How to Install Cement Backerboard for Floor Tile

by Roger

The days of grabbing a three dollar bag of “thinset” and sticking floor tile right to the plywood in a bathroom are long gone (for professionals, anyway). For a proper tile installation you need a proper substrate. One of the most readily available are cement backerboards.  These include products such as Hardiebacker, Durock, Fiberboard, wonderboard and a host of others.

When properly installed on your floor it is an ideal tile substrate for a quality and lasting installation. Notice I said typed “properly installed”? Laying them down on the floor and shooting drywall screws through them does not constitute proper installation.

Choose your weapon. I prefer Hardiebacker or Fiberboard. Whichever you choose make sure you get the proper thickness. With rare exception the 1/2″ variety would be the best choice simply because I like to overbuild stuff. With proper floor framing and deflection ratios, though, you can use 1/4″ to minimize height differences. This is not to say that 1/2″ adds significant sturdiness to your floor – it does not.

Dry fitting Backerboard on floor

Dry fitting Backerboard on floor

You need to realize that cement backerboards, or just about any tile flooring substrate, does not add deflection stability to your floor. That is the up and down movement in your floor when you walk, jump, or use a pogo stick on your floor. The backerboards will not significantly diminish that movement. This needs to be addressed by adjusting your floor joists and framing – not by adding stuff on top of them. If your floor is bouncy without the backerboards it will still be bouncy with them.

Bouncy is not good for tile. (There’s a sentence I never thought I would say type.) I will, however, address deflection ratio in another post.

Start by ‘dry fitting’ all your pieces. This simply means cut and lay your pieces into the room without attaching them. Get all your pieces cut, holes cut out, and doorways undercut to fit and lay everything in there just like it will be when installed. This saves a load of time, mess, and headaches.

Backerboards dry fitted into room

Backerboards dry fitted - notice gaps in seams

The joints in backerboards should be staggered. that just means that none of the seams should line up across the room and no four corners should be placed together. By staggering the seams you add strength to the installation simply by not having a significant weak point in the substrate.

You also want to leave 1/16 to 1/8 inch gap between each sheet – do not butt them together, and around the perimeter. If you butt them together you leave no room for expansion. The backerboard will not expand, but your walls will. If everything is butted tight and your wall expands into the room guess what happens. That’s right, your dog may burst into flames and no one wants that! It will also cause your floor to pop loose and possibly ‘tent’ or peak at the seams.

Beneath the backerboards you need thinset. Just about any thinset will work but you need to have it there. skipping this step virtually eliminates the purpose of preparing your substrate for tile – you may as well go grab that three dollar bag and start setting tile now. You need it – really.

Installing thinset beneath backerboards

Installing thinset beneath backerboards

Now that you have them all laid in there properly pick one side of the room to start on and pull a row out. You should only pull out one row at a time to place thinset beneath. That way you can replace them easier and in the proper position. If you pull out the entire room you may get to the last piece and discover everything has shifted 1/2″ and the last piece needs to be cut again. Not really a big deal but you won’t realize it until the backside of it is covered with thinset and you now need to pull it up, wipe the thinset off the wall from pulling it up, cut it, clean the thinset off your saw, snuff out the flames engulfing your dog (again), and replace it. It’s a bit easier just to pull one row at a time.

You need to trowel thinset onto your floor. I cannot overemphasize this (well, I could but you’d get sick of hearing it). This step is imperative for a proper tile installation. The thinset is not meant to ‘stick down’, adhere, or otherwise attach your backerboard to your subfloor. It is simply put in place to eliminate voids beneath your backerboard. Once laid into the thinset bed the floor becomes a solid, fully supported substrate for your tile – that’s what you want.

If you have an air pocket or some certain spot in your floor that is not level or flat with the surrounding area and you simply screw your backerboard onto it this will create a weak spot in your floor. Constantly stepping on that spot will, over time, loosen the screw and your floor will move.

When your floor moves your grout cracks. When your grout cracks your tile may become loose. When your tile becomes loose your tile may crack. When your tile cracks your dog will burst into flames – again. Put thinset beneath your backerboard. And put your dog out.

Installing thinset beneath backerboards

Installing thinset beneath backerboards

Once you have the area fully covered with thinset you can lay your backerboards into the bed of thinset and screw it down. DO NOT use drywall screws! Let me repeat that – THAT! Drywall screws are not made, nor are they sturdy enough for your flooring. You will either bust the heads of the screws off or be unable to countersink them into the backerboard. Hard to get a tile to lay flat over the head of a screw.

There are screws made specifically for cement backerboards. You should be able to find them at any hardware or big box store. They have grooves on the underside of the head which will dig into the backerboard and create its own ‘hole’ in which to countersink the head as it is screwed in. How cool is that?  If you look closely at the photo you can see the ‘grooves’ beneath the head. They are more expensive than drywall screws – just so you know. But you need to use them.

Backerboard screw packEach manufacturer has their own specific spacing instructions for screwing down the backerboards – follow them – really. Some say every 12″ and some want every 6 – 8 inches. The board you use will determine the spacing. (And its right there on the sticker so don’t tell me you couldn’t find it.)

Start your screws in the center of the board and work out. This eliminates undue stresses on the boards. If you screw all the way around the outside and it is not perfectly flat you are going to have to release that pressure somewhere and it

Backerboard screw

Backerboard screw

won’t happen until you have all that pretty tile on top of it. Working from the center out eliminates that. It would probably never, ever be a problem but if you’re anything like me your installation would be the millionth one for that one in a million occurrence.

Backerboard placed into thinset and screwed down

Backerboard placed into thinset and screwed down

Your floor is probably too thick (should be) for the backer screw to actually penetrate into the floor joist. If not, or just to be safe, do not place screws into the area above the floor joists. The plywood or chipboard which makes up your floor will expand and contract at a different rate and, more than likely, in different directions than your joists. If you screw your backer into the ply and into the joist six inches over it will cause inconsistent movement – no good. Do not screw your backerboard into your joists.

After I have all my floor down I will go back and double the screws around every seam. Just put another screw between every screw along the seams. It helps me sleep better at night.

The last thing you need to do is tape your seams. Get an ‘alkali resistant’ mesh tape – similar to drywall tape – and place it over all your seams in your floor. Then mix up some thinset and trowel it over the tape with the flat side of your trowel. Just like taping and mudding drywall. This will make your floor one large monolithic structure and lock it all together. You want alkali resistant tape so it will not break down due to chemicals present in most thinsets. I do not have photos of this because I do it as I set tile.

That’s it! Congratulations, you now have a perfect floor for your perfect tile installation. When installing floor tile – or any tile for that matter – the most important aspect of the installation is always the preparation. Everything beneath your tile is important, if any one aspect is done incorrectly it may compromise the integrity of your installation. Take your time and do it correctly, you will be much happier for it.

Now go put your dog out.

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Bob watson

Thanks for this great information. I have done the hardibacker install as described. Seams taped etc. I notice that some of the joints between the hardy are not 100 flush. When I lay a 12 inch tile across it there is a slight rocking across the span of the seam. Will the thin set compensate for that when I lay the tile or do I sand the seams somehow?

Reply

Lynda

our bathroom floor has one layer of linoleum over the plywood subfloor. Can we put backerboard over the linoleum or does the linoleum have to come out?

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Jarred

Roger, thank you for your time first of all, your devotion to helping ppl that aren’t all paying customers (aside from those who buy your books) is incredible, I can only imagine how successful your business is. :rockon: These questions may be answered elsewhere, if so I apologize. 1. Is it ok for me to stand on the substrate after laying it on top of the thinset in order to screw it down? 2. How long after properly installing the substrate can I begin tiling?

Reply

Roger

Hi Jarred,

1. Yes it is.
2. Immediately.

Get to work! :D

Reply

severein

How to tell thinset is under a cement board since the installer of it did not respond to that question?
Thanks S

Reply

Roger

Hi Severein,

Ask him again. :D Unless you have a heater vent in the floor you can remove and see the layers there is no way to do it without pulling up a tile and cutting it out.

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Sevieren

A tiler removed our small bathroom tiles and put down a water resistant cement backer with screws with square holes in them which are flat. We were told by another tiler since it was a small room not to be concerned with how it was put on the subfloor. It is an elevated room due to a basement below it. The Kitchen floor had vinyl removed and now most is cement backer also screwed in about every 6 inches and also staggered joints. We were told that one should be replaced so that thinset can be put below the cement backer as it was a much larger room. Then the cement backer then thinset with the tile set into it. When called other tillers they insisted not cement backer be used excepting on walls. What is the answer. It is like 1/2 dozen here vs 1/2 dozen there. Was told by an experienced tiler that cement board is fine as long as it had thinset below and above it. Would that work? We are looking at 12 x 24 porcelain tiles and plank sized one for a bathroom.

Reply

Roger

Hi Sevieren,

Provided you have thinset beneath the backer as well then you are just fine. But it needs thinset beneath it – no matter how small the room.

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John Walls

I’m laying down cinder block as the support for an 8″ tall fireplace hearth.
I’m using the large 8x16x4 blocks. The plan is to thinset the cinder blocks to the concrete slab and to each other. Then lay tile directly to the top and front face of the cinder block. Similar to what you describe in your book for shower curbs.
1)Will this work OK?
2) The eBook for showers says to use the gray bricks (I assume they are cinder block) without holes. Why without holes? Just for strength?
3) Is it OK to thinset the tile directly to the cinder block or do I need Hardi? The eBook didn’t say, but I’m assuming no Hardi required–right?
4) Should I leave 1/8″ expansion gap between the cinder block and the firebox face?
5) Should I leave 1/8″ expansion gap between the tile and the firebox face?
6) If I do leave an expansion gap, what do I fill it with? Probably too hot for silicone.

Reply

Roger

Hi John,

1. Provided you have flat surfaces for all your tile.
2. So you have flat surfaces for all your tile.
3. You can bond directly to the cinder block.
4. Probably.
5. Yes.
6. Silicone. It is rated for heat from fireplaces.

Reply

Vasili

Hello Roger,
I am laying tile on a concrete porch area that was carpet before. I have cleaned it and removed the glue. Using an orbital sander sanded the area to rid of all residue. It is a flat and level area. Do I need to roughen the surface for porcelain tile with PM thinset?

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Roger

Hi Vasili,

Provided it soaks in water when you splash it on there it’ll be just fine.

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John Walls

Roger,
Please help me understand the whole tile-to-concrete floor bonding issue. In the past I always thought the idea was to get a good bond between a concrete slab and tile via the thinset—i.e. thinset needs to stick well to the concrete. This is sort of implied in the above post. But now we have these decouplers that seem to do just the opposite–i.e. make sure the tile doesn’t stick to the floor, but floats over the floor. Am I misinterpreting the function of Ditra and similar products?
There are even sales presentations that compare modern decoupling products to the wisdom of the ancients that laid tile on sand—not much bond there.
I’ve even had tile guys tell me that if you have a tile crack in a slab-on-grade tile, that you should pop it up, put duct tape (yep, duct tape) on a likely floor crack that caused it, and then reset a new tile with thinset. Then the tile is supposed to float over the floor crack and not crack again—– because the duct tape disbonds the tile from the floor crack. Now if that were actually true, why not just duct tape all floor cracks before you start laying tile? I find this hard to believe, but you get the idea.
So, do I want to bond or not bond?

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Roger

You are comparing a chemical bond with a mechanical bond. Bonding directly to the slab is a chemical bond – the cement crystals bond into both the slab and the concrete. Ditra floors do not ‘float’ over the slab. The ditra is chemically bonded to the slab and the tile is mechanically bonded into it. It allows movement in the slab to dissipate before reaching the tile. But it isn’t floating.

And no, duct tape, while it can work, is not a proper way to do it. So I guess the answer to your only question is yes, you are misinterpreting the function of uncoupling membranes. Not necessarily the function, but how they work mechanically. You always want a bond to the concrete – either directly to the tile or to an approved membrane.

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John Walls

Thanks, Roger. I think I may finally get it.
1) Mechanical bond– Ditra (fleece side)-to-thinset
2) Mechanical bond (sort of)—– Ditra square pans on the top side-to-thinset. Thinset not really stuck to the Ditra here because it’s slick polyethylene, but somewhat captured simply by the geometry of that interface. This is what provides the decoupling.
2) Chemical bonds—-Thinset-to-slab, and Thinset-to-tile.
3) So, the tile isn’t chemically bonded to anything but thinset. The pattern of square pans in the Ditra keeps horizontal tile movement in check. Vertical movement of tile is restrained only by gravity? The little square pans look like they may have a slight indent of the vertical leg–maybe that helps with vertical restraint?

Is that right?

Reply

Roger

The waffles in the ditra are dovetailed, that’s what locks it into the ditra and onto the floor.

Reply

mike

Any advice for tiling a cement basement floor? Do I need cement board on top of the cement floor? The floor is flat.

Reply

Roger

Hi Mike,

No, you do not use backer over cement. If you want a membrane I would use something like ditra, but you can just go over it with a good thinset if you want.

Reply

Mimi

Roger,
Is the installation of the cement board the same when the floor is tongue and groove flooring? This is an old house with no sub floor. There is no bounce to the floor. I will be removing the old layers of linoleum down to the boards. I thought I would want to line up the seams over the joists, I am glad that is not so.
Also thanks for explaining why the slate floor we installed in the kitchen 25 years ago has cracks in the grout. Not planning on redoing it, but at least I know exactly why every time I see the cracks.
Thanks,
Mimi

Reply

Roger

Hi Mimi,

Yes, it is the same.

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Glen

If I don’t have a dog, will the wife’s cats burst into flames? Not that that would be bad thing………

Reply

Roger

Hi Glen,

We can only hope…

:D

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Daniel

Roger,

I am replacing a linoleum floor with tile. I had removed the linoleum and glue , but was worried when I put my hardiboard in my floor would be too high. So I removed the 1/4 in ply down to the sunroof and now I am dealing with hundreds of staples on the floor. I can hammer them down to be flush with the floor for the most part are they going to be a problem or will the thinset make up for any that don’t quite get hammered flush? Should I remove them instead?

Reply

Roger

Hi Daniel,

You can pound them down.

Reply

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