least a month after the slab finished curing — hence my concern.

    The tile pros did not mention the cracking before they left for a week to wait for the tile. When I called to ask about a crack isolation solution, they mentioned that they typically use felt paper. Everything I’ve read online says this is wrong even in dry climates like SoCal, because it isn’t a strong enough bond between tile/thinset/floor.

    So, I asked about other solutions, and they offered me an $1,800 quote on Mapei AquaDefense. This product *seems* to recommend an optional polyester reinforcing fabric for cracks, but the retailer doesn’t sell it and this the tile contractors can’t install it. $1,800 is rather expensive for $700 worth of roll-on product anyway, so I’m planning on just doing it myself since we’re way over-budget. The question is:

    Is it better to use AquaDefense + fabric on just the cracks (3″ of fabric and 18″ of liquid on each side of the crack, extended at least 2 ft beyond where the crack stops), or just the liquid across the entire floor? AD+membrane on cracks only would be about $200, vs $800 for the liquid-only solution on the entire floor (I suppose I could add the fabric strips to that for another $50). I don’t think my budget could handle nearly $500 for fabric for the whole floor on top of the $800 for liquid.

    Thoughts, oh wise floor elf? Hope I didn’t make your head hurt as much as mine.

    • Roger

      If they are no more than 1/2mm then you should be able to use just the AD over the cracks. If you want to add fabric it definitely won’t hurt. They sound like spiderweb cracking to me, which is just inconsistent curing rates throughout the depth of the slab causing spiderweb cracking. But if it transferred through the glue then you need crack isolation of some sort.

  • Tony Mazz

    Hi Roger,

    I got a grout question, yesterday I was attempting to seal my grout lines and notice the grout breakaway. So that would imply that the grout didn’t fully fill the gap between the two tiles. What did I do wrong? I used Mapei unsanded grout with polymer as the grout lines are 1/16″ and the tiles are 24″x24″

    I assumed that when I used the float that the gaps were being filled, how can I be assured that my grout lines are being filled with grout? I was hoping to paste some pics but not sure I can here?

    Any help or advise will be great thanks Tony Mazz

    • Roger

      Hey Tony,

      It could be a number of things. You can upload photos to the site here: photo upload page Just be sure to put your name and any specific questions you may have in the description box so I know who it belongs to.

      • Tony Mazz

        Hi Roger, not sure if my pictures unloaded correctly, let me know later if they did. Its titled – Grout line Tony Mazz
        I believe there was 4 pictures I’m uploaded wanted to know what my next step should be regrout (by removing all the new grout) or what. I’m surprised the grout came away so easily just by running the wheel bottle of the grout sealer. Anyway let me know thanks Tony

        • Roger

          Hey Tony,

          When you upload photos you need to let me know, I don’t get automatic notices. I use to, but I HAD to limit the amount of email I get, I just couldn’t keep up. And my wife actually forces me to do other stuff, it’s horrible… :D

          The issue is not with your grout, it’s with the grout installation. Those are what are called ‘grout bridges’. They are due to not getting the grout forced fully into the entire length of the grout line. In some areas going over it will result in grout just hanging onto the sides of the tile at the very top (face) of the tile without any grout below it. You can’t see it, it’s not your fault, it just happens if you are not very diligent when grouting. The good news is you can usually just go over those areas with more grout to fill them up, they should be fine once you do that. Let it cure then finish sealing.

  • Cecil Cook

    There is a product that DAP manufactures called flexible floor patch and leveling. On the bucket it says good for ceramic tiles. This sound contradictory to me. Since movement is bad for ceramic tiles, why would you put down something flexible in the gaps of the plywood subfloor? Am I misunderstanding something here? It first states linoleum tiles, which I can understand. Then it says ceramic tiles, almost as an afterthought. Until I read your thoughts on cracked tiles and floor seams I was going to use it. Now I’m taking it back. Or so I think. What do you think?

    • Roger

      Hi Cecil,

      It depends on what you’re using it for. You normally want something flexible in the gaps between sheets of plywood on a subfloor. It serves two purposes: wood expands and contracts naturally, the gaps are needed to allow for this movement without the sheets expanding against each other and ‘peaking’. Basically that means it will expand and rise up at the seam because that expansion has no where else to go. Filling the seams with that will allow for that movement. It also will keep thinset out of that gap as you set your sheet membrane or backerboard over it.

      Most patches state that they are ‘flexible’ to an extent. What that means, in the substrate context, is that it will compress and/or expand without causing vertical pressure or movement against the back of your floorcovering, like the plywood example I’ve stated above. With all that said (typed), if you are using it to fill gaps between sheets of plywood for your subfloor you can just use regular silicone.

  • SJ

    I’m hoping you can take some time off from one of your benders :dance: to help me with several questions. I am in the midst of a tiling project of my grand room (approx 25 by 30 feet) and have been trying to follow the instructions from the James Hardy website verbatim. My subfloor is sound and the setup (prior to hardibacker) seems to meet all the criteria (plywood grade, joists), deflection criteria). I installed Hardibacker 500 (1/2″) in a staggered fashion as instructed on thinset as indicated using correct type of screws. However, I think I made two errors. I thought I was supposed to tape the joints at the same time I was laying the hardibacker. NOTE: I have NOT yet installed my tile. So, while I DID use the 2″ alkali tape, I ran out of the Custom Blend thinset as I started installing the tape…here’s where I went stupid…instead of going to the store to buy more thinset to fill the joints and seal over the tape, I used drywall mud instead fill and to seal over the majority of the alkali tape, not the actual thinset. Second…I did keep the hardibacker 1/8″ away from walls…then (not sure what I was thinking), I filled in the space between the board and the walls with mud. Now I’m thinking when my walls expand this could cause the CBU (and tiles) to tent…and the dog to explode. Thoughts/suggestions? As I mentioned, I have not yet laid my tile. Thanks. SJ

    • Roger

      hey SJ,

      The drywall mud needs to go – both on the seams as well as around the perimeter. When your walls expand the mud will be the first to crack – it’s weaker than the in-plane stresses needed to compromise the backer. But it may eventually cause problems there. The stuff on the seams can be sanded off – the alkali in the thinset is going to compromise it eventually.

      DO NOT use any more custom blend. It’s fine for beneath the backerboard, for anything above it, including bonding the tile, it isn’t worth the bag they package it in. Get versabond for the rest of it.

      • SJ

        Thanks Roger. I just need to follow up to clarify…to remediate the problem, I need to sand out the mud that was installed between the seems (which currently has the mesh on top of it) and then also between the edge of the CBU and the walls. Then, I would guess when I’m then ready to install the tile (with Versabond), I can buy more tape and versabond between the CBUs/tape them/install the tiles all at the same time? The error of my ways was using custom blend under the CBUs, but then mud in the seams/on top of the mesh. Thanks, SJ

        • Roger

          You need to remove the mud from between the backer and the walls and at least the mud on top of the seams and tape. It is not imperative that you remove it from between the backerboards, although that would be ideal. Replace it with versabond (over the seams) and leave the area between the backer and walls open. The custom blend is fine beneath the boards, nothing above them, though.

  • Kris

    I am a remodeling contractor and have a question for you. Found you while looking for information on impact damage to a framed house and crack floor tile over plywood underlayment and grout.

    I have enjoyed your blogs and comments. What I am looking for is information on how crack can show up later after the impact and they will only get worse. I can see the cracks but tough to catch in a photo. they are becoming more visible as dirt settles in them.

    Not a tile installer but know enough to be dangerous

    • Roger

      Hi Kris,

      It is likely either lack of a perimeter joint (tile directly against perimeter walls), lack of a soft joint or a combination of both. The impact placed in-plane stresses on the installation in the tile itself as well as placing unnatural stresses on the plywood underlayment which, as they settle back of relax, will compromise the positions of the tile relative to the substrate.

      An installation over plywood is a VERY exacting process without external stresses. If you add an impact, even in a properly constructed installation (which they rarely are) it is pretty much guaranteed to fail. I would make sure there is a perimeter joint and remove grout out of the lines every 20′ or so in both directions. This allows for natural expansion and contraction and may help release stresses currently built up in the installation.

  • Jim Edson

    What can you tell me about polyurathane grout? What about the latex additive available? What is it’s purpose

    • Roger

      Hey Jim,

      I can tell you that polyurathane grout is only used in concrete structural installations to provide flexibility to expansion joints. :D I think you’re asking about urathane grouts? Urathane grouts were created to provide a grout with a bit of flexibility – which normal, unmodified cementious grouts do not provide. As far as I know they work just fine – just not in wet areas like showers. Depending on the manufacturer you may be able to use it in exterior applications – I simply don’t know that much about them because I do not use them.

      Liquid latex additives (I assume you’re speaking of grout additives here) were created both to provide a bit of flexibility to your grout as well as (depending on the specific product) to provide sealing and stain-resistance to the grout. I don’t use them, either. I have tried ‘grout boost’ and I don’t like it. It causes my grout to fire off (that just means get hard or begin a hard cure) much faster than it normally does. This shortens the working time of the grout and, I feel, does not lead to grout lines as smooth or clean as it normally is when I grout.

      Now all this does not mean they don’t work as advertised – they may or may not – it simply means that I don’t use them so I really cannot speak to that. I do know true professional contractors who use products such as grout boost and swear by them – I just don’t like them. I prefer to mix my grout with water only and apply a penetrating sealer after the fact – I know how and why that works and am very, very comfortable with it.

  • Candace

    Wow! You got it and it is amazing that you did in only 100 rereads :oops: Us southern girls tend to ramble under stress.
    The sink is one piece with drain boards attached and yes it is 3/8 higher. I looked at Schluter trims and believe that would be perfect for my problem. Now I just need to know how to shop for it. the area that needs transitioning has no corners so is there an end cap that I should ask for, I was a little confused by their information. I did find a dealer though in my area.
    Thanks for all the help sorry for the confusion. Love all the info on your site and am really glad I came here before I laid my tile because I had bought pre mixed thinset oops. Back to Home depot tomorrow for dry kind.
    Thanks again! Have a blessed weekend.

    • Roger

      Hey Candace,

      The Rondec line has caps for the ends. They are normally used for the tops of wainscot and applications of that sort so they should work for your application. Any of the counter trims have end caps as well. You may want to call that dealer and see if they have one or the other of those lines before heading down there. Most only stock the very basic trims but can usually get whatever you need within a week or so.

      • Candace

        Looking at it now from a different angle, I think too much :-? , can the pvc version of the rondec line be mitred without a cap? thinking maybe I should carry this all the way down the new cabinet and around the corner to give it a completely finished look and installing on the other cabinet just so there is continuity/sameness with them. Hope this makes some sense.
        Thanks again!

        • candace

          I guess I was having a duh-duh-duh dumb last question. I figures that one out own.
          Thanks for all the info. I know just from reading, if I ever have a major tile job, I am hiring someone with a lot of experience.
          Have a great Halloween beware of all the little ghost and ghouls.

          • Roger

            Hey Candace,

            Glad you have it worked out. :D For anyone else yes, the pvc rondec can be mitered but it tends to look like mitered plastic when done like that. It looks much better with the corner and end caps.

            You have a great halloween too! I’m gonna dress up like a smart-ass tile guy or a platypus – I can’t decide…

            • Candace

              The kitchen counters are finished.Whew!! Not to bad, obviously not professional, probably make you cringe but it’s a rental some jackass will tear em up or out and all that stressing will have been for not.
              I just stopped back by to say Thank you one more time I did use ends not mitres by the way. I think I will stick with cake baking and decorating I’m much better at it and I do not forsee tiling as a career option in my future.
              if you ever need tips on baking a cake or making a gumpaste rose I’m your gal.
              Thanks again!

  • candace

    I had my neices husband build me new kitchen cabinets”yay”. Long story short I have a cast iron farmhouse sink with porcelain overlay and adjoining drains. When I did the dry run with the granite tile I was laying I discovered that the cabinet was off by the depth of the granite. Any ideas what I can do to transition and join these two areas and save my granite? Can’t afford new cabinets and he cut these down a little bit to try and help but no room left to cut off .

    • Roger

      Hi Candace,

      If I’m understanding you correctly (I must have read that a hundred times :D ) your cabinets are the depth of the granite – 3/8″ I’m assuming – higher than the sink. Is that correct? And you are looking for some sort of transition from the granite down to the sink? Is the sink level with the top of the cabinets (without the granite)? If so then you can either simply bullnose the edges of the granite tiles which will give you a rounded edge from the top of the granite down to the sink or you can look at Schluter trims and see if they have something that may work for you.

      Or I’ve completely misunderstood what you are trying to do, which wouldn’t surprise me :D . If it is something different or either of the above won’t work just let me know and we’ll get something figured out for you. Everything can be solved!