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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  • Trey

    Huge problem. I’ve contracted a flooring company to pour a concrete-like overlay on our conventional floor- thinking this would be the equivalent of tile but with an epoxy-cement mixture as microtopping rather than mortar bed, tile, and grout.

    To achieve levelness throughout the house there are two rooms where they put cement board directly on 2×8 plank subfloor. No mortar bed. (disaster right?). Not only that, but the boards were not staggered and they only used about 12 screws per 32” x 60” board. How do I know? We saw cracks immediately and cut an access hole out to potentially stiffen the floor from below by adding 3/4” plywood in each bay attaching to underside of the planks and seated with 2×4 rails.

    Now that everything is installed and we have those problem area, I am wondering if I can screw through the 3/8” topping, 1/2” cement board, and into the 3/4” planks, with 1-1/2 backer screws, in the pattern we should have created with the backer (grid every 6”). Our plan then would be to do another 1/4” of the overlay to cover the screw heads…

    I know this is rickety, but it seems the plywood from below will give the joists stiffness (tho expand at different rates), and the screws from above will reinforce the cement board-subfloor connection- eliminating the many number of gaps that surely exist currently. We could even screw every 4”… doesn’t matter as we would cover this monolithic sandwich with the overlay.

    • Roger

      Hi Trey,

      You’re trying to put a band-aid over an improperly installed product. First of all, this is the company’s problem, not yours. Secondly a 1/4″ layer of product is not going to prevent cracking, and screwing into plywood is not a long-term solution, it won’t last with the forces placed the way they are on a floor (constant pressure and release). Screws need to be placed into a joist to solidify any flooring. With a second layer of plywood for a tile installation you want to NOT screw into joists, but it’s because you want the allowance of horizontal expansion. Cementitious products, even as an overlay, do not allow movement without cracking (but you already see that).

      Make the company remove it and install it correctly, it’s simply shitty work and I do not believe that particular ‘fix’ will last long-term.

  • DEBI BAKER

    First, thanks to one of your tile books I bought, my daughter and I made a shower in one of our bathroom, formed the shower pan with mud, red guard and surface mounted drian, etc…. works great.

    We are now repairing an existing bathroom here with different limitations and needs. The floor in this part of the house is tongue and groove fir, thick. We need to keep any added substrate height on the floor to 1/2 ( not including tile or thinset, just all added substrate) to match up with existing height of tile in the hallway ( the hallway is realy thick, cant get it anymore, quarry stone that was mortared directly onto the tongue and groove subfloor in 1972. We need to tile in a new shower stall and bathroom floor. The shower will be curbless and have same tile as bathroom floor.

    I bought a Kerdi sloped shower pan and linear drain, and after realizing that water could get around the sides of this, I am now going to have it slope to the back wall of the shower, high end at shower stall opening. So I bought a Kerdi sloped ramp to get back to floor height. The shower is 3’x3′ .

    I had planned on having 1/2 durarock for bathroom floor, screwed down onto the tongue and groove subfloor. But now I wonder about the whole substrate. I bet that kerdi slope does not want to go straight onto the tongue and groove floor, nor possibly the preformed shower pan. Main question is what to have as substrate ? Would it be different under the Kerdi than the rest of the floor ? Or, should the whole floor get 1/22 inch plywood and then kerdi on that and 1/2 backerboard for the rest ?

    2nd question is, for such a small shower, we are going to get kerdi board for the walls, and then new drywall will butt up against it on one wall ( used to be wood paneling). should the substraight/backerboard go on the floor before the drywall ? I know Kerdi says for the shower stall itself to put the walls in then the shower pan. But, if I need plywood first, wouldnt it be plywood, then walls, then shower pan ?

    Thanks

    • DEBI BAKER

      in the 4th paragraoh, I meant to say, should I put 1/4 inch plywood on whole floor and then kerdi on that and 1/4 inch backerboard on the rest ( 1/4 ply, 1/4cement to be 1/2 )
      ?

      • Roger

        Hi Debi,

        First I need clarification on one thing: When you say tongue and groove is it T&G plywood, or planks?

        If it is plywood you can install 1/4″ backerboard over it (with thinset beneath), then ditra, or just install 1/2″ backerboard. If it is plank flooring you will not be able to put anything over it that will be exactly 1/2″. You will need to install 1/2″ plywood over it then a suitable substrate, the smallest of which would be ditra at ~3/16″. You can also order a product called greenskin, which would only add about 1/16″ over your plywood.

        You CAN NOT use 1/4″ plywood beneath anything with tile, it simply is not stable or durable enough and really adds absolutely nothing to your flooring.

        The ramp is made to go from a specific height, up and over into the shower pan installed on the same initial height. So you do not need different heights inside and outside of the shower. And yes, you would install the plywood first, then the walls, then the shower pan.

        • DEBI BAKER

          Yes, it is planks, tongue and groove fir, solid 2 by 6 wood and it feels stable. Upstairs bathroom floor I put 1/2 inch backerboard on the tongue and groove ( plank) subfloor and then tiled and it has been wonderful for 15 years so far….. But this Kerdi stuff is different. SO, 1/2 inch plywood under the shower and ramp, then install kerdi to its instructions …. But, the rest of the floor too ? The upstairs floor is very stable.

          • DEBI BAKER

            How does the plywood get secured to the subfloor ?

          • Roger

            It isn’t about stability (or strength) of your existing planks, it is about movement. You can go over the rest of the floor with whatever substrate you want, but it needs the 1/2″ ply over it. Ditra will give you a shorter height, greenskin even more. But you’ll still be a bit over the 1/2″.

            • DEBI BAKER

              I am looking up the greenskin product. My understanding of ditra is that it is not good for the smaller mosaics pieces ? How is plywood substrate attached to the tongue and groove boards ? Screws of wht type and at which distance ? Thanks for the advice

              • Roger

                The limitations on ditra are tiles no smaller than 2 square inches. You want to screw the plywood ONLY into the planks, do not screw them into the joists. About every 18″ is sufficient. I use 1 1/2″ deck screws.

  • Mike

    This is pretty straight forward, but what about taping and sealing vertical joints between two tilled areas that could be wet, such as a shower curb to floor, shower seat, or whirlpool tub? Theses areas will almost always see water, and I can’t see caulking them like the other post suggests.

    I planned on taping the seams and using a roll on waterproofing near the tub and extending the membrane from the shower out a foot or two (Kerdi).

    Thoughts?

    • Roger

      Hi Mike,

      You can absolutely do that, it won’t hurt anything. It’s not normally required, but more waterproofing in that application is just fine.

  • Kimberli Brooks

    We installed 1/2″ cement boards two nights ago using modified thinset. We realize now we didn’t screw down some of the boards (we’re newbies). Can we still screw them down now that the thinset has hardened? We’d rather not have to pull them back up. Thanks for your insight.

    • Roger

      Hi Kimberli,

      Yes, you can absolutely screw it down now.

  • Maury P Vargas

    Hi Roger quick stupid question #20: there is a NTCA vid in YouTube where they push glass into notch-troweled thinset and they rock it right and left to show how the air pockets eliminate and coverage climbs to 80-95% versus swirls or sit back buttering. Now, a hardibacker/cement board is way to large for one to effectively wiggle around and close those ridges. I am assuming that’s the screw’s function? To push down that hard on the board to kill all of those little air pockets which makes things “unsupported”? Also I assume there’s no need to backbutter the hard right? Thank you Roger

    • Roger

      The thinset beneath backer does not need to have full coverage, it just needs contact. You’re comparing a 3×5 foot piece of cement to a 12″ square piece of porcelain. Porcelain requires full contact, it may chip. There will be no point load on the backer once it’s covered.

      • Maury P Vargas

        Thanks Roger makes sense

      • Maury P Vargas

        HEY Roger, sorry for being a pain on the human seat cushion, but in reading for the 20th time your step by step instructions you mention the backer screw should not go all the way through joists because “[…] your floor should be [too thick] for your screws to actually penetrate all the way through” but the ones I bought say on plywood it SHOULD go all the way through take a look at the pic, what do you think? Oh and I just found out something “Elfy”, responses are rated at 1,666 how weird is that. Thanks!

        • Roger

          Screw where the joists are not. It can go through the plywood, just not screwed into the joists themselves.

      • rick

        I applaud your patience.

  • Maury P Vargas

    Roger, have you used Wedi-type washers with cement boards? I want additional strength, would a 1.5 mm screw head height non-flushness with the board create issues while 1/4” notching and backbuttering ? I think it CAN cover it right? The thing is, some RockOn brand screw heads do not seem that solid. A 1” washer could really make a difference in diametral pressure. Sorry for stupid questions. How about a regular washer? Thanks

    • Roger

      You are an engineer, aren’t you??? :D No, I have not done that. You shouldn’t either. The rock-on screws are just fine. Stop trying to build a tank and tile your floor. :)

      • Maury P Vargas

        Ok your answer gives me confidence, so that’s perfect. Thank you. I guess 100 screws per hardi sounds ok? And on the back of my Rock-On 2-1/4” inch screws it says it has to go all the way through on plywood for “optimum performance” does that sound like it? I made a test where I screw in 3/4s” onto 1” ply and the other all the way through and tried to leverage it with the hammer claw and it seemed like the latter did better in “upward” tension, both ultimately failed and the screw did not give, neither it lost friction coefficient between the Hi-Lo trademark threads, meaning it didn’t slide out like on regular wood, but instead the ply layers crunched and unglued like a chocolate waffer cookie.

        Last question promise: can I put something NOT Tar paper between the plywood and thinset,, before screwing the hardi back on? Just so that in the future I somehow need to take a peek or remove entirely, I won’t rip layers of plywood subfloor: like rolls of a plastic bag construction grade type? I know you build things to never rip them off, but just curious if that would create a situation where the thinset solidified underneath would “float” in between screws.

        Sorry for dumb questions it’s my house just want to make sure I’m doing things the “Elf” way. Not an engineer, but just a dad who likes to read a lot. I’m an illustrator so if you’d like a a token of my appreciation I can draw a cartoon of the Elf or yourself or what ever you want! Let me know, You have my email from “get news subscription”

        • Roger

          Do not put anything between the plywood and thinset.

  • 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!

      • Maury P Vargas

        HI Roger a quick question:
        1. if the thinset sandwiched between the plywood/particle board and the upper substrate cementitious board layer is not made to adhere in any way but just “fill air pockets”, can a plastic bag layer or similar membrane be used over the wood right before applying the thinset? I am not sure the plywood was phenolic glue water resistant and I know particle wood chip boards absorb moisture, I am not happy knowing some moisture of the thinset will transfer to the plywood and maybe swell and balloon all around. Let’s say I know for a fact it’s not water grade plywood, I cannot rip it off and can’t add other layers on top of water resistant one. Is this acceptable? I have applied 4 coats of polyurethane car-like liner tough stuff like the one used for truck beds. Thoughts? Would this create a *mold sandwich*? I have been terrorized by that term and saw it on one of your shower articles but not sure if it applies to flooring…

        2. Lastly, can the thinset sandwiched be strengthened with soluble fiberglass fibers? They look like tiny blond 1” baby hair and claim to add mortar strength to any cementitious mix, plus I guess I could add acrylic white glue that also helps avoid cracks by expansion/contraction, hey why not add an alkali resistant plastic mesh while I’m at it? Thanks so much take care 🖐

        • Roger

          Hi Maury,

          1. It may cause issues with mold. The issue is that any moisture that gets below your tile will go absolutely nowhere. The moisture from thinset IS NOT nearly enough to cause any problems with your substrate before it cures – even OSB.

          2. I don’t know, I do know NONE of the thinset manufacturers would recommend it. Why would you do that? You have extra cash burning a hole in your pocket? :D There is no reason to do any of that – you’re filling empty space. It’s easier to do that with cheap sand and cement than with titanium shavings, no? They both accomplish exactly the same thing – they fill empty space. If you’re concerned about point load (if you’re an engineer), the weight distribution is more than adequate after the cement board is placed over it. In other words (if you’re not an engineer), once cured any ‘strength’ difference is moot under your cement board – wouldn’t make any difference if it were portland cement or diamonds. :D

          • Maury P Vargas

            OK makes sense. Thanks so much, I do read ALL the comments and your replies prior to duplicating efforts. It would be nice to place them under FAQs so you are not answering our silly questions twice !😋 . Yes I was worried applied pressure on one point under the backer would some how create micro-cracks in the thinset and wanted to avoid having “loose stuff” underneath as much as possible. Thanks again. I read the Flexbone answer and think that product is great, but no sell in Costa Rica, I guess I could try and order it but weight shipping costs to other countries kills me. I once ordered a Demco inertia activated hydraulic trailer tongue and ended up costing twice the $100 it took to send it to FL, just because Costa Rica taxes are 🥜 nuts! Thanks so much take care, gotta love the FloorElf! 🧝‍♂️

      • Maury P Vargas

        HI Roger, when I drop a coin or screw on top of the hardibacker in between the screw heads (empty area) I hear a hollow sound. Is that because 130 screws per board is too few? I cannot thinset more than the 1/4” trowel. I’m afraid that will crack tile or as you say there is no “point load” and it’s distributed equally. Once the tile is set. Cheers thanks again. Hey we should think about “Elf-Built” type of floor certification, we can do lab tests and all. Could take about 9-12 months in private lab. Take care

        • Roger

          Hi Maury,

          You are hearing the sound of the board itself, it is not indicative of a coverage (or lack of) issue beneath the backer. It’s fine.

  • Megan Casey

    Hi,

    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

            Thanks!

  • 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).