The Importance of a Properly Installed Backerboards

by Roger

Cracked floor tile due to improper substrate preparationI know there are some people out there (not my regular readers like you!) that read what I write and think to themselves ‘okay, but I’m sure that won’t happen with my installation’. So periodically I’m gonna post things like this that show exactly what happens when things aren’t built correctly. And yes, it will happen to yours, too, if the proper steps aren’t taken. If you care to see more train wrecks you can check out my ‘flawed‘ page wherein I post photos of absolutely horrible tile installations which I’ve torn out and replaced.

See that crack in the tile right there? (The line down the center is not a grout line – it’s a crack. You can click on it for a larger version) That bathroom floor is less than eight months old. It was installed with hardibacker over the subfloor and thinset. At least that part is correct, but that was about it. There was no thinset beneath the hardi and the seams between the sheets were not taped and thinsetted. To a lesser extent the correct screws were not used in the hardi – they committed the cardinal sin of using drywall screws in the backerboard. Yeah. Wrong.

So, back to the crack. (Never thought I’d ever type that!) As soon as I walked in and saw it I knew exactly what was wrong and I knew why. The crack was in an absolutely straight line – a dead giveaway that the crack is likely over a seam in the backerboard which wasn’t taped. If you read my post about how to correctly install flooring backerboards you will see that there needs to be thinset below the boards, the seams need to be taped, and the proper screws need to be used. None of which was done.

And here’s what was beneath it: Improperly prepared substrate beneath cracked floor tile

If you click on that photo (I dare you!) you can see the crack follows the seam of the backerboard exactly. Without the tape on the seams the individual boards may move in different directions and, without the support beneath from thinset, they will move independently and eventually crack your tile or, more commonly, your grout lines first.

When you tape and thinset your backerboard seams it will lock the two separate sheets together and any movements in the substrate (seasonal micro-movements, completely normal) will all move as one and in the same direction. This won’t cause any stress on your tile.

I simply pulled up all those cracked pieces and chipped the old thinset out of there, installed proper screws along the seam, taped and mudded the seam (when I say ‘mudded’ the seams I mean thinset) and reset new tiles and grouted it up. Once that grout cures it will lighten and it will look brand new.

Repaired floor tileSo all these little things like ‘tape and mud your backerboard seams’ that I throw out there may seem like it’s just overkill or taking extra precautions which aren’t really necessary – well, they are necessary. And this is why. This will also happen on a shower wall if your seams are not taped and mudded. If the boards move differently it causes uneven stress on your tile – it needs to release somewhere. Ninety seconds worth of work to tape the seam to begin with would have prevented this – just do it! (damnit)

If preventing cracked tile isn’t enough motivation for you maybe this will: all of my regular readers know what happens if your tile or grout crack – your dog will burst into flames! So if you don’t do it for your tile, do it for your pooch. Not only are flaming dogs bad for, well, the dogs, but they tend to run around and catch other stuff on fire too! You don’t want that, do you?

TAPE YOUR SEAMS! :D

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anthony

Roger,

Can you explain reason for “ditra or strata-mat rather than backer?”

Thanks,

Anthony

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Roger

They provide uncoupling from your substrate so any movement doesn’t transfer up through your tile installation, as well as moisture and vapor management beneath the tile.

Reply

Jimmy

I am adding additional framing to my tub surround before backer installation since I have 24″ centers. My concern is that the new lumber seems at least 1/16 wider than the existing framing. I suspect that it’s moisture content is many times that of the existing Arizona bone dry ones and it is swollen. How long should I wait before leveling the walls to avoid any problems from later shrinkage? Thanks!

Reply

Roger

Hi Jimmy,

More often than not a piece of lumber will acclimate to the existing environment within a week. It may still lose moisture or shrink a bit after that time, but not significantly so.

Reply

Jimmy

Thanks. I took off the high points with a plane and will use construction adhesive to act as a continuous shim on the studs. Brilliant!

Reply

Lindsey

My husband installed 1/4 haribacker boards and two areas are spaced farther than 1/8 inch apart it’s about an 1″ as long as we use thinset and the tape between will this be an issue?

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Roger

Hi Lindsey,

No issue at all, unless you run out of beer. That would be an issue. :D

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Anthony

Roger,
We bought a new home 7 years ago and the 18×18, diagonal travertine tile floor started slowly cracking about 9 months ago. It started with one short, straight hairline crack 2 feet in length and grew to some 20 feet over 3 months. Over the last 6 months hairline cracks formed perpendicular to the first crack making several 3′x5′ patterns over the entire floor. Hardibacker was likely used, but need to confirm. The cracks are inverted (not tented) and difficult to see unless seen from an oblique angle. You can run your hand over the cracks and can hardly feel them since there isn’t any separation. There are three such floors with these same hairline cracks in rectangular pattern, spanning from 5 feet to 18 feet. The joist/subfloor system likely meets L/720 requirement for stone (to be confirmed). Any insight as to why the floor took some 6 years to fail? Are the inverted cracks signs that hardibacker seams weren’t taped or tape joints deteriorated over time? Builder is in denial about workmanship given the 6 year period to fail.
Please help!

Thanks,

Anthony

Reply

Roger

Hi Anthony,

It’s simply lack of traffic (the reason it took so long). It is either seams not taped, lack of thinset beneath the backer, or a combination of both. I guarantee it is one or the other, probably both. Vertical movement causes the boards to move independently at the seams, walking on it causes the vertical movement. If it was in a hotel lobby it would have failed in 30 days, in a residence it can take a long time. But that’s the reason it’s doing that.

Reply

anthony

Roger,

Thank you for your insight, it’s extremely helpful! Before the travertine cracks were visible (even with close inspection), inverted stress lines appeared in the tile as though the tile was being pulled down at points directly over the cement board seams. The tile gradually cracked along these stress lines and still appears inverted as opposed to tented. Any thoughts on why the cracks are inverted instead of being tented?

Anthony

Reply

Roger

I would guess that your backer seams are located between joists, are not taped and there is no thinset beneath it. Walking on it caused differential movement, cracking the thinset beneath those areas causing a weak point then the tile gave out without full support beneath them.

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Anthony

Roger,

The plot thickens. Subfloor is 23/32″ thick, joists are 16″OC with support every 6-9ft so if my calcs are correct it meets or exceeds L/720. I carefully mapped out all the tile cracks and found they form 4ft x 8ft patterns (not 3×5) throughout the tiled floor areas. I then verified that the subfloor panels are located EXACTLY beneath the cracks. The travertine tile is 3/8″ thick with an underlayment (can’t tell if its’ backer and thinset or just mortar) that is 11/16″ thick for a total thickness of 1and 1/16″. Would appreciate your perspective.

Thanks,

Anthony

Reply

Roger

Then is sounds like:

1. The seams of the backer (if there is indeed backer) are not offset from the plywood panels
2. You only have a single layer of plywood beneath the tile over the joists
3. If there is backer the seams are not taped and there is no thinset beneath it.

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Anthony

Roger,

The crawl space was extremely damp during construction and I doubt that the OSB subfloor panels (and travertine tile for that matter) were allowed to acclimate or dry out appreciably. I know this because some sections of the hardwood floor developed significant cracks between planks two years after installation – yes, it’s the house from hell ! It was determined that the 3/4″ oak planks were not properly acclimated and the crawl space below the failed sections was extremely damp during construction. The builder tore out these hardwood sections and replaced them and also installed a moisture barrier in the crawlspace. The hardwood floor restoration turned out extremely well and has remained so the last 4 years. If the OSB and tile were installed with a high moisture content and slowly dried out and shrank, particularly after the moisture barrier was installed, would it likely be a contributor to tile cracking?

Anthony

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Roger

It always can be. That coupled with regular seasonal movement and load weights on the floor. It’s normally a number of factors rather than one single reason.

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anthony

Hi Roger,

What would be your recommended installation method for Travertine Stone Tiles (3/8″ thick, 18″ on diagonal)? Please include cement board type and thickness, thinset type and thickness – below and above backer. Assume OSB subfloor meets L/720 deflection rating.

Regards,

Anthony

Reply

Roger

Hi Anthony,

OSB, 1/2″ ply, 1/4″ hardi with thinset beneath it (I would use ditra or strata-mat rather than backer), medium-bed mortar with a minimum 3/8″ notched trowel. Grout, sealer, beer.

And yes, 1/2″ ply is still required even if a single layer of osb meets L/720. Because I knew that was gonna be your next question. :D

Terry

I used the Buildex Backer-On screws on my HardieBacker but some of them aren’t 100% flush… close, but not all the way (not a measurable amount, but I can feel a slight bump with my finger). Is this going to be an issue when I lay my tile? Would it help if I put some of my fiberglass tape over them? Your input would be greatly appreciated.

Reply

Roger

Hi Terry,

It will be fine. The thickness of the thinset will more than compensate for a ‘slight bump’. If it doesn’t you aren’t using enough thinset or a large enough trowel.

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Terry

Thanks for the quick reply. I’m using 1/4 inch trowel, will that be good enough or should I consider 1/2 inch?

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Roger

The 1/4″ should be more than enough to go over the heads of the screws. Depending on what size tile you have and the flatness of it you may want to consider a larger trowel. 1/4″ is fine for smaller tiles, maybe up to 12″ square if they’re flat enough, otherwise you may want to try a 5/16″ or 3/8″. The correct trowel is the one that gives you complete coverage.

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Terry

One more question if you don’t mind. I just ripped up some old tile in my foyer and it was laid directly on the subfloor. The carpet that was in the hallway was over some old laminate tile that is on the subfloor. I’m doing tile on all of it now. There is about a 1/16 inch difference (the laminate tile is very thin) between the two areas. Will the thinset under the hardiebacker compensate for that difference or will I need to do something else?

Reply

Roger

You can definitely make up for that with thinset beneath the backer.

Reply

Caitlin

Hi Floor Elf,
I came across this post while trying to research hardibacker installation. I was wondering if it is necessary to level the floor before installing as you as described?

Thanks!

Reply

Roger

Hi Caitlin,

It does not necessarily need to be level, but it does need to be flat. Flat floors

Reply

Wiley

Hey home are you I was reading your post and am interest. I have the worst tile install I think ever in my home 4yrs old. At this point not only grout popping out and tiles cracking but I can lift out full pieces of tile. I have noticed that when I have looked under some of the pieces even the concrete backer board is cracked. I’m not sure what this means. The home inspector came out and stated no evidence of foundation issues. Any ideas I’m at a loss. Thank you in advance

Reply

Roger

Hi Wiley,

You have movement in your substrate (backer has not thinset beneath it, it’s not screwed down well) or you simply don’t have a proper bond between the tile and backer (the tile will come up with little, if any, thinset on the bottom of it – or it will come up with no bond to the backer).

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Scott

Roger
I came across this forum as I was searching for info. Since I know all these answers from my past extensive research it is nice to get confirmation I have done everything I’m supposed to.
You are a great person for taking the time to answer everyones questions. :rockon:

Reply

Roger

Hi Scott,

Normally easier to confirm before doing, but I do stuff backwards too! :D

Reply

Laurie

Hi Roger, thanks for the quick answers. We have the walls and ceiling done with kerdi board and all of the seams and washers covered in kerdi band. We bought the shower kit and will do the floor this weekend. It’s a 4′ x 4′ foam pan. The concrete floor is out of level by 1/4″ to 3/8″. The Schluter site just says the floor must be level prior to installing the foam pan.

1. Can we add more thinset to one side of the floor to level it or do we need to scrap the pan and do a mud bed? I’m afraid if we try to level it with thinset as we’re laying it, we’ll break the foam trying to pull it up if we don’t get it level or we’ll end up with air pockets under the tray.

2. Can we level the floor with thinset and let it dry then thinset the tray down?

3. Can we thinset a 1/4″ piece of cement backer to one side of the shower floor, let it dry then thinset the floor and set the tray?

I’ve read your book on how to build a mud base and am sure we can do it but thought I’d run the above by you first as they sound a bit quicker for DIYers. But, I want to do it right. Thanks for your help.

Reply

Roger

Hi Laurie,

Number 2 would be your best option. You can level it first, let it cure, then install the pan. If you do need to remove the pan from thinset do not pull straight up on it, slide it as you are pulling it up. It makes a mess, but it won’t harm the pan.

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Laurie

Thanks Roger, we leveled the floor with thinset and will install the tray this week. Here’s a dumb question. Do we install the finished shower head pipe and pipe for the handheld showerhead before we tile or after? There are plain galvanized pipes with caps on them sticking out of the wall now. We haven’t installed the kerdi pipe seals yet. Thanks for your help.

Reply

Roger

Normally after.

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Laurie

Hi Roger,

The shower is waterproofed and we did a 72 hour flood test with no leaks. I bought porcelain mosaic sheets of 1″ x 2″ subway tiles for the floor. Is there a 2″ x 2″ minimum tile size for use with the foam floor or is that just for Ditra? I can’t find a minimum tile size limitation on the Schluter site except for use with Ditra. Thanks for all your help. We wouldn’t have gotten this far without you sharing your experience and advice.

Reply

Roger

Hi Laurie,

Technically yes, 2×2 minimum over the foam pans. Realistically, though, you can install those 1×2′s over it and you’ll be just fine. You won’t have your warranty, but you weren’t concerned with that anyway. :)

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Laurie

Hi Roger. I read your kerdi shower book but you don’t mention using kerdi board so I thought I’d ask you about it. We are getting ready to tile a 4′ x 4′ x 8′ tall shower that has 4 framed walls and a glass door that will be hinged to the jamb. We’re going to use 1/2″ kerdi board over wood stud framing (after straightening the studs per your book). We’re also tiling the ceiling. Here’s my questions:

1. The kerdi boards seem a little flimsy, have you used them? They’re very expensive but seem like they’d be a lot easier and faster than cement or drywall backer with kerdi membrane for waterproofing. Just waterproof the seems of the kerdi board with kerdi and thinset and it’s done.

2. Should we install kerdi screws with washers 12″ apart or closer than that? Our framing is 13″-16″ on center but in a couple of spots is 19″ (where original plumbing was and where it was rerouted to).

3. Should we use kerdi fix at the corners and ceiling (changes of plane) and where the horizontal sheets meet before waterproofing the joints with kerdi and thinset?

4. I’m assuming that we need to attach kerdi board to the wood door jambs using the same screws and washers. Since we don’t know where the hinges will be installed, I’m concerned that the placement of the washer could interfere with the door hinges on a frameless shower door. Should we just use cement backerboard and kerdi membrane on the jamb where the hinges will be located or would placing the washers as close to each side of the jamb as possible leave enough room for the hinges? The door is on a 2 x4 stud wall.

Thanks for your time.

Reply

Roger

Hi Laurie,

1. Yes, 90% of my showers now get kerdi-board. It is faster, easier and cleaner than backer or regular kerdi. Once tiled they are just as durable and stiff and any backer.
2. 12″ apart is just fine.
3. It’s not required, but I do.
4. Use kerdi-fix to attach the board to the studs then put a screw three inches from the bottom, one in the center and one at the very top. Your hinges won’t hit them.

Reply

Tom

Any suggestions for seam cracks on properly taped and thinsetted seams?

I’ve got a floor with cracks showing up around one particular piece of backer….literally up both sides…tiles in center all fine of course….but I did tape and put thinset on very tight backer formation….

The previous sub-floor was of the old metal mesh screening variety thus very thick….nearly 2 inches….I had my frame guy go under the floor in the crawl and reinforce crosswise on the joists….

I’ve got a bag of ultra-flex and am planning of re-taping and mudding then using the flex bond thinset….but since I’ve never had this happen I’m still a bit gun-shy about going ahead without more information.

Advice or thoughts….?

Thanks

TF

Reply

Roger

Hi Tom,

You said “a very tight backer formation”, does that mean they were butted to one another? And do you have perimeter expansion gaps in your backer and tile?

Reply

lauren

Oh, also.. i’m tiling around a bullnosed window jamb.
Can I stop the backerboard before the curve, bridge the edge of backerb and jamb with thinset, and wrap the tile around the curved jamb, applying tile directly to block?
There is not much clearance for material between the window hardware and block jamb.

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Roger

You can paint redgard directly onto the block. Provided you have redgard over it yes, you can bridge the edge with thinset.

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lauren

Hi Elf,

I am putting mosaic tile in my tub surround, and the long wall is cinderblock, which I have removed most of the paint from. The plan is to use 1/4 wonder board over said wall. Should I use Redguard on the wall before fastening cement board?
My husband wants to use contractors glue to hang the wonderboard; I say thinset, and then a few of those expensive blue cement screws.
Any thoughts?

Thanks,
Kellie

Reply

Roger

Hi Lauren,

Thinset and tapcons would be the best way. Install the redgard over the backer after you get it installed.

Reply

GW

I am very new to tiling. I used Simple Set Thinset Mortar underneath my hardibacker 1/4″ boards. Now I am reading that Simple Set thinset is really mastic and not thinset mortar! Am I ok? It’s a floor installation in my entryway. I live in Wisconsin, so we will get snow tracked in. (Which is why we are tiling that area!)

I have not laid the tiles yet.

Reply

Roger

No, it’s not ok. You need powdered thinset that is mixed with water. You can try it, but you may end up taking it up and repairing it. Better to do it now.

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GW

Thanks for the answer. Not what I was hoping to hear, but I want to get it done right and cracked tiles will drive me crazy if it comes to that….

So, I need to remove the hardibacker screws and pull up the hardibacker, remove the (stupid) simple set pre-mixed thinset which is really mastic and put down real thinset, put the hardibacker on again and re-screw it in….. do I have that right? And I will be able to re-use my hardibacker right?

Reply

Roger

Yes. To both.

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Darren McQuay

I have this exact problem!! Hardie backer 1/4″ was laid over top of linoleum and screwed down, (no thinset between) he did tape and thin set seams, but that did not stop movement.. I have approx. 20 tiles with straight line cracks through them. The tile is 6 months old.. Can this be repaired as well?? Or does it all need to be ripped out?

Reply

Roger

Hey Darren,

The problem lies with lack of thinset beneath the backer. Backer can be installed successfully over vinyl, but it absolutely needs thinset to fill any voids. The only way to fix it permanently is to remove it and use backer with thinset beneath it. The good news is that the backer will come up easily. :D

Reply

Darren

Also the same guy installed hardwood for me, an it’s tight! I’ve read a 3/4″ gap is fairly standard, do you think the hardwood is playing a partial cause in the tile cracking? Or separate issue?
Thanks

Reply

Roger

It could be part of the problem, but no thinset is your main problem. There should be a 1/8″ gap (NOT 3/4″) between your hardwood and tile and it should have silicone in it.

Reply

john

Can I run Hardi backer into the mud bed of the shower? You have mentioned on the site that you can. However, the Hardi installation manual states you are supposed to run it 1/4″ above the bed and caulk.

It would be much easier to run it into the bed, above the pre-slope. The bed will hold it against the studs, it is easier to mark the level line on the backer board, and I am concerned about the mud pushing out into the stud areas.

Reply

Roger

Hi John,

Yes.

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Charley

First off – This site has been very helpful to me on my shower rebuild project – so thank you – I do have a question related to this article above. I just put up and patched the hardibacker seams and screw heads using thin set and the required tape in the new shower I am building. All the screw heads are counter sunk. After the thin set dried its kind of sandy and If I rub my hand on it it kind of sands off. Im not sure if this is how it should be or if something is wrong with it. I plan on putting redguard over everything but IM wondering if I should be concerned about the thin set issue.

Thanks for any help
Charley

Reply

Roger

Hey Charley,

It will be a little sandy, it’s normal. As long as it isn’t falling off the wall it’s not a problem.

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Laurel

We decided to go with tile for the shower. We’re looking at a limestone basketweave for the floor and porcelain on the walls. Is limestone a bad choice for a shower floor, I’ve heard it is porous and requires a lot of maintenance and special cleaners? Is there anything different about installing limestone correctly over kerdi? And my final question, do you have an opinion on the kerdi trays since you just spent some time at the Schluter facility? Thanks for your help.

Reply

Roger

Hi Laurel,

The particular limestone you choose has a lot to do with it. You need to contact the manufacturer to ensure the limestone is not water sensitive. They should be able to tell you.

My opinion of the trays hasn’t changed. They are a good product and work well, I just don’t use them. I’ve even seen someone drive a forklift over one with tile installed on it (true story), They just aren’t a product that fit with most of my projects.

Reply

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