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

  • Glenn

    Summary. Lifted basement ceiling tile to run coax. Saw my drain from tub getting ready to go. Finally have excuse to gut bathroom and trash PINK tub!
    Found you searching floor prep for tile. Fantastic site, and fun too! Thank you.

  • Dan

    My dog just burst into flames. ,,,!! Lol thanks so much. I was under the impression that the thinset held the backer in place. Now I know better and it all makes much more sence. My dog thanks you.

  • Eileen

    How thick should I lay the thinset under 1/2 backer board, Its a bathroom 6′ by 5′ ?

    • Roger

      Hi Eileen,

      It depends on how flat it is. If it’s fairly flat a 3/16″ x 3/16″ square notch trowel will be fine. The correct trowel is the one that gives you complete coverage.

  • susie

    First shower install for me; very confusing with soooo much contradiction in the “right way” to do things!!!! Help me little floor elf! So…I have used this waterproof edging in the corners and around the bench I installed utilizing permaboard (it’s called tile tape by strait-flex). I applied with thinset on back and front similar to how you would drywall in corner, and feathered this out neatly, it has since dried and now my corners are “waterproofed”? Well, now thinking about it (in hindsight I might add), I’ve just made for myself rigid corners on plane changes. I applied the product as instructed. Did I just prove how stooopid I am? Do I rip it out and start anew, silicone sealing corners as you would suggest and grouting the tile? Please help me fix this!!!!!!!
    Susie-q

    • Roger

      Hi Susie,

      Your corner is fine. I recommend silicone in the corners to compensate for swelling in the backer, not for movement in the backer. You want both walls to move in the same direction, which is why they should be taped and mudded (with thinset) over the silicone corners. You’re just fine with that, just make sure you silicone the tile in the corners – they swell also.

      I’m more concerned about only waterproofing the corners of permaboard (permabase?). Cement backerboard is not waterproof, you still need waterproofing either behind or over the entire surface of the board.

  • Matt

    I just want to say that this site is amazing, thanks for putting up all this super helpful info!

    I have a question about tiling a surface that will not be walked on, may see some water(but not likely to be “submerged” although accidents do happen), and is horizontal. It’s inside a window box in my kitchen behind the sink. I want to use the cement board as a substrate because it seems to be the right material to use, and because the previous tile was installed over a couple layers of drywall and it wasn’t exactly flat and level when I removed it. Since this won’t be walked on do I still need the 1 1/8th” layer of ply? I don’t think I have the height for that, so I’m hoping that I can either just screw the backerboard(it will be a single piece) directly into the 2×4’s underneath or just use a single layer of 1/2″ ply over them first.

    • Roger

      Hi Matt,

      Yes, you can just screw the backer right to the 2×4 and tile over it.

  • PAUL M.

    Hi Roger,
    My daughter is having her bathroom taken down to studs and I am concerned about the floor and what is the proper underlayment from studs to tile. A contractor is doing the job and I want it done properly(obviously). It is a 1925 home and I do not know what the floor will be like when they take up mud job.
    Can you give me info from from floor joists to old sub floor and what should be over old sub floor, plywood grade etc. Top to bottom.
    Thank you,
    Paul

    • Roger

      Hi Paul,

      It should be the joists with a double layer of ply above that (AB) at a minimum thickness of 1 1/8″. A proper tile substrate such as backerboard or a sheet membrane (like kerdi) over that. Another mud bed is also an option.

      • PAUL M.

        Thanks Roger. Should the plywood be exterior and when you say 1 1/8″ does that include the 1×6 boards that are on the joists?(older homes have this instaed of plywood.
        Thanks again

        • Roger

          Yes, it should be exterior. If you are leaving the 1×6’s just put a single 3/4″ layer of ply over them, then your tile substrate.

  • Jon

    Great web site! Did shower walls with glass inlay that came out great based on your advice. I have a narrow bathroom tag I paln to tile with 12x 12 ceramic. The floor is linoleum with luan and 3/4″ ply. Can I put backer board over the linoleum? The floor is stiff. Thanks, we owe you many beers etc. :shades:

    • Roger

      Hi Jon,

      Yes, provided you put thinset beneath the backer and screw it down well. The tile will bond to it properly but you need to ensure your floor has proper support for the tile installation. You said it’s stiff, so it should, but I can’t guarantee it. Going over the line, though, is just fine.

  • Richard Burgoyne

    Roger —
    Best tile web site out there. I am re-doing a bathroom. Linoleum and gunked-up plywood removed. I am down to 1 inch plywood subfloor on top of 16 inch center joists. The plan was to put 1/4 inch hardibacker and then tiles to minimize the huge difference in height with the hardwood floor just outside of the bathroom. You say over and over that an additional 1/4 inch of plywood is needed in this setting, but given the height difference I wonder if I can use a coat of REDGARD on top of the hardibacker to allow for flex and make that additional layer of plywood unnecessary? Or I could even put a layer of REDGARD on top of the existing subfloor, then hardibacker, then an additional layer of REDGARD. Whaddyathink? Richard

    • Roger

      Hey Richard,

      As long as you get a layer of thinset beneath your backer you should be just fine. I wouldn’t even worry about the redgard. Normally with most smaller bathrooms 1″ of plywood is realistically fine. I just prefer to adhere to the 1 1/8″ minimum standard across the board for most general information. The real world doesn’t always conform to ideal standards. :D One of the biggest reasons I do that is that most people will interpret it to mean that their 5/8″ or 3/4″ layer of ply should be fine as well. I don’t want to give that impression, but your 1″ should suffice.

  • Kirk C

    Thank you Roger for the great and thorough explanation of this process. I also love how you incorporated your wittiness into it! It’s great to laugh while you learn amen!

    Sincerely,
    Kirk C

  • Mark

    Just wanted to add a note to your readers. I began a “first time” tile installation in my masterbath (100 sq ft). It required backerboard over wood and I laid porcelain 12 x 12. After reading up on the “how to’s” on your site, followed the advice. I wasn’t sure I would get a level floor using backerboard with taped seams….I was happy to find that even with screw heads that were not entirely flush with the backerboard and seams that raised up 1/8′ above the board, the mortar easily made up for the difference. What I learned on my own in the process, (1) lay out your tile in another room if you are planing to match the flow of the pattern (2) get extra spacers when doing steps – the quarter round edge alone required 1/2 a bag (3) thinner grout lines look better than thicker – suggested 1/8″, but that is harder to install when cuts are tight. (4) mixing mortar with a power drill mixer is hard, better to do smaller amounts which requires weighing your powder before adding water – PS- used a concrete hammer drill to mix a full bag, it worked, but it was tough and I could never have done it with a regular drill. (5) most important – get your advice before you start. Read Roger’s website on all prep and mixing, backerboard instructions, etc.. Being successful on the first try is sooo much better. Thanks Roger for providing this info – made our holidays sweeter!!

    Mark :-D

    • Roger

      Thanks Mark! I always appreciate when people point out things like the spacers and mixing thinset. These are things I NEVER think about because I do them so often and I have more spacers in my truck then I’ll probably ever need in my lifetime. :D

  • Omar

    Hey, I am planning on installing 12×24 porcelain tile over 1/4″ hardiebacker. The subfloor consists of 3/4″ OSB glued and screwed to 2×8 joists 16 inches on center. The span is about 9 ft. There is solid blocking between all joists as well.

    Should I put 1/2″ plywood over the OSB before the thinset/hardiebacker installation?

    Thanks,
    Omar

    • Roger

      Hi Omar,

      Yes. A proper tile installation requires two plywood layers with a total of 1 1/8″ minimum thickness.

  • Dave

    Thank you. Time to get to work…

  • GW

    Roger, Thanks for your article. I just laid my hardibacker and used thinset underneath and the appropriate screws to screw it in place. I have a question.

    I used thinset in the seams, then the tape and then more thinset and let it dry for 4 days. Now I’m reading that it’s really better to tape as you lay tile. I do have “hills” at most of my seams now (I think my thinset was getting a little dry at that point.) Should I remove the thinset and tape and redo it as I tile? Any suggestions on how to remove it if I should?

    • Roger

      Hi GW,

      Unless your ‘hills’ are more than 1/8″ then you can compensate for them with the thinset beneath the tile as you install it. You can do it before or as you install tile, both ways work. If you need to flatten it out some you can just sand down the seams with sandpaper.

  • Dave

    Hi Roger,

    Is there any reason you should not nail down the 1/4 hardy backer with nail gun and ring shank nails?

    Thanks!

    • Roger

      Hi Dave,

      It doesn’t hold as well long-term as screws, but no, there’s no reason not to other than that. It is an approved installation method. They must be galvanized nails.

  • Matt

    Roger,

    I’m remodeling a 5’x10′ bathroom on 2×8 floor joists spanning 13 feet with a crummy 1/2″ subfloor. The current 1970s mosaic tile is on a 3/4″ mud bed which seems sturdy. I want to rip everything out and tile using 12×12 marble. I’d like to recreate the mud floor to minimize any floor height differences as the adjoining hardwood floor is only 3/4″ above the subfloor. Does this sound like a good plan? Any chance you could post a blog on mud floors? What kind of material will I need (metal mesh size, type of concrete)? Should the mud floor continue under the tub?

    Thanks,
    Matt

    • Roger

      Hi Matt,

      Yes, a mud substrate would be perfect. You would need metal lath, roofing felt, sand and portland cement. It’s mixed just like deck mud, the roofing felt goes down first, lath over that then deck mud screeded flat. Mud floors are on the list of posts – it’s a long damn list. :D

  • Jeremy

    Greetings! I am installing 12×12 porcelain ceramic tile. My joists have 3/4″ planking on them, and I put 1/2″ ply over that. I will be putting 1/4″ hardie on that (with thinset!) Everything good so far? :shades: Ok, I have never used thinset before and it makes me nervous because it is cement… But anyway, I secured the planking to the joists with 2 3/4″ exterior deck screws. I then used liquid nails, and more of the same screws securing the ply through the planking into the joists. Did I mess up with expansion issues??? :eek: Other than that, following your substrate article, I should be fine, yes? Much thanks for all your info I have read so far! :rockon:

    • Roger

      Hey Jeremy,

      Ideally the second layer should just be fastened to the first – not the joists. But it isn’t gonna make your floor fail by itself. Everything else is fine.

      • Jeremy

        Thanks for the reply. I screwed the subfloor to the joists to get rid of the squeak and because the planking was not accessible… I’m hoping it doesn’t affect anything. I also probably didn’t leave a large enough gap between the subfloor and the wall, but there is a gap on one side of the room…

  • Steve

    3 questions:

    1. For a bathroom floor / mudroom floor. (2×10, 16″ OC, sistered joists, 9 foot span, Advantech 3/4″ (well, really it’s something ridiculous like 45/64)….I really need another 1/2″ on top? I will do it if I need it, but it seems pretty damn stiff to me as it is.

    2. CBU or Ditra? Ditra sure seems easier to work with. It’s a small area, so cost isn’t a real consideration — anything else I should be thinking about .

    3. What about waterproofing considerations for a bathroom and or mudroom floor? (Seeing how tile and grout aren’t waterproof) I’ve never really understood that..most places I have lived would leak water (harmlessly down into the unfinished basement) when the floor got more than a little bit wet. What to do about that? Use ditra and kerdiband the seams? Where does the water god, though? I guess you could tell me to not let it get wet, but I haven’t figured out how to do that yet. Especially in a mudroom when 4 pairs of snowy boots are deposited. Maybe I should make one huge shower pan for the whole first floor?

    • Roger

      Hi Steve,

      1. Standards dictate that an additonal 1/2″ be added to a single layer plywood floor to support a tile installation. Realistically you can normally get away with the single layer provided you have a good membrane over it.

      2. If you are sticking with the single layer use ditra.

      3. Entirely a personal choice. Short of a flood the water will simply evaporate over time rather than leaking water into your basement.

      • Steve

        Roger,

        Thanks (again), and a follow-on question.

        Two complicating factors for my installation:

        1. Mosaic tiles, so I think Ditra is out.
        2. Under floor heating (WarmWire – I have read your 2 posts on this)

        Q1: Is the combination of radiant heat and mosaic tiles dificult? (Wire bumps + no Ditra => hard to make “flat flat flat”)
        Q2: Does this make sense for the layers: 3/4″ OSB/Advantech -> 1/2″ plywood -> thinset -> 1/4″ CBU + screws -> Warm Wire -> SLC -> thinset -> mosaic tile
        Q3: What kind of plywood? 15/32 CDX? (Based on your previous comment I’m inferring that the plywood layer is mandatory, assuming I can’t use Ditra)

        • Roger

          Hi Steve,

          Correct, no ditra. The tile needs to be larger than 2″ square.

          1.Not difficult at all, you just can’t use ditra.

          2. Yes, that’s the correct method except you need the primer for the slc over the backer before you put the heating wires on. So it’s 3/4″ osb – 1/2″ ply – thinset – 1/4″ backer – PRIMER – Warm wire – slc – thinset – tile.

          3. Yes, the plywood is mandatory. It needs to be a minimum grade BC. The CDX a lot of times have voids behind the plugs leaving a hollow area in your ply layer.

          • Steve

            O.K. One final question.

            “I” (she) changed my (her) mind again. Floor is not going to be mosaic, instead 12″ ceramic tile.

            Does that change anything for me?

            1. With Ditra now back on the table I’m wondering if I can skip the 1/2″ plywood (while still using SLC and warm wire) That damn half inch matters.

            2. Any SLC you recommend? Anything to stay away from?

            • Roger

              Standards dictate a double layer of plywood at a minimum 1/8″ thickness, no matter what is going above it. That is what they want. That said, you can likely do the heat and slc with ditra over it without a problem. I like laticrete’s new nxt series levelers – good stuff. However, the custom’s brand that you get at home depot is very good stuff. Make sure you use the primer over a single layer.

  • Ken

    Hi there, this article was really good, and I could even laugh at the wonderful writing style. Now I will go out and make my purchases and hope for a wonderful tile installation and since I have no dog and WILL use thinset, I might be ok! Thanks a ton for this one.

  • TR

    Thanks for such a detailed How to. I have a question regarding putting backer board on concrete. I have been reading forums online and talking to the “Pros” at big stores about my Deli-ma. I have a concrete floor and I’m looking to tile it. Problem is, i have asbestos tiles, which im told not to touch and tile over it. I don’t like the idea at all. After reading for hours and hours online i cant find the right answer. Do you think its “OK” to put backer boards on top of the existing asbestos tiles??? You insight will be really appreciated. (Im tight on budget and cant afford a professional company to remove tiles est is 5K)

    • Roger

      Hi TR,

      Backerboard is not made to be used over concrete substrates, only wood. You can tile over them but the only way I will do it is to use laticrete 255 (or other thinsets made specifically for use over those types of tiles) and put ditra over the top of it.

  • Ken

    Hi Roger,
    Read your comments regarding cutting out square holes in tile and not having the thin pieces break off. You stated that the holes should marked and the tile set and left overnight, and then come back the next day and cut the holes. My question is, what brand and model right angle grinder do you use, if in fact that is the tool that you use. Also should I create a dam with plumbers putty and water while cutting the tile, or is it OK to just cut the holes without the water.
    Ken

    • Roger

      Hi Ken,

      I use a dewalt. Not sure offhand which model, it’s just the regular 4 1/2″ model. No need for the water, they spin fast enough to cut it dry, just make sure you have your vacuum hose going at the same time.

  • Craig

    Hi there,
    If using Cement Backer Board, do I absolutely need a 1 1/4″ plywood subfloor?

    Thanks,
    Craig

    • Roger

      Hi Craig,

      It depends on your floor framing and what you are installing over the backer. Cement backerboard adds absolutely zero stiffness to your floor, it is not a deflection substrate.

      • Craig

        Hi Roger,
        We’re installing 6 x 24 inch porcelain tile. It’s being installed on probably 5/8 or 3/5″ plywood which are on floor joists.

        Also, is Fiberock any good?

        Thanks,
        Craig

        • Roger

          In that case then yes, you need 1 1/4″ ply. You have a large format tile which needs a stiffer, and flatter, flooring substrate. Yes, fiberock is very good.

  • Blair

    Need to extend my hearth a couple of inches in depth but this means the base will be concrete and wood. Is it ok to have the backer board over both then tile on top.

    • Roger

      Hi Blair,

      You need to be able to mechanically fasten (screw) the backer into the substrate. If the concrete portion is less than three inches or so then it’s fine. If it’s more than that then you should use a membrane which is installed using thinset, like ditra or strata-mat.

      • Blair

        Thanks for the advice very cool of you. If I use the ditra, then tile on top of that?

  • John

    Hi Roger,

    Thank you for your website which contains tons of valuable information.

    I am about to tile my kitchen with 12×12 tiles (I removed the vinyl and particle board from the original subfloor).

    The existing subfloor is now 1 inch thick plywood (in the rest of the house I already installed 3/4 inch hardwood floor on top of it).

    Ideally I wanted to put 1/4 inch thick hardibacker under the tile so that the hardwood floor and tile will be level in adjacent rooms (1/4 inch of hardibacker + 1/4 inch tile + 1/8 inch thinset below hardibacker + 1/8 inch thinset below the tile = 3/4 inch total).

    Should I go for the 1/4 inch or is it safer to use the 1/2 inch hardibacker?
    Thank you for your help.
    Regards,
    John

    • Roger

      Hi John,

      1/4″ is just fine. Make sure you have thinset beneath it to fill any voids.

      • John

        Thank you ! :)

  • Vicki

    Hi,

    We’re getting the tile floor replaced in the bathroom of our circa 1926 house. The existing floor is mudded (apologies if I’m not saying this right) or in a mudbed – i.e. there is no plywood subfloor, which was a surprise to us. The joists have been chiseled to a point to accommodate the wire mesh. Anyway, one contractor we had come out said he could, after ripping out the existing floor and mud bed, remud and then lay tile. From what I’ve read, this is a particular skill but he says he has a lot of experience doing this and does it all the time (I will be checking references) Alternately, he said they could reinforce the joists and do a cement board substrate.

    My question is is there any reason to prefer one method over the other? He told us they would cost about the same. We don’t know what to do so any advice would be very welcome!

    • Roger

      Hi Vicki,

      A mud bed is still the most solid, most durable substrate for a tile installation. Given the choice I would choose the mudbed.

  • Tim

    Hey Roger! My current floor setup is 2×10 plywood I-Beam Joists 24″ OC with an 11ft span, 3/4 ply subfloor.

    I will be installing 12×24 porcelain tile (this is all going in the master bath).

    I am planning on installing 1/4 hardibacker with Laticrete 252 Thinset on top of the 3/4 ply and screwed down per manf specs and then my porcelain on top of that.
    I have 2 questions:

    1. Should I add any additional bracing to the joist i.e. cross blocking?
    2. There are a few areas of the subfloor that dip.. can I use a self leveling cement under the hardibacker?

    • Roger

      Hey Tim,

      1. No, but you should add an additional 1/2″ or plywood.
      2. Yes, unless they are less than 1/4″, in that case you can just make up for it with thinset beneath the backer.

      • Tim

        So for the self leveling cement would I just let it do its thing for the low spots, allow it to dry and then install my thinset and screw down my 1/4″ hardibacker?

        • Roger

          Yes.

          • Tim

            Will the leveling cement crack when putting the hardi screws through it?

            • Roger

              Yes. But if you put thinset beneath your backer as you should it won’t make any difference.

              • Tim

                Ok so here is now my dilemma. If I use 1/2″ ply on top of the 3/4″ subfloor, then 1/4″ hardibacker with thinset and then my tile… I am looking at a pretty good size transition to the bedroom which is carpet. Any idea on a way to make the transition not look like a sore thumb? I guess if I didnt use the 1/2″ plywood I would be fine but.. I need to stiffen up this floor. Any ideas oh wise elf?

                • Roger

                  You can use a reducing transition, schluter makes quite a few different ones. A marble threshold is fairly common in certain parts, and readily available. You can also ramp the carpet up to the tile a bit to make it less of a transition.