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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Mark

I had to replace old baseboard using thicker base board which I had to remove some older tile to make more room then cut new tile and replace the pieces. When I took out the old ones a layer of the backer board peeled off do I just put thin set in thicker to keep level with excising tile or cut out backer board completely where the three pieces of tile was and replace the backer board which would cause a thin line gap between exciting tile.

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Roger

Hi Mark,

You can just fill it with thinset. That will work fine.

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Brett

Hi Roger,
We are installing Hardiebacker 500 on plywood subfloor. Several pieces are down and dry already.The thin set oozed into the seems between each piece when we screwed it down but it wasn’t necessarily conciously ‘packed’ into the seems…and it is now all dry. It was scraped pretty flush with the surface while wet so there is no ridge but the gaps were not cleaned out. Is it too late to tape the seems corectly to get the desired strong bond (making all cement boards move as one)? Should the thin set be notched out somehow before taping so the tape has more thin set holding onto it rather than a surface mount to a dry thin set with a wet thin set coating? Any advice would be much appreciated. Thanks!

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Roger

Hi Brett,

You’re fine to just tape and mud over that. The new thinset will bond to the existing, which has already bonded to the backer.

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T

Hi,
Enjoy your posts. You may have already answered this and if you have I apologize. Can you use 1/4″ hardibacker on the ceiling of a shower before tiling the ceiling?

Thanks-T

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Roger

Hi T,

No, you need to use 1/2″ on the ceilings. 1/4″ doesn’t have the thickness to hold up tile, it may pull through the screw heads.

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Terry

Hi, i’m laying porcelain tile (6×24) on my bathroom floor on top of 1/4 inch hardiboard. Seams are taped and mudded on top of thinset and recommended screw and recommendations d spacing. Is redguard necessary?

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Roger

Hi Terry,

No, it is not.

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Barbara

Hi Roger

I had my 20×20 tiles installed about 18 months ago in my bathroom and am just hearing a strange sound when I walk on one of the tiles as I enter the bathroom. No cracks in tile or grout. I do know he used a wet bed to lay the grout.

Since 18 months have passed, I am not sure if this is a settling issue or if it is something I need the contractor to come back and check out.

Thanks so much for your help.

Reply

Roger

Hi Barbara,

I have no idea what a wet bed for grout is – that has no definable meaning. If you are hearing ‘crunching’ then it is indicative of unbonded tile. I can not begin to determine what the cause may be without exact knowledge of your substrate. Do you know what he set the tile over?

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Caroline

Hi Roger,

We tore out my existing subfloor in the bathroom and replaced the half inch plywood with 3/4 inch. The floor joists are at 16″ on center. There is a tiny bit of movement at the seams. We are planning to install 1/4 inch hardiebacker with thinset over the plywood, and then tile with travertine. Will it be ok or do we need to reinforce the plywood at the seams between each joist? Should I tape the plywood too?

Thank you,
Caroline

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Roger

Hi Caroline,

It will be fine once you get the thinset and backer down. Be sure to offset the backer seams so they do not fall directly on the ply seams.

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Rita

Hi –
I’ve been having problems with my tile contractor wanting to skip steps. I had to tell him I wanted all the cracks taped and mudded (he hadn’t been going to do that step otherwise), and that we’d need to also redguard the bench. He did the taping and mudding for most of the shower, but when When I got back 20 minutes later, I found he’d redguarded the bench, but hadn’t put on any tape or mud! …Is this fixable? At all? What should we do?

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Roger

Hi Rita,

You can tape and mud over the top of the redgard.

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Nate

House built in 1946. I’m installing penny tile in a small upstairs halfbath. I removed old linoleum to reveal original hardwood floors. I don’t want to remove the hardwood floor because of the time and trouble. My question is do I need to installed plywood on top of original hardwood (to compensate for expansion contraction) then hardi then tile or can I get by with just hardi on top of the original hardwood? Thanks!

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Roger

Hi Nate,

You should be fine going with thinset, then hardi directly over those floors.

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vicki

How do you fix the marble floor with the crack? Do you have to remove the whole floor?
the seams between the sheets were not taped and thinsetted. Do not know if the correct screws were used in the hardi.

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Roger

Hi Vicki,

What marble floor with the crack? If it’s just one tile you can replace it. If it’s in multiple areas then the entire floor likely needs to be removed and a correct substrate installed first. Kind of an open-ended question. :D

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Craig

When taping and mudding, do I fill the gap between the two pieces of hardie board or am I just tryin to get the tape to stick without filling?

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Roger

Hi Craig,

On the flat seams (same wall) yes, you want to fill the gap. In the corners and changes of plane you do not – you just want to have the tape bonded to the surface.

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Bev

I have two more questions. First, my contractor installed the tile in my new home by putting mud directly on the plywood subfloor then the tile. I questioned whether there should be waterproofing of some sort on the plywood first and he said no, everyone does it this way, just seal the grout. There is tile in the kitchen, bathrooms and foyer. Should I pull all the tile up and put Schluter Ditra down first? I regularly get water puddles in the kitchen around the dishwasher, outside the shower, and in the foyer when there is snow/rain. I don’t want my subfloor to rot. Second question is concerning my shower. It leaked like a waterfall in the basement whenever the shower was on. This same contractor did the shower and did not seal the seams in the hardiboard and also cut the rim off my custom shower base. I pulled the shower apart down to the studs and plywood subfloor. I am putting in hardiboard and Kerdi membrane on the walls and an EPS Kerdi shower tray on the floor. Please tell me if I need to hardiboard under the tray, or if I should just put thinset on the plywood subfloor and then the tray.

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Roger

You should just put thinset on the plywood then the tray. If he did that with the mud under regular floor tile – he’s partially correct. But there should be a cleavage membrane and wire lath beneath the mud bed as well.

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Bev

You say to use the proper screws but it is impossible for me to get backer board screws to be flush with the board. It is also impossible to get the board to set flush against the studs all the time. Is it ok to use deck screws because they pull the board in?
To get the screws in deep enough so that they don’t stick out I have done the following: Drill a hole through the board with a bit smaller than the screw diameter, then go over the holes with a larger drill bit to indent the board where the screw head will sit. Then put the deck screws in. The screws went in past the surface of the board on most of them. This is a shower wall and I will be putting Kerdi membrane on the Hardieboard.

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Roger

Hi Bev,

Why is it impossible? They are made specifically for the board. Deck screws will not screw in flush, they can not dig into the board. If the board is not sitting flush on the studs then you need to straighten your studs by shimming or sistering them first. If not, your board will simply follow your out-of-whack studs and you won’t have a flat surface for your tile.

If you want to drill those holes first yes, you can use the deck screws. It’s just a much harder way to do it.

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Maria

Hi
Is it possible to sandwich two hardies with thinset (a 1/2″ and a 1/4″) to create a 3/4″ board instead of furring the wall?
Or even using two boards straight up instead of furring with wood strips?
Thanks

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Maria

Oh, never mind :) I see you answered someone else that he could double up two 1/2″ so I guess I could sandwich them.. that saves me so much time! Thank you for a great site!

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Roger

Hi Maria,

It’s easier to furr out the studs, but you can just put up the two layers if you want. No need for the thinset between layers.

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Lili

Oh my God. My floor is a plywood flooring ( floating floring) My contractor didnt put sinset under my bakerboard and i think he used drywall screws and just screw it with A DRILL!!!!!. He said he was licensed and insured and knew better. Afrer job was finished i double checked online and got 2 opinion ( i wish i did it bedore i hired that idiot). Can i try to get some compensation for his sloppy job or contact any department in Florida to stop him from doing ” so called awesome installations for orher customers?”

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Roger

Hi Lili,

I have no idea what your local laws are. Contact the building department in your city and they can answer those questions for you.

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peter

Hello,
Ive laid backerboard about 1/4″ gap from porcelain glazed cast iron tub tub. I’ve cut tile about an eighth inch gap from tub (haven’t installed tile yet). I’m taping the backerboard joints with thinset, and wonder about the deep gap next to the tub. Am i leaving an eighth of an inch gap only to be filled with silicone caulk (if so what exact kind should I buy – too many variations at big box have confused me)? Do i try to fill the gap all the way down to the subfloor? It seems like something should go there. Need clarification, thanks! :?:

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Roger

Hi Peter,

Yes, it just gets filled with silicone. Although there are thousands (seems like it, anyway) of different ‘silicones’ out there, 100% silicone is 100% silicone, they are all nearly identical.

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John

My bathroom has pine flooring with no subfloor and 18-20 inch centers on joists. I cut out the hardwoods in all the main areas, blocked every 12″ and glued n screwed 3/4 plywood. I will use 1/2″ hardi over thinset. My challenge is that where plywood butts to some of the old floor at edges, closet,etc I am 1/8 inch low on plywood. I need a spacer or furring board. I read a suggestion of using composition roofing shingles. Just about the perfect thickness, easy to cut. I would put down thinset then shingle then thinset then hardi then screws. Seems to make sense. Thoughts? By the way… When you tape seams, what kind of tape ?

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Roger

Hi John,

The shingles will work just fine. Use alkali-resistant mesh tape and thinset to tape your seams.

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Jay

Thanks so much for the quick response..i need to pick your brain a little more now :rockon:

Theres a 3/8″ negative slope in one corner of the kitchen. Could I use self leveling cement over the hardibacker after its down?
I assume prime the HB before doing so?

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Roger

Hi Jay,

Yes. And yes. :D

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Jay

This is a great writeup!

I recently cut and spaced out 1/2″ hardibacker over a 3/4″ plywood subfloor for a kitchen with 12×24″ tiles. In a few spots the gap is greater than 1/8″, in fact in a few spots its around 1/4″. Am I asking for trouble with these gaps greater than 1/8″?

I have yet to thinset, screw and tape. Should I use something other than thinset in these gaps?

Any advice is much appreciated! Thanks!

Reply

Roger

Hi Jay,

Nope, just tape and mud those seams and you’ll be just fine.

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Keith

Roger,

The CBU installed around my tub surround showing that the tub isn’t exactly level. It’s a 1960s porcelain cast iron tub which i didnt move. I guess it’s my fault for not raising the tub up after removing and rebuilding the surround (prior water damage from before I bought the house last month) anyway, there’s a .4 in gap on one side and a .25 on the other of my short walls in the tub surround, leaning towards the inside wall. Should I just silicone up to prevent wicking, and then thinset over, and then tape/redgard down to the tub? I can’t quite get my head around how the water is going to travel and how I should compensate. If I redgard the front of the backer board there will be a .5in gap available, then there’s silicone, and a trough between the backer board silicone and the front tile caulk to collect water… I can’t really get a good example of how to alleviate this juncture online anywhere… Would you be able to help me think this through?

Thank you so much!

-Keith

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Roger

Hi Keith,

Completely fill those gaps with silicone, let it cure, then paint your redgard right down over it. There is always space behind the tile where water runs into, that’s why you leave weep holes.

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Lynn

Roger,
The Durock was very uneven (up to 1/2 inch but mostly 1/4 inch or less) in several areas between studs. Unfortunately, this wasn’t discovered until after the initial layer of redguard was applied. Is it okay to skim coat low areas with thinset over the redguard (already done on 2 walls) on the backerboard and then reapply redguard again? Or will this create the dreaded vapor sandwich? Oh, if only we had found your site sooner…Eagerly awaiting your reply. Thanks.

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Lynn

p.s. The Durock is installed on the studs around a bathtub.

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Roger

Hi Lynn,

Yes, you can do that. It won’t cause any problems provided your thinset is fully cured before the second layer is applied.

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Lynn

Hi Roger,
Thank you so much for your quick response! :) Have a great weekend!

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Evelyn Handy

Roger,
I had a and I quote ” experienced” man to install my backer board when i went to install the tile around my shower I noticed that on the left side of the back wall that is was off about 1/2 inch where the wall is in to far. I don’t want to install the tile until I know what to do to fix the problem, any suggestions. I did install the tile on the right upper quad of the the shower before discovering the problem.

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Roger

Hi Evelyn,

The best (and correct) thing to do is remove that piece of backer and shim it out so it is flush like it needs to be.

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Josh

I’m tearing out my old shower tile now and on the half wall that has the bathtub on the other side, they had hardiebacker then tar paper then wire lath then mud and then mortar and tile. Would I be able to put two 1/2″ pieces on backer board together to get the same size I had before or was there a reason they did that the way they did? They had the kerdi orange waterproofing on the wall in some spots and membrane on others it looks like and there’s no water damage I have found so far.

Thanks

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Roger

Hi Josh,

No idea why they would do that either. Yes, you can double up the backer if you need to.

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Daniel

Daniel,

I’m tiling the bathroom floor. The floor is still not level even after installing the hardibacker with mortar underneath. What could be done to fix the leveling issue?

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Roger

You’re Daniel, I’m not Daniel, I’m Roger. :D

It depends on how out of flat it is. It doesn’t necessarily have to be level, it has to be flat. If it is more than about 1/4″ out of flat you need to either skim over the low areas to flatten it out, use a self-leveling cement, or use a medium bed mortar. Anything under about 1/4″ can be compensated for with thinset as you set the tile.

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Daniel

Roger,

I installed my backer board and did my best to level out the floor with the mortar underneath the boards. My floor is still not level, there’s some places where the tile teeter totters on the floor. What do you advise before laying down the tile? thanks in advanced!

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Roger

Hi Daniel,

I assume you’re talking about the perimeter of a shower floor? If so you can sand down the deck mud to make it level. If not you’ll need to let me know what type of floor you’re talking about.

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Lee Burnham

After reading all your information you may have already answered my question. My tile guy says he uses mesh over the wood subfloor and then thin set to bond the mesh to the subfloor and then when dry uses thinset to bond the tile to that. No backerboard at all. Is that a good idea?

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bobby the builder

that would make it a seamless substrate would it not ? so i would say yes..

Reply

Roger

Seamless substrates leave absolutely no room for expansion, which every layer beneath your tile will do. It’s not a good thing to have beneath tile unless there is compensation for movement in the layer itself, such as with deck mud.

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Roger

Hi Lee,

Nope, that is unlovingly referred to as a ‘jersey mud job’. Just google it, they normally don’t last long-term.

Reply

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