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.

{ 1721 comments… add one }

Leave a Comment

  • Kent

    Thanks for the article. Remodeling my bathroom and have ripped up all the old flooring straight down to the subfloor directly on top of the joists. Do I need to put down a think piece of plywood to put the thinset on? I’m not sure I would want to put the thinset directly on the subfloor because then it would have to be replaced if we ever wanted to redo the tile in the future.

    Thanks.

    • Roger

      Hi Kent,

      Another 1/2″ layer of ply should be sufficient.

  • Timothy

    Roger – I have some 4 x 8′ hardi cement board smooth siding left over. Can I use that as floor underlayment?

    • Roger

      Hi Timothy,

      No, hardi does not recommend that. I have no idea what they do differently to the siding product, but they are fairly adamant about that.

  • Dave

    I am installing 3/4″ oak over 5/8″ plywood on top of our concrete slab. In the rooms that will be tiles do I install 1/2″ cement board over the concrete to minimize the elevation differences in the height between the tiled flood and the hardwood floor? Thank you

    • Roger

      Hi Dave,

      You can use a mud bed, ditra xl or slc. Backerboard should not be used over concrete.

  • Matt

    Great article! I gutted our bathroom that was made in early 80s. We pulled up the original vinyl floor before we knew about the asbestos worries. Not sure if it is asbestos or not. I’m going to install new subfloor for shower and toilet as it was rotted. The other half of subfloor is ok and 3/4 inch thick but has that adhesive from vinyl squares. My question is can I thinset over the plywood with adhesive then install cement board or do I need to put a 1/4 plywood over 3/4 then cement board? Or rip it all out and start fresh? Thanks so much :rockon:

    • Roger

      Hi Matt,

      Provided your floor is adequately strong you can go right over everything with thinset and backer.

  • c

    Hello. I am putting backboard down in your kitchen and wanted to know how much space to I leave between the wall and backboard. The subfloor is about “1” from the wall. Which I believe is to far from wall? So my questions are
    Should I add to subfloor to fill wall gap?
    What is the recommendation measurement for wall gap?

    • Roger

      Hi C,

      1/4″ is just fine. You should fill that remaining 3/4″ gap with more wood or thinset.

  • chukker

    Thanks for the instructional: clear, concise, intelligent. :evilb:

  • Matt

    Hi Roger,

    Really good stuff here. I’ve experienced first hand what happens when you skip a single step in this process (i.e., – Hairline cracks on more times than you care to count).

    I just moved into a new house and am ready to tackle the floors in the mudroom and adjacent laundry room. I can already see that there is asbestos tile under the old green carpet in the laundry room. I planned on covering over it with thin set and cement board as you’ve described.
    The problem I’m running into is that the subfloor consists of two different materials because of a previous addition to the home. Half of the laundry room has a plywood subfloor while the other half is concrete slab. I have no idea how to approach this while keeping the floor level for tile installation.

    • Roger

      Hi Matt,

      You’ll need to use the same type of underlayment over the entire area to remain level, cement board won’t work. You can go over all of it with something like ditra, just be sure to place a soft joint over the break in the two different substrates.

  • shawn

    Hey do I still need to lay down concrete board if I’m laying tile down on a concrete slab?

    • Roger

      Hi Shawn,

      No.

  • Jeremy

    Roger:

    Great article! Thanks for this.

    I am renovating my kitchen. We have just pulled up old tile on the floor and it has a substrate down underneath it. The old substrate is glued down and it has staples. Under that is old laminate sheet from what we fear is from the 50’s (fear being the potential asbestos glue holding it down). Under that is the sub flooring. There are parts of the laminate that were already torn up and exposing the sub floor underneath. I would say about 25% is exposed.

    I am trying to get the old tile and substrate up so I can put the new cement backerboard down and then the new tile on that.

    Question: Can I put new backerboard directly over a laminate/subfloard following the steps you laid out?
    If So, how smooth do I need to make it (a lot of the glue and some small chunks of the old substrate are left behind).

    Thanks.

    • Roger

      Hi Jeremy,

      As long as you can get any inconsistencies down to less than 1/8″, then use a 1/4″ trowel to place thinset beneath your backer, it’ll be just fine.

  • Kevin B

    This is the best explanation I’ve read yet I how to prep a floor for tile!

    My kitchen has a couple of dips (1/4″ in places) and I was wondering if I should use a combination of 1/4 & 1/2 board with leveling mud to bring those dips level?

    • Roger

      Hi Kevin,

      I would use one or the other, just have more thinset under the areas that dip.

  • Brian

    I just pulled up our old kitchen tile and see that they did not leave any spacing between the hardiebacker but did leave the 1/4″ space between the backerboards and walls. The floor is probably 10+ years old and I only moved in 3 years ago. They used thinset underneath the backerboard but did not tape the seams. There was a crack in a straight line under 2 tiles that were directly over a backerboard seem. Here’s my question…Will taping the seams even though the backerboard is butted together solve the problem or do I need to do something else? Subfloor is 3/4″ plywood and joists are 2×8 16″ OC. The backerboard where the crack happened with one backerboard with a lot of foot traffic and the other having a fridge sitting on it (no foot traffice).

    • Roger

      Hi Brian,

      I would carve out about a 1/16 – 1/8″ gap between them, then taping and mudding over that.

  • Hank

    Okay Elf…THAT’S IT! You’re obviously having far too much FUN here! From now on; NO TALKING, NO LAUGHING!
    Actually, I applaud and appreciate the concise and helpful information you provide. I’ve been a carpenter for over 30 years, which of course means I already know EVERYTHING, but now I discover that there’s someone out there who knows even MORE! I find that really annoying. I’m willing to let that slide because you ARE doing a good job. However, I would thank you to stop conspiring with my dog, and quit sending him those trick re-igniting birthday cake candles! Every time I put him out, he turns, grins and bursts into flames again!
    That’s very funny you guys; NOW KNOCK IT OFF! I’m gonna have to restrict his phone privileges and if the cat will ever give me the password, I’m going to install parental filters/blocks on the computer. Maybe that will bring a halt to the bad influences lurking about in this cyber-space world.
    Until then, keep up the good work, have fun and remember: “Never get good at anything you HATE doing!!”

  • Darrell

    thank you very much for the great advice…. and the humor made it more of a good read… again thank you I hope I can do this in a mobil home…. I tore up the osb and replaced it with 3/4 inch plywood… if you have any advice, please email me….

    Darrell

  • Emily

    Hello,

    Thank you for the article. I have old asbestos tile over a concrete slab. In doing some reading (on the internet, so I’m not quite sure how accurate it is), I am under the impression that covering the asbestos tile is better than taking it out. I also read that I’ll need backer board between the asbestos tile and the new tile I want to put down. My questions is how do I screw it down? I don’t want to disturb the asbestos tile any more than necessary. Also, because of the concrete slab, do I need 1/2″ backer board or will 1/4″ suffice?

    Thanks so much!
    Emily

    • Roger

      Hi Emily,

      You need backer over it on top of a wooden substrate. Over concrete you want some sort of membrane like ditra or strata-mat. Backerboard can not be used over concrete.

  • Glen

    Hi Roger.
    We are redoing our bathroom floor. We took up the old tiles. Can we use mortar instead of thin set under the backer board?

    • Roger

      Hi Glen,

      Yes. Unless you mean something like mason’s mortar – then no. It needs to be tile mortar.

  • Peter

    Hello Roger,

    After you screw down the 1/2″ backer board on thinset, is the height still 1/2″ or do you need to allocate an extra 1/8 or 1/6th for thinset?

    I’d like to match my tile to wood flooring, and need to buildup 1-1/2″ on top of 2″ concrete floor with sleepers (with radiant floor heating). Two layers of 1/2″ backer board would work, but if layers of thinset add height, then I may not be able to match levels ?

    Peter,

    • Roger

      Hi Peter,

      It’ll normally be less than 1/8″, depending on the trowel size. You can use 1/2″ and 1/4″ backer instead?

  • Ivan

    Roger,
    I am trying to redo my small bathroom floor and tile half the walls(which are painted). The shower is already tiled and with a tub. My house sits on a concrete slab and I am halfway done busting up the current tile floor, and from what I can see there is concrete underneath or some sort of hard thin set. Also the tiles are a pain to get busted up… also it doesn’t help that they are like 2x2in.

    Question is:
    I assume that I can lay the new tile on top of the concrete/mortar slab?
    Also can I scar and sand the walls real well, than put on wall tile?
    Any proper “half wall” tiling height?

    Thanks!

    • Roger

      Hi Ivan,

      Yes, you can go over the existing slab. Yes, you can scuff up the walls and install tile on them. Normal height is just above the switches, wherever they lie. Normally 4′ – 4’6″.

  • Scott

    Hello Roger,
    Once the concrete board is installed, due you need to provide time for the mortar to dry before you start tiling? Or can you begin to tile immediately since the concrete board is firmly secured by the screws?

    • Roger

      Hi Scott,

      You can tile immediately.

  • Eric

    Hope I’m not wearing out my welcome, but one more question. I have torn out a shower insert and we are going back with a tiled shower. I have done pans, curbs, etc. But we are using cement board for the walls which will be tiled, but I know that can’t be finished very well. So should I use the cement board up to the tile height and then use standard sheetrock (or maybe greenboard) above it – with the joint just below the edge of the tile maybe??

    • Roger

      You can do that, or you can simply prime the backer that is exposed. Once primed you can finish it just like regular drywall.

  • Eric

    I am redoing my girlfriends bathroom – home was built in 52 and bathroom has been remodeled a few times. It if very sturdy, but I am dealing with a few things. I have pulled up the underlayment or top layer of floor which is 3/4″ plywood – this was due to (not dog) but cat pee issues which had gotten beneath the vinyl flooring and well – that’s just nasty and it had to go. Underneath, the original subfloor which is (oak boards run diagonally) is still present in a few areas, but the majority of the bathroom has had 5/8″ play which has replaced that. All is very sturdy and tied to 2×10 joists. It is actually very level and strong all the way around. So my plan is to go back with the original thickness 3/4″ plywood for underlay and then the cement board over top. Does this sound like the way to go? I had thought about dropping down to 1/2″ ply and then the cement board over – maybe the 1/4″ to get back to the same height, but its really not a concern anyway, since all pluming locations are being moved anyway for a new bathroom lay out, so I can set them whatever height I want. So in my thinking – go back with the 3/4 ply and then the 1/2 cement. Thoughts?

    • Roger

      Hi Eric,

      That would be perfect. You can go back with the 1/2″ then 1/4″ backer if you want, either one, or a combination of either, will also work just fine.

  • Paul F

    Great article. Very helpful for the project I am about to undertake.

    I’m about to tile my bathroom floor with some run-of-the-mill Home Depot tile. I’ve got enough Hardie siding panels left over from siding my house (4′ x 8′ fiberboard sheets) to use as a substrate, if these panels would be suitable for this use. Would you recommend these as a substrate? It seems like they are the same reinforced material as the backerboard products I see made specifically for that purpose.

    Thanks.

    • Roger

      Hi Paul,

      I honestly don’t know, but I doubt it. I would not use a product not made specifically for tile, you’re just rolling the dice doing that.

  • Gail

    I have laminate over wood subfloor that will be replaced with tile. A couple of tile people have said we can go over existing laminate whole others have said we need to tear it up. Your opinion??? Thanks!

    • Roger

      Hi Gail,

      Really, someone told you that??? Wow.

      It absolutely needs to be removed.

  • Ruaa

    Thank you. Both floor and dog are well.
    Well-written.
    Russ

  • Lisa

    Hi, Roger – Do you have a similar article showing the proper installation of ceramic floor tile over plywood backerboard? Or might you be able to point me to another online resource that provides that information? I did try to search your site for it but only find the cement backerboard info.

    You’ve already kindly answered my very long questions in your grout cracking section. My builder insists that plywood is fine for backerboard in a bathroom or laundry room and even had the tile install company bring me the ANSI spec book to prove it to me. When I mentioned the 2 3rd party inspections, my conversations with tile stores and your site, plus my hours of googling and viewing install videos (which all recommend cement backerboard over a plywood subfloor), they implied that everyone else was steering me wrong and that they alone were experts in the field. They seem to believe that regrouting my 3 bathrooms (again) is going to fix the grout cracking (which they continue to blame on humidity and house settling of my 8 month old house).

    Thanks again for all your help. My apologies for taking advantage of your obsession with responding to all of the comments posted on your site :)

    • Roger

      Hi Lisa,

      No, I do not. It can be done, there is a spec for it. However, it is a HIGHLY SPECIALIZED installation. What that means is that there can be absolutely no leeway in following the steps and if one minor thing is missed it will fail. Even down to the specific type of plywood, which I’m certain they are not willing to purchase since it would actually be cheaper to use backerboard over what you have rather than use the proper type and grade of plywood.

      They are giving you excuses – that’s it.

  • Sharon

    I’m still not clear what I have to use to properly support my new bathroom tile floor. The bathroom floor will butt up against 3/4″ hardwood which is installed over 5/8″ plywood.
    I’ll be installing Warmly Yours heat and Noble CIS crack isolation – all will add height. Here’s my start point: Bathroom subfloor is 5/8″ ply over 2X10 joists at 16″ oc.
    My Question:Do I need another 1/2″ plywood underlayment in addition to cement board or would 1/2″ Hardibacker be adequate over the 5/8″ subfloor?

    • Roger

      Hi Sharon,

      You need another 1/2″. Cement board does not add any structural strength to your floor.