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?


Previous post:

Next post:


My installer was suppose to tear out my old ceramic tiles, lay down backer boards and then install my 3/4″ marble. He insisted it was not necessary to tear out my tiles and started to lay the marble 12×24″ directly on the marble. I am really weary of this and had them stop the project until I get a few tile experts opinion. Please let me know your professional opinion.


Steve M

So far this is the best post I have found about this topic to ask my question.
What about thinset under Durock (or similar) on concrete? I just can’t find much info on laying tile on existing concrete or how to prepare the concrete for accepting tile. I am new to your site so maybe it is covered, my apologies if that is the case, please point me to that post.
I am installing 9×24 wood plank looking porcelain tile on a slab-on-grade at my back door in a 4’5″ x 6’7″ area and am worried that the tile will be too low after carpet + pad is installed next to it.
Carpet pad at 1/2″ + carpet 3/4″ = 1 1/4″. Tile is 9 mm thick (~.35 or 1/3″) and using 1/2 x 1/2 x 1/2″ trowel at 45 degree angle yields approx .35″ depth of thinset (about 1/3″) for a total of .7 or just shy of 3/4″. I am concerned the carpet will be too high at 1 1/4″ so am strongly considering how to add a bit of height to the tile. I do not want to use any SLU here, I had to do that in an uneven section of the same floor area by the garage entry (my “new” house was built in 1964 and has some cupage around door threshold). Can I (or should I) use 1/4″ Durock or other backerboard to bring the height up a bit? Dealers choice? Just leave it and install tile on floor as is?
Using Durock I would have a nominal amount of thinset to fill voids and adhere it (hopefully? or fail?) to concrete floor plus 1/4″ Durock/cement/backer board + the 3/4″ of thinset and tile on top to make 1.1″ which would seem a better fit and only 1/4″ shy of carpet height. I am not sure screwing Durock down to concrete as is done on plywood is a good idea.
I have researched plenty of Schluter products for transitions but it seems that if I can just have the carpet tucked right next to tile that is a nice clean transition.
I have the opposite problem with SLU pad (4’x9′) mentioned by garage door – I added 3/8″-1/2″ of variable depth to level floor area. After tons of internet searches for carpet-tile height transitions I finally stumbled across an informercial video for carpet shims. I think these will work very well for bringing the carpet up to the tile. Unfortunately this tile does not have edge pieces so I need to either have the carpet right there to be tucked tightly against it or finish with an add on edge piece.
This also same area transitions into a powder room that is on a different poor (And still about 1/8″ higher – which I am hoping to transition with thinset and will be using ATR to “level” tiles and avoid lippage.) After reading about soft joints I am now concerned that this transition should have a soft joint even though it is not a large run. Four feet to joint from edge of pad then into powder room for another 4ish feet.
Can an underlayment such as RedGard or ditre solve that for such a short run or it nothing to worry about? (ha ha watch out dog!) I am planning on using RedGard for both locations for crack prevention (likely one coat vs. 2+ for waterproofing). Realizing I can do whatever I want, just trying to avoid tile installation failure.
Whew – that is a lot of questions.
Thanks for any advice, answers or suggestions on any of the above!
– Steve M


Jessica Bruce

I am about to begin a project that is a little more on the artsy side. I want to create a strip of mosaic flooring between my living and dining room. I will be using small river stones, and metal gears. The problem I am facing, is that the backer board will lift this section of flooring up higher than what is on the sides. This will be a high traffic area, so I want to do it right. The transition will be from carpet, to the mosaic, and then hard wood. Is there a thinner backer that I could use, or will I just have to pick and choose the thinnest rocks I can possibly find? Also, when I tape and mud the seams, do I do this the same way as fixing drywall?
Thank you for your time!



Hi Jessica,

Yes, it’s the same as drywall (the taping). You can also use 1/8″ WEDI, 3/16″ kerdi board, ditra, stratamat, or 1/4″ backerboard. If you want the absolute thinnest – google ‘greenskin’.



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.


Leave a Comment

;) :wtf: :wink: :whistle: :twisted: :suspect: :shades: :roll: :rockon: :oops: :lol: :lol2: :lol1: :idea: :guedo: :evilb: :evil: :eek: :dance: :cry: :corn: :cool: :censored: :bonk: :arrow: :D :?: :-| :-o :-P :-D :-? :) :( :!: 8)