How to Install Cement Backerboard for Floor Tile

by Roger

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

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

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

Dry fitting Backerboard on floor

Dry fitting Backerboard on floor

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

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

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

Backerboards dry fitted into room

Backerboards dry fitted - notice gaps in seams

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

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

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

Installing thinset beneath backerboards

Installing thinset beneath backerboards

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

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

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

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

Installing thinset beneath backerboards

Installing thinset beneath backerboards

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

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

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

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

Backerboard screw

Backerboard screw

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

Backerboard placed into thinset and screwed down

Backerboard placed into thinset and screwed down

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

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

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

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

Now go put your dog out.

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Steven

I want to tile a screened in porch. It currently has the typical 2×4 decking. I got it flattened out pretty good but was wondering if I need to sheet it with plywood before the cement board goes down? What would be the minimum thickness of plywood I could get away with. Thanks, Steven

Reply

Roger

Hi Steven,

Yes, you need a minimum of 1/2″ plywood over that.

Reply

Jiffy

Here’s my thoughts and I know it’s a long answer for you so Thanks in Advance.
I’m planning an elevated deck in northern WI. (9ft above ground and 364 sq. ft.) it’s not covered and will be with all the elements northern WI can dish out. Even my dog.
My thoughts are to use 1/2″ durock on top of 3/4″ AC2 plywood and the joist spacing below is 12″ on center. I will be attaching the durock as per your methods above.

Am I going out on too much of a lim or do think this can be done?
All your advice you can give me would be Greatly appreciated.

Thanks
Jiffy

Reply

Roger

Hi Jiffy (if that is your real name :suspect: ),

It can be done, but there is a lot more than just backerboard involved. Google the noble deck system, or the schluter deck system. All drainage and flashing also needs to be taken into account, as well as what waterproofing membrane you choose. The structure definitely sounds doable, though.

Reply

Leon

Hi Roger,
The bathroom floor is 5 ft x 9 1/2 ft. The floor currently consists of 2×5″ lumber boards, placed across the shorter side of the room.
I would like to know if I still need to add plywood before 1/4 ” wonder board and if so’ what would be the ideal plywood thickness required.
In the end, what would be the estimated final thickness of the floor (plywood+wonder board+tile) and how would I make the transition from the bathroom finished floor to the other room?
Thanks for your help; great post.

Reply

Roger

Hi Leon,

Yes, you still need 1/2″ plywood over that. Your total thickness with 1/2″ ply, 1/2″ wonder board and tile would be approximately 1 3/16″ plus the thickness of your tile. There are several ways to transition from tile to a different height. Schluter makes several reducing trims, you can use a marble threshold, you can buy or fabricate a wooden threshold, etc. Never compromise a tile installation due to transition height differences.

Reply

Steve_in_Denver

Never compromise a tile installation due to transition height differences.

(I thought that was worth repeating)

Reply

Roger

Me too! I’ve typed it about 1000 times. :D

Reply

Leon

Thanks Roger (& Steve).
Two final questions: should I glue or cement the plywood to the wooden substrate or just screw it in? How far apart should the screws be placed?
Thanks again

Reply

Roger

Just screw it into the layer beneath it (not into the joists) at 8-12″ on center. No cement or glue.

Reply

Eugene

Can I put the backer board down over vinyl flooring or do I need to take it up? The vinyl has been glued so well I tried to take it up under the stove and could not get it all up.

Reply

Roger

Hi Eugene,

Provided you have a good layer of thinset beneath the backer and you screw it down properly it can stay. As long as it doesn’t have luan or thin plywood beneath it, if it does that needs to be removed.

Reply

Jim

Roger,

I’ve installed 1/4 in Hardibacker over my plywood floor pretty much as you suggested with thinset and proper screws. However, I noticed that my floor is still not perfectly flat. To test things out, I dry laid out my 13 in square tiles across the floor, and noticed that some of the tiles can be “rocked”, particularly where the two hardbackers meet. There are high/uneven spots under a few of the tiles which, when rocked, can produce upto1/4″ “lippage” on one or the other adjacent tile. (Maybe I could “split the difference and end up with 1/8″ on both ends.) I would like to know if you think that this amount of non-flatness can be accounted for when applying the tiles with thinset, or if I should consider using a self leveling concrete to mitigate this. Thanks

Reply

Roger

Hi Jim,

Absolutely it can be fixed as you are setting your tile. It can also be fixed with slc. Completely your choice. The slc will make it go much more quickly, but it isn’t required.

Reply

Steve

You didn’t mention caulking the perimeter. Isn’t this important ?

Reply

Roger

Hi Steve,

No, it isn’t important nor is it required. As long as you have expansion room around the perimeter you’re fine (provided you don’t fill it with thinset as you set tile). If you want to you can, but it’s not necessary at all.

Reply

mike

Hi Roger,
I would like to know if I put 1/2″ or 5/8″ plywood and then 1/4 ” wonder board over the existing 5/8″ particle board that is already down over the joist , will this be ok. I’m tiling with 18″x18″x 1/2 ” thick Travertine tile.
Thanks
Mike

Reply

Roger

Hi Mike,

It should be fine provided your floor framing is solid (and you have thinset beneath your wonder board – but you already knew that…).

Reply

mike

Hi Roger,
Just wanted to add to my last comment about my floor for tiling. The particle board on the floor right know over the joist is 3/4 ” inch I think not 5/8″. Is this ok to add plywood on top of it and then the wonder board ?

Reply

Roger

Hi Mike,

Not if it’s particle board. You need to replace it with regular plywood or at least osb. I missed that in your first comment. If you have any more please click the ‘reply’ icon at the bottom of the previous comment, it will keep your whole question thread in one place.

Reply

mike

Hi Roger,
My mistake it is OSB on the joist. So adding plywood and then thin set then the wonder board should work I guess. Should I screw the plywood down over the OSB onto the joist or does it not matter to hit the joist? I know for wonder board you say not to screw into the joist.
Thanks

Reply

Roger

Yes, that’s fine over osb. Just screw into the underlying layer with both the ply and wonder board – not into the joists.

Reply

mike

great love your site and help

Reply

mike

Hi Roger,
I have lifted the tiles off the floor and they just popped off. It was a porcelain tile and who ever did it never buttered the back of the tiles as the paper bar code were still intact and not even damaged. The tile and grout were always lifting so it seems they never held to the cement as the tile where brand new looking still on the back. I would like to know since the skim coat over the wire mesh he used shows no crack. I did notice that he did put the proper thickness of plywood down but it seems he might have screwed it over the OSB into the joist but I see some screw between the joist as well.
My question is could I screw over the skin coat that is already down sending screws to the plywood into the OSB to make sure its between the joist, thus not having to put down the wonder board since no cracks or skim coat is cracking off. Or do I skim coat under the wonder board then screw down on top of already laid wire mesh skim coat. Other choice is do I tear up skim mesh coat and then apply wonder board on top of exposed already plywood.
What do you think?
Thanks
Mike

Reply

Roger

Hi Mike,

If it is a skim coat over lath it is what we call a Jersey mud job. They’re junk and they fail – period. As you now know, that’s why yours likely failed. It needs to be removed and a proper tile substrate put over the wood, then tile. It will NOT last over that skim coat.

Emailing me asking me to answer your question will not get you and answer more quickly – day job and all.

Reply

mike

Hi Roger,
Thanks, I only emailed as it has to be done tomorrow and I have done skim coat over mesh in the past and it has lasted. I was not sure this time since plywood was put over OSB and this job was done by someone else. So I will tear up the mesh and screw down the plywood then but the skim coat with the wonder board as I have done from your instructions in the past. Thanks again for you quick reply

Reply

mike

Hi Roger,
Could I apply the wonder board screwed down over the mesh and skim coat ?

mike

Hi Roger,
I took your advice and removed the old mesh and skim coat, I also removed the old 1/2 plywood over the 3/4 ” OSB. I will replace with new 1/2 ” plywood and use glue and screw down over the OSB avoiding most joist. Then I will apply a thin layer scratch coat under the wonder board making sure my screws don’t hit all the joist also. Will be good or should I remove the 3/4 OSB before I do all this or like you said before OSB is ok. Just asking thinking 3/4 plywood might be stronger.
I noticed with OSB with a person over 250 lbs the OSB has a little movement but I guess all wood floors do with a lot of weight , don’t want grout to pop. What do you think replace OSB or leave it and maybe put some bracing between joist are sitting on walls in basement so span is only 5 feet wide with 2×8 joist.
Thanks
Mike

Reply

Roger

Hey Mike,

Leave the osb, it’ll be fine with a 1/2″ over it. Also, just screw it, don’t glue it. Other than that you’re fine, you’ll have a nice solid floor.

mike

Thanks Roger

Reply

Wendy W

I’m laying 18″ porcelin tile in a bathroom mobile home, which also has a washer & dryer in room, is there anything else I should do when laying the hardibacker? :rockon:

Reply

Roger

Hi Wendy,

You need to make sure your floor meets the minimum requirements to support a tile installation. If it’s a mobile home then it’s likely particle board under your floor – that needs to go. You need a minimum of 1 1/8″ plywood over your joists before you install the hardi.

Reply

Jerry

hello I am installing backer board on a cement floor over old glue residue , after installing with thinset on cement will the board still have to be screwed down to concrete floor thanks for the help

Reply

Roger

Hi Jerry,

Yes, it still needs to be screwed down. The thinset is only there to support it fully, not bond it to anything.

Reply

Ellen

Hi Roger,
As I prepare to lay the backerboard, is it ok to spread the thinset on the plywood if the plywood still has glue residue from the vinyl flooring I removed? I read something that said it was ok as long as the thinset was labeled ok to apply over cutback adhesive, but I feel better hearing yes or no from you! Thanks so much.

Reply

Roger

Hi Ellen,

Yes, and you can use any thinset you want. The thinset doesn’t need to bond to anything, it’s only under the backer for support.

Reply

Steve

OK , this is a 15′ X 15′ room and seems like a lot of work for one person. So can this be done in sections, or does it need to be done all at once in a single application?
Thanks for the help.

Reply

Roger

Hi Steve,

It can be done in sections.

Reply

Todd

I was putting down tile and used my cordless nail gun with the backerboard thinset underneath, barbed nails that are about 2 inches long… then put a screw “maybe 4 a sheet and put them first” nails were just so much easier. I liked your comment on avoiding joists.. I think I tried to hit joists thinking that would be better.. anyway.. it’s all good.

Reply

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