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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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 ?

Reply

Roger

Hi John,

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

Reply

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?

Reply

Roger

Hi Jay,

Yes. And yes. :D

Reply

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.

Reply

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

Reply

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.

Reply

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.

Reply

Lynn

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

Reply

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.

Reply

Lynn

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

Reply

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.

Reply

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

Reply

Roger

Hi Josh,

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

Reply

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?

Reply

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.

Reply

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!

Reply

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?

Reply

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.

Reply

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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