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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Q

I am tiling a new construction bathroom floor. The deflection on the floor is L/480. The subfloor used was 23/32 OSB. Ditra says it is OK, to go over OSB, but I wonder if I should use Plywood or hardibacker, thinset,suntouch mat, Thinset, Tile
Or just OSB thinset, suntouch mat, thinset, Tile.

I would like to match a 3/4 inch floor in the adjoining space.

Thanks,

Reply

Roger

Hi Q,

They do tell you that, but they also assume you know (even with l/480) that you need an additional layer of 1/2″ ply over it before any of the other stuff you’ve mentioned. So ply then whatever tile substrate you choose.

Reply

Mike Mike

Hi Roger,
Want to use 1/4 ” Wonder Board. Can I apply this with a scratch coat on top of 5/8″ chipboard or should I apply a 1/4′ layer of plywood over the chipboard first then add scratch coat then wonder board on top of new plywood.
Thanks
Mike

Reply

Roger

Hi Mike,

You need to install another 1/2″ of plywood over that. The minimum thickness for your flooring substrate should be 1 1/8″. And the term scratch coat is not what you want. A scratch coat is an initial coat, usually of mud, which is allowed to cure to create a surface for bonding. It is done on wall mud applications. You just want a layer of thinset installed beneath your backer in order to fill any voids and provide a solid surface.

Reply

Ragan

Hi Roger,
I got my backer board all cut and ready to install but while removing the pieces after dry fitting. The piece around the toilet flange torqued and cracked. My original piece had a hole cut in the center for the toilet flange but now there is a crack extending from the toilet flange hole out to the edge. Is this piece kaput and needs to be tossed or can I still use the piece even with the crack in the board and simply treat it as I would a regular seam? (the crack DOES NOT follow any floor joints – is actually perpendicular to them). Any insight would be appreciated!

Ragan

Reply

Roger

Hi Ragan,

You can use it just fine. Just be sure to tape and mud over that crack and get the sides of it screwed down well.

Reply

waaa hoo

Question.

I currently have a plywood / thinset /cementboard / SLC primer / loose heating wire / SLC bathroom floor staring at me.

Can I redguard the SLC prior to setting tile for an added layer of protection?

Will the red guard effect the heating wire or vice versa?

Thanks.

Reply

Roger

Yes, you can redgard it. No, it won’t affect the heating wire. No, the heating wire won’t affect the redgard.

Reply

Mike Mike

Hi Roger,
I would like to know if you can give me advice on outside patio stones. I have a old interlocking patio/walkway and would like to replace with new patio stones variable size pattern. Can I lay overtop old stones if I put a new layer of sand on them and should it be compacted as well or just dry fit without gas compactor. If this is ok how thick of sand layer should I use . Thanks Mike

Reply

Roger

Hi Mike,

You can go over the existing, but you’d need a minimum of 1 1/2″ sand strata layer over the old. It should be tamped down, but a gas compactor is overkill for that and will actually be counter-productive. It will compact it down until the compactor hits the old stones since there is not a layer beneath the sand into which it can embed. It’s like trying to compact sand over stone with a hammer, it just won’t work.

Reply

Thomas

Roger,

Can’t believe you answer every question. I’ve got 2″ thick T&G diagonal planks in my bathroom over 4″ wide beam joists 16″ OC. What do you recommend under the tile?

Thinset, backerboard, thinset?

Some other combination?

Reply

Roger

Hi Thomas,

I can’t believe it some days either. :D

1/2″ ply, thinset and 1/4″ backer or uncoupling membrane like ditra, thinset, tile, grout, beer.

Reply

Thomas

Alright, I’d seen that Ditra before, now I’m sold. Forgot to mention, though, that I’m putting in an electric heating mat. Looks like that gets embedded in the latex thinset on top of the ply, then another layer of thinset under the Ditra, then ANOTHER layer over the Ditra.

Whom do I sue when my dog gets cancer from all that thinset? And should I use the 1/4″ Ditra, and 1/4″ X 1/4″ notch trowel?

Reply

Roger

Don’t feed the thinset to your dog, and don’t have him breathing it in when you’re mixing it, and he’ll be fine. :D Not sure what 1/4″ ditra is that you are referring. There is regular ditra ~1/8″, and ditra XL at ~1/4″, I assume that’s what you mean. No reason for the larger ditra, regular will work just fine. If a 1/4″ trowel gives you full coverage then yes, use it.

Reply

Tom in NJ

Hi Roger. Just demo’d old floors down to joists. Plan was 3/4 t&g ply glued and screwed then 1/2″ hardiebacker set in thin set and screwed. Joists are 16″ oc. Is this a proper prep for ceramic.Thanks great site

Reply

Roger

Hi Tom,

Standards actually dictate a double layer of ply totaling a minimum of 1 1/8″ thick under your 1/2″ backer.

Reply

Rhonda

We recently had our foyer, 1/2 bath, dinette and kitchen tiled by a “professional” flooring company. We went with 18 x 18 rectified porcelain tile…subfloor is ditra. Tile was laid in a brick lay pattern. (all of the preceeding, by the way, recommended by the flooring company). Many of the tiles have serious ledging issues…also lots of corners sticking up. We had the installers back to look at the job, they are blaming it on our floor joists. Also, after installing, they trim on all door jamb areas is “up in the air” about 1/4 inch. Until we complained about this, they were going to leave this as is.
I guess my question is…where does the blame for the uneven tile job lie? With us as owners of the house with the supposed “bad joist” areas or with them as the professionals? We are meeting with them again next week and I want to know what recourse I have with them to get a job that I am happy with.

Reply

Roger

Hi Rhonda,

Your joists make absolutely no difference at all. It is up to any professional flooring contractor to get the tile substrate FLAT BEFORE installing tile. There is no excuse for not doing so. Specs indicate no more than 1/4″ in ten feet nor more than 1/16″ in two feet for flatness. Your recourse would depend both on your local building code requirements concerning contractors, the licensing board and the contract you had with the company. None of which I can help you with, but you can call those sources and find out.

Reply

Jim

Hi Roger,
I’m doing a 50 SF bathroom floor. I chiseled up all the tile and removed the 3/8 plywood underlayment. I’m getting ready to lay backer board and was wondering if it makes any sense to put a waterproofing membrane down first. Two reasons are, first, a bathroom is a wet environment and two, the next guy to tare up my tile, hopefully not for a very long time from now, may have to tare out the subfloor as well. By the time he got the backerboard up, the subfloor would be shot. I’m thinking some felt paper stapled down, then thinset, then backer. Does that make any sense?
Thanks, Jim

Reply

Roger

Hi Jim,

Felt paper beneath the thinset and backer will be just fine.

Reply

Jim

Roger,
I did a search on your site and didn’t find any reference to felt paper which leads me to believe that this is not a standard practice. I’m not looking for extra work if it’s not necessary.
I really enjoy your site, my go-to place for tile info! :rockon:
Thanks, Jim

Reply

Roger

Hi Jim,

You don’t need it under backer. It is needed as a cleavage membrane under mud beds, but under backerboard it’s not necessary.

Reply

Pete

Thank you! My dog thanks you too; he had the fire dept. on speed dial…

Reply

Bill

Can you lay tile on top of freshly laid backer board or do you need to wait for thin set to dry under backer board ?

Reply

Roger

Hi Bill,

You can tile immediately.

Reply

curt

Roger, I searched numerous sites for answers to some questions I had to include cement backer board manufacturers sites. Your site not only answered my questions but your answers also provided the rationale. You explained that the backerboard screws should not be screwed into the floor joists. Believe it or not, the manufacturer sites were silent on this issue. You did a great job of providing the necessary detail and rationale for a proper backer board installation. Thank you!!!! Your efforts are appreciated. After reading your instructions, I feel I have enough info to do the job and do it properly.

Reply

JACK CORBACIO

HI,I HAVE 3 1/2″ PLANK FLOORING ON A DIAGNAL AND I HAVE SCREWED IT DOWN WITH 2″ FLOORING SCREWS.IVE DOUBLED UP FLOOR JOISTS TO ELIMINATE THE BOUNCE.NOW I WOULD LIKE TO USE THE HARDIE BOARD BUT NEED YOUR ADVICE ON MY NEXT MOVE.I BOUGHT 3/8″ THICK PORCELAN TILE,4 PIECE PATTERN.TILING KITCHEN,HALLWAY,BATHROOM IN SAME TILE AS IS CONTINUES FROM KITCHEN INTO HALLWAY TO BATHROOM.FLOOR JOISTS ARE ON 16″ OC AND 12′ SPAN.SO CAN I SPREAD THINSET OVER PLANK FLOORING AND SCREW HARIE BOARD OVER TOP ID LIKE TO USE 4+8 SHEETS.THANKS JACK

Reply

Roger

Hi Jack,

Not over plank flooring. While you can, there is a high rate of failure when the tile substrate is placed directly over single planks. Every seam in the plank is a different substrate, and may move independently of the one adjacent. There needs to be a layer of plywood over the planks.

Reply

JACK CORBACIO

HI ROGER,THANKS FOR YOUR REPLY.WHAT THICKNESS OF PLYWOOD DO YOU RECOMMEND,IM THINKING 3/8 PLY SPRUCE OR IS THER A STONGER PLY LIKE FIR?

Reply

Roger

You need 1/2″. It can be any species.

Reply

JACK CORBACIO

HEY ROGER,1/2″ PLY,BUT WHAT ABOUT HARDIE BOARD?IM GOING TO BE ABOUT 1″1/8 HIGH AND PLUS CEMENT IF I USE 1/4″ HARDIE.

Reply

Roger

You’ll need to get a transition strip of some sort or switch the hardi out for something thinner like schluter ditra.

Reply

JACK CORBACIO

THANKS ROGER FOR ALL YOUR ADVISE,HAVE A GREAT DAY.

Reply

Jerry

Roger

Should the lanolin be removed before putting down backer board, even if there is no bubbles in it? If so what is the best way to remove it? (elbow grease, back breaking labor)

Jerry

Reply

Roger

Hi Jerry,

Normally yes. The problem is with the layer of luan which is normally beneath it. And yes, elbow grease and back breaking labor. And yes, it sucks. :D

Reply

Chad

Hi Roger,
Can I install deck mud on top of Durock to fill a low spot? If so, do I need to use thinset in between the 2 layers? This seems to be much cheaper and easier than SLC. Thank you as always, Chad

Reply

matt

Roger-

I am hiring a tiler to tile an upstairs condo that has a plywood and gypcrete floor. What is the proper addition to this substrate before the tile?

One guy says thinset, decoupling membrane, thinset, tile. One guy says hardibacker (not sure how he is going to fasten it to the gypcrete).

Could/should hardibacker be added above the gypcrete? Say with thinset?

What is the correct layers of substrate? Plywood, gypcrete, ? , ?, ?….

Thanks so much!

Reply

Roger

Hi Matt,

The guy that says thinset, membrane, thinset and tile is correct. You can NOT install hardi over gypcrete.

Reply

matt

Thanks so much for your reply Roger. Best!

Reply

Eileen

Roger,
Going to tile my kitchen, I have 3/4″ plywood floor, would it be ok to use 1/4 backer board instead of 1/2 because I don’t what my floor height to be so high. thanks

Reply

Roger

Hi Eileen,

Not really. You need a minimum of 1 1/8″ plywood beneath your substrate. You can use 1/2″ and use a thinner tile substrate like ditra.

Reply

Tom

Same situation… a 3/4 plywood subfloor. What is the best way to go? Build up the subfloor by adding a layer of 1/2″ plywood (screwed down and not having seams overlay the seams of the 3/4″ plywood) to get to the 1 1/8″ substrate thickness and then using 1/8″ Ditra as the tile substrate. Or, using 1/2″ backer board (Hardi or actual cement board) and then using the Ditra. Two ways to skin the cat, which one is best?
Note: the floor is currently 12″ tile that is over 15″ old that was laid directly on 1/2″ cement board (can’t tell if it is nailed, screwed and/or thin set to the ply. Will only know once I begin removing it) and there are no cracks or grout failure. Below are trusses that are 19″ on center.

Reply

Roger

Hi Tom,

The 1/2″ plywood would be best.

Reply

Pete

Hi Roger,
I’m demo’ing a 30s.f. bathroom floor (tile,plywood,everything down to the joists). Will reinforce joists as needed.
Then what? 1 1/8″ of ply & ditra, or, is hardibacker required as well?
I had been thinkin’ ditra was a substitute for hardi. Thanks, Pete

Reply

Roger

Hi Pete,

You’re correct, 1 1/8″ plywood then ditra. Ditra does take the place of backer. I have used both twice and made the mistake of using one as an example of in-floor heating here on this blog. Both are not required, there were extenuating circumstances with that project.

Reply

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