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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  • Maury P Vargas

    Hi Roger, I guessed you have helped so many homeowners, pros and newbies alike that you should receive the Internet Knighthood Medal of Honor! So anyway back to my question: 1. How thick should the thinset under the cement board be in order to fill in those air pockets. Say is there a 1/8 -1/16” tops tolerance?
    Also 2. I am treating a balcony that gets CostaRican 200 avg day of windy rains per year as an outdoor shower. I’m doing the liquid flexible membrane on top of the meshed cement boards in order to create that waterproofing; I’m doing three applications of an anti-positive feedback water permeations product called “Aqua Proof” from LANCO. It’s used for watertightening swimming pools. But because the balcony’s back part hits against the wall, I’m using a 6 inch galvanized metal flashing screwed into the backer board and on the back into the Densglass with drywall screws later covered with Polyurethane roofing caulk. My question is: can’t all the grout be replaced by polyurethane caulk since the whole 300sq ft balcony is being treated as a giant swimming pool floor? I mean this would allow for far better expansion of tiles, movements and instead of eventual cracks it will all kind of be bouncy between the tiles and help? Epoxy water resistant grout is ultra expensive here and I figured I would need only about 10 tubes a 10usd each. Look at the pic, my bathtub, it been caulked between the resin and the shale stone and 3 years later looks like new no filtrations and it’s easy wipe. Thanks for everything cheers

    • Roger

      Hi Maury,

      1. If your biggest gap is 1/8″ then it should be a minimum of 1/8″. In that case a 1/4″ square notched trowel will suffice.
      2. No, you can not. Actually, you can, but it won’t last. No caulk, silicone or sealant is made to be permanent (or a wear-layer). It will, over time, lose it’s elasticity and begin to shrink. It won’t happen over three years indoors, but in 5-7 it will. In an outdoor application such as that, with all the expansion and contraction, it will happen much sooner than that. You also don’t ‘need’ epoxy, you can use regular cementitious grout.

      • Maury P Vargas

        Thanks Roger you are right!

  • Megan Casey


    I’m laying 1/2″ cement board over ply, and there’s 1.5-2″ trench around the perimeter of the ply (Old bathroom in a bar.) Trench depth varies from 1/2″ to 1.5″ and the cement board will lay over it. I have 3 questions:

    Can I fill the trench with thinset when I’m thinsetting the board on, or does it need to be done in advance of that? By the time the walls are rocked and tiled, the trench will be mostly covered.

    After I thinset in the cement board, can I continue working over it and start rocking the walls?

    And how/when do I seal the seem between the sheetrock and the cement board? It’s not a shower bathroom, but drinks get spilled and men tend to miss when they’re drunk.

    Thank you. And great articles.

    • Roger

      Hi Megan,

      Do not fill that with thinset, it’s a waste of thinset. :D Pack it with deck mud first. You can let it cure first (24 hours) or just go right over it.
      Yes, you can rock the walls immediately.
      Your best option to seal is kerdi-band. Half on the floor, half up the wall.

  • Eric Stafslien

    Thanks, I like revisiting your site. Even though I have a few tiling jobs under my belt, I just like confirming what I learned before. Cheers – Eric

  • Gena

    Thanks for your response earlier Rodger. I have attached a picture of a problem that I’m not sure how to fix, or the best way. This is the doorway of a bathroom that I’m hoping to lay ceramic tile in. The old bathroom floor was completely removed and replaced with AdvanTech but as you can see there is a pretty big gap between it and the wall. That rectangle piece you see is old subfloor, particle board I think, and it continues under the wall. What would be the best way to get this ready for ceramic tile?

    • Roger

      Hi Gena,

      You didn’t say what you are using as your tile substrate. Normally you would be fine filling that with thinset and setting your substrate over it, but it really depends on what you’re using.

      • Gena

        I’m using 1/4″ Hardiebacker.

        • Roger

          I would fill that with thinset and let it cure to get it close to flat, then the thinset you use beneath the backer will fill it in and solidify the space over that when it cures.

          • Gena


  • Gena

    Hi Roger, Should I wait 24 hours after installing the backer board before I lay the tile?

    • Roger

      Hi Gena,

      No, you can set tile immediately.

  • Chuck

    Roger, I am rebuilding a 50’s pink bathroom into a non-pink newer than 50’s bathroom. Most of the floor tile popped up easily leaving the mortar bed intact. Around the toilet the tile adhered so well that the mortar bed was damaged. I have cut out the damaged part and plan to use concrete board there and in an area where the old vanity was built in. Subfloor is in good shape. It seems likely there will be differences in height as the mortar bed is between 1/2 and 1 inch. What can I use to make it all flat?

    • Roger

      Hi Chuck,

      Self-leveling cement (SLC) would be your best and easiest solution. You can also level it all out with deck mud in the lower areas if you just want to patch it.

      • Chuck

        Thanks for the quick reply. So are you saying I could just use deck mud to replace all the parts of the old bed I took up as well as the area where the vanity was? Anything go under that, or do I just fill with the mud? No barrier or wire?

        • Roger

          Yes, regular deck mud. No barrier or wire, but put thinset down first to bond the new mud to the old.

  • Manuel Luz

    Don’t know if you still reply to theses but I have a bathroom with a new subfloor. We had to hike the floor up pretty good to get it level. Now the difference between the bathroom and hallway is pretty steep. Friend told me I don’t need thinset between the subfloor and cement board and that it’ll just add more height to my problem. Was wondering what your thoughts on it? Bathroom floor roughly 8×8, going to use roughly a little less than 3 sheets of cement board. The floor is pretty level as is so was considering not using thinset.

    • Roger

      Hi Manuel,

      You absolutely need thinset. Absolutely.

  • Chris

    The kitchen I’m tiling has a 5/8″ *plywood* (not OSB) subfloor. I installed 3/8″ exterior grade plywood over the subfloor and screwed it down (1 inch base now). I then grabbed a bag of regular unmodified mortar (by mistake) and cemented the Wonderboard to the sub floor, nailed AND screwed. I realized a day later that I didn’t use the modified thinset. I’m hoping I will be okay laying the 12 x 12 ceramic tile tomorrow. What do you think?

    • Roger

      Hi Chris,

      I know this is late, but you’re fine. Under backer the type of mortar doesn’t matter. It’s only there to fill voids, it only has to cure hard, not bond.

  • Ursula

    Thanks for the great instructions!

    I am renovating an 25 sqft bathroom. Currently, the tile is glued directly to the 5/8 in subfloor.

    My questions:
    Should I install 1/2 in backerboard directly onto the subfloor, or would it be better to first add 1/4 in plywood and then 1/4 in backerboard – or would there be a better third option? Adding a total of 1/2 in would make it flush with the bedroom floor.

    Recommeded are 1 1/4 in backer-on screws. These will be too long with my current options. Is it acceptable if the screws poka a bit through the subfloor?

    Should the backerboard also be installed beneath the shower pan (it will be a heavy acrylic shower pan from Kohler).