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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Dave

Hi Roger,

First, Thanks for a great site! I happened upon your article on how to properly install cement backer board and then I was hooked which led me to get your 3 book pkg. Outstanding info!

I am going to be installing 13″ x 20″ porcelain tile in a cabin open floor plan approx. 25′ x 25′ and (1) windowed wall floor to ceiling, wall to wall so I know I need control joints. I have 2″ x 10″ joists 16″ o.c., 15′ span, and 5/8″ osb subfloor. I will be putting 7/8″ pex hydronic radiant floor heat attached to the underside of the osb.

I was all set to put down 1/2″ hardiebacker over my 5/8″ osb, till I read your article on ditra and started researching that. My question is given the radiant floor heat and marginal 5/8″ osb. If this were the Floor Elf’s house, what would you recommend, hardiebacker, or 1/8″ or 5/16″ XL ditra?

We have 3 dogs all of which my fiancée loves more than me…. So if any of them burst into flames… It ain’t gonna be pretty!
Thanks, Dave

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Kim

We pulled up the cracked tile floor and the cement board that had not been laid properly (using thinset). This was followed with a week of scraping up the original linoleum. I’m so happy with the clean up of it. OSB never looked so good. Years ago I tagged along with a professional tile setter for about a year and learned so much just by being there. Now that I’m ready to put down tile on my own floor I needed to double check my memory on the details. I was so happy to find your post, and even happier to find that everything I learned then is spot on. My friend was a “A-class” person who did “A-class” work! Hope I haven’t wasted your time with my reflection. Just wanted to say thank you for providing accurate info/affirmation. My husband is now standing back on this project ~ I believe he’s made a good choice.

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Betty Cluck

I understand that a sheet of plastic, etc. should be placed over the sub-floor before applying the thin set. We have some 15# tar paper left over from our roofing project, would it be okay to use the tar paper for that water barrier?

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Jody

Hey. I’m putting in two new baths.. New sub floors.. Then thin set ,cement board, for tile base.. I have not read anywhere to put plastic over the sub…I would think you would not get the bonding with the thin set and it would not dry properly..

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Kim

The thinset is not for the purpose of bonding the cement board, rather it is used to fill space for there are no air pockets or hollow space between the cement board and sub floor. If you read most how-to, it will be explained.

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mark

Excellent article!

A question though, on thin set under the CBU: is the thin set intended to fill the sag the occurs, between joists, in the particle-board sub-floor? In other words, should I be ensuring the sag, between joists, is filled level, between the joists, with thin set?

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Mary-Louise Hansen

We’re tiling our kitchen with ceramic tile. There is a 3/4″ subfloor and we cut pieces of 1/4″ Wonderboard to fit over this. The problem is, there is radiant heat tubing under the floor. The shortest cement board screws seem to be 1-1/4″ and we are afraid we would risk puncturing the tubing. Roofing nails come in a 1″ inch size, but it won’t be possible to counter-sink these. Should we try the roofing nails, get 1/2″ Wonderboard and re-cut the pieces, or add a 1/4″ layer of plywood between the Wonderboard and the sub-floor?

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Ted

Hi! Great article, informative and comedic.

I am going to install an 1/8″ mesh electric underfloor heater between the cbu and tile. The instructions day to just thinset right over it with at least 3/8″ trowel when tiling. Any tips on how to maintain all that hard work getting a perfect substrate and then adding 1/8″ mesh in only pay of the flooring area? Thought about pouring self leveling underlayment all over it.

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Jay

Roger, great article and very helpful, if read first. We are laying a new 6×24 plank tile floor in our kitchen, adjacent family room, hall and bath. I used 1/2″ hardier backer but did not use thin set under it. I have screwed it down every 6-8″ with the proper screws. A friend who does tile work is laying the tile for us. He did start taping seams w/thin set and tape and trying to level areas that are needed. However, today he came back and pulled up all the tape the started using a grinder in most areas along the seams. Is this normal? And I’m worried about not having the thin set under hardie now. Any chance do to the thickness that I might be ok?

Thank you,

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Andy

OK WAIT!! We just remembered we took our original purchase of plywood back because my brother-in-law had access to some free 3/4″ plywood. Sorry, we did the subfloor awhile ago and I did not remember. I just went to the basement and see a stamp that says Rosen***(something I cannot make out) Resin Hot Pressed Exterior Glue Sheathing or sheeting…it’s faded and part goes under a joist.

Does that sound like the right grade?

The Roseburg web site says Exterior does not permit grade D. http://www.buildsite.com/pdf/rosebur…log-381110.pdf

We are going to block anyway, just to be safe.

LAST QUESTION – Can we skip the extra layer of plywood now? We have determined we have 3/4 Resin Hot Pressed Exterior Glue Plywood, and we are going to block.

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Roger

You can if you want to. Same answer applies. :D

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Andy

We screwed up the prep for tiling our bathroom floor by not thin setting between the plywood and durock. We are ready to do it again, but have been told from someone on another forum that the new 3/4″ CDX plywood we used is probably not adequate. The plywood goes under the tub, which is already set and walls tiled, so no option to take it up. We are wondering if we should glue and screw another 1/4″ of plywood on top to give more support before placing the new cement board.

House was but in 1954. Bathroom is approx 5×8. 2×8 joists, 16″ on center, 10′ length.

Should we add the extra plywood to increase rigidity or go ahead with the 3/4″ and thinset and screw the Durock?

Thank you.

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Roger

Hi Andy,

The issue with cdx is the grade of plywood and possible voids between the layers. That said, if you go over it with thinset and backer it SHOULD be just fine. I’ve never had a problem with it. Not saying you won’t, just saying I never have.

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Kat

Im new to construction so im working for a builder well ive had to put my dog out three times and the builder is super cheap he wants to cut corners all day- using liquid nail to install Backer board ?? Is that even legal and use drywall nails as well!! What do i do??

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Roger

Hi Kat,

Don’t know if it’s legal in your area or not, it shouldn’t be anywhere. Roofing nails are absolutely acceptable, drywall anything is not. Honestly what you should do is find a builder who gives a shit (they are hard to find, unfortunately…)

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Suz

Wonderful article!! I’m well into fixing a professionally laid tile floor that cost me $2,000 just over a year ago. They laid down the backer board with thin set and regular NAILS, no tape or joint compound. They basically laid ceramic tile over a bunch of Swiss cheese. So, after a year of mopping this floor, I’ve got water damage and mold. I just wish these idiots had read your article!!! Here’s to doing it right the first time.

Oh, company said that the year warranty had expired–anyone have ideas on recourse??

Reply

Roger

Hi Suz,

You may want to check with your local building department. They can tell you what, if any, recourse you may have. It varies wildly.

Reply

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