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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Alon Abramson

Hey Roger,

Thanks for putting together the site. The posts and the ebooks have been enormously helpful as I (slowly, ever so slowly) rehab my house. I have a tiny (18 sf) bathroom that’s just getting tile on the floor that I know I could knock out in a day, but I have a question… Do I need to let the thinset under the backerboard set up overnight before I lay my tile over it or can I lay the backerboard, fasten it down, and then immediately lay tile over it?

Thanks!

Alon

Reply

Roger

Hi Alon,

You can tile over it immediately.

Reply

Jess

Hello, i am about to build floor up for tile installation and I was given advice yesterday that id like to confirm or ignore.

I have 3/4″ diagonal planks over joists for substrate. My aim is to get as close to my existing hardwood as possible and that is only 3/4″ above the planks.

My orginal plan was to lay down 1/2″ plywood(glue and screw? , attach electric wire heating, pour slc, thinset then tile.

However i met a tile installer yesterday that said I could lower the profile even more by screwing the planks to the joists for extra measure, then thinset and lay 1/4″ cement board directly on top of planks. Then install heating, slc, and tile after that. This could potentially save my a little thickness but i wouldn’t want to do it at the risk of my tile(8″×32″).

In either case i was planning to use an anti fracture membrane as well.

Is this advice sound? Or should i stick with my original plan? Basically is comes down to 1/2″ ply vs. 1/4″ backer.

Thx in advance!

Reply

Roger

Hi Jess,

He’s correct, you can do that.

Reply

Axel

Roger,

What did I miss? Feedback time.

Fortified my 7/8″ plank substrate with an additional 3/8″ and screwed every 6″. Overlayed a primer and Self Leveler to minimize peaks and valleys upwards of 1/4″. Once applied the result was a little under a 1/8″ tolerance. Deflection was not a concern, but level/flatness was as I am laying 12×24 tile. Applied thin-set with 1/4×1/4×1/4 and then 1/4″ backerboard. Screwed as instructed. Meticulously followed every step. I then went to check flatness. I was so excited to see how my efforts had paid off. Unfortunately I found new peaks and valleys. I jumped on the high spots. I even unscrewed one board and tried scraping high spots and back-filling low spots. It was very frustrating. It was to be my moment to bask in my hard work. It turned out to be a moment to avoid using words that would get my mouth washed out with soap. Any thoughts of what I missed or did wrong?
Here are some thoughts I had: Being meticulous comes at a cost of time. I noticed that the thin-set under the high spots was gritty or dry, thus not displacing. And the high spot appeared to be where my self-lever was located vs over the wood substrate. Different dry rates?
The final result is now a little over 1/8″ tolerance. Dare I use a self-lever again on top of the hardiboard before setting the tile?

Thanks for your valued insights.

Reply

Roger

Hi Axel,

In the high spots, where you used the self-leveler, the leveler leached moisture from your thinset leaving it ‘gritty’ and unable to flow as it did in the lower spots. With 1/8″ difference I wouldn’t worry about it at all, you can make that up as you set your tile provided you use a large enough trowel. 3/8″ or larger should take care of that for you. Do not put more self-leveler over your backer, it’ll just end up being a big mess.

Reply

Bob Watson

Hey, many thanks for all the great info.
Question 1: what trowel size do you use for the thinset for the backer board?

Question 2: After applying the ‘self-leveling’ to the plywood floor, I have a couple of spots now less level. Can I just use more leveling compound to level those spots?

Thanks

Reply

Roger

Hi Bob,

1. Normally a 1/4″ square-notch
2. Yes.

Reply

Rick

I need your advice. Is there a proper way to install three 18 inch square by 1/2 inch thick granite tiles for my hearth so they will be flush with my 3/4 inch hardwood flooring? The subfloor is 23/32 T&G OSB 16 inch on centers. If I use 1/4 inch hardiboard substrate I have no room for the thinset below the hardi or the tiles. If I use a 1/4 inch bed of thinset and no hardiboard, will my subfloor be adequate to support the installation without cracking the granite tiles? What would you recommend?

Reply

Roger

Hi Rick,

You don’t want to go right over the subfloor. You can use ditra (1/8″) or something like tavy thin-skin or greenskin (1/16″).

Reply

Rick

Thanks Roger for the info. Any idea where I could get a small amount of Greenskin, 18 inches x 51 inches? Everywhere around here (Naperville, IL) sells it by the roll. Thanks for any help you can give me. Rick

Reply

Roger

Hey Rick,

Here you go. Greenskin ICE

Reply

Rick

Thanks for the info Roger. Great solution. Take care. Rick

Reply

Axel

Hi Roger,

Thank you for an excellent and informative website.

You shared the following paragraph on “How to install cement backerboard for floor tile”.
“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.”

The question I have applies when the backerboard abuts another material like greenboard. I have a wall that is 1/4 backerboard. The other 3/4 is greenboard. The tile is continuous along the entire wall. The last 1/4 is part of the shower wall, thus the backerboard. Do I tape and thinset the greenboard just like the backerboard?

Reply

Roger

Hi Axel,

Yes you do tape and thinset the greenboard as well.

Reply

Dave

Roger,
Thanks for your help. Should I tile right over this substrate, or do I need to then install a Dita membrane over this?

Reply

Roger

Hi Dave,

As you did not ask your question as a reply to either my answer or the original question I have absolutely no idea what substrate you may be speaking of. If it’s an answer I gave you please put this under it by clicking the ‘reply’ directly beneath the answer. I (literally) answer an average of 50 questions per day. While I would like to remember every one, I’m gettin’ old, it doesn’t work any more. :D

Reply

Christina

Great explanation, Axel! Im prepping a bathroom floor for tile and your post is super helpful! I now look forward to tackling this pronect and am a lot less intimidated about it! 😃

Reply

Patrick Preisinger

I enjoy your writing. I also appreciate that you explain WHY or WHY NOT to do certain things. For example, I’ve seen the admonition not to screw into joists, but never an explanation of why. Thanks! Very Helpful!

Reply

Axel

Roger

My question is about high and low spots. I seem to only find info on low spots, unless it is a concrete floor and instructed to ‘grind it down.’ What do I do with a wood floor that has a high spot. Do I self level all around it using the 1/8′ per 10′ rule? Which would mean the entire small room.

Reply

Roger

Hi Axel,

You can use slc or you can sand it down. Keep in mind if you sand it down you need to ensure you have proper thickness of ply left before your backer goes down. If you use slc you can forego the backer and just tile directly to it.

Reply

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