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?


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Hi! Thank you for your blogs.

When laying the backerboard down onto the thin set – do I aim to have the seams as close together as possible? Or does it not really matter since I WILL tape and thin set the seams?

Do I screw in the hardie backer screws while the thin set is hardening beneath the backer board? Or wait until the thin set is completely dry?

Thank you.



Hi Aimee,

You want the seams offset by at least 1/3 of the board. You normally screw it down while the thinset is wet, but you can do it after it cures.



Hey I laid hardieboard (1/2″) in my bathroom without thin setting it down. Will I be okay with just taping and mudding the seams and gaps? Or should I pull it up piece by piece and thin set each board and re-screw each board back down?

I want it to last



Hi Zach,

If you want it to last you need to pull it up and put thinset under it.

And yes, I know that isn’t what you wanted to hear. Sorry.



Hi, great forum. I am tiling my bathroom with 6×24 wood look plank tile. I have read that a medium bed mortar is recommended for the larger tiles. Do you know if this is the case and if so, can I also use the medium bed thinset under the durock as well so I don’t have to buy two separate bags? Also, what size of trowel do I use for the thinset under the backer board?




Hi Eddie,

Medium bed mortar is always a better choice. And yes, you can use it beneath the backer as well. The size of trowel depends on how wonky your floor is. Normally a 3/16″ square notch will do just fine.


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