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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Tom

Can I lay backer board over old ceramic tile using thin set?

Tom

Reply

Barry

Roger — I’m a DIYer who firmly believes in a by-the-book approach to projects, especially those like laying tile where the cost and trouble of fixing a bad job is just too horrendous to contemplate. Within the last year, I retiled a 5×5 3/4-BR with 6″x24″ planks and am now about ready to start with a second BR about twice that size. All along when I needed a break I would peruse the net to get info about tiling and, to be honest, I never realized that there were so many differences in opinion. I just left another site where it was about 2-1 for the “no thinset under hardibacker” position. I’m with you, however, in what I see is a MAJOR reason for using thinset, to wit: it fills in those small depressions you will always have when doing tearout. I’m not talking about big depressions, just little hollows about 1/8-inch deep or something. They’re not big enough to think about using a leveling compound, but enough to make we want them gone. So, again, I plan to let thinset under the hardibacker take care of it, and your suggestion to let the backer-board set on top of the thinset until the thinset dries makes perfect sense. I had never elsewhere heard that suggestion — is it common? Even the Hardibacker instructions talk about screwing the panels down, but doesn’t say “when” — immediately, after the thinset sets up for a few minutes, or after it’s dry. Your other suggestion to lay screws from the inside out also makes sense — curious that I’ve seen that nowhere else either. One other question: Everyone talks about having a nice, level underlayment, but, frankly, that’s hard to do with a remodel. Something is always slightly warped. How much of a departure from perfect flatness can I fix with thinset? In other words, how warbly can the surface be before I should take more severe measures to fix (i.e. like tearing out and replacing all the underlayment)? Many thanks for your insights.

Reply

Fred

We have an old bathroom, floor is about 4′ X 9′. It’s been covered with carpet. Want to carry our hall tile into the bath now. There is about 1/2 ” difference. Don’t want to remove the old tile floor. What can I glue the 1/4 wonder board to the tile with? Was thinking of liquid nail or tile grip. Thanks for any suggestions.

Reply

Mike

Roger,
First, I really enjoyed your article. As a retired firefighter, I had a real laugh from the “dog bursting into flames” line. I thought ‘now there’s one run I never had’. lol
I’m about to start construction of my new home. The house will have a rather large covered back porch. Since the lot has a view, we’ve decided to have a deck on top of the porch. So the roof of the porch will double as a sun deck. We are considering tile as the finished flooring for the deck. The deck’s floor substrate will be wood covered with plywood. We plan to take extra measures to insure as little deflection as possible occurs in the deck’s flooring. The deck will be subject to all the elements. Including direct sunlight.
I live in Northeast Florida and we have a lot of sun, rain and high humidity. Temperatures here range from the mid to low 30s to very high 90s. Occasionally, we may have a freeze. Our last snow and hard freeze was Christmas of 1989.
Question: Is your article, How to Install Cement Backerboard for Floor Tile, applicable to outside use?
Thanks for your help.

Reply

Rudy

Hi Roger,

Slightly different than Brad’s situation: it’s a 1962 bathroom I’m redoing for Mom & Aunt. Yellow pine tongue & groove with some gaps between the boards here & there. I will screw them down into 2×10 joists 16″ OC as most old nails wonked out. It’s a small bath 8×8 and remodeling will have 3×7 ceramic tiles. The exterior end of the bath has 1.5″ thinset + tile and the door end around 1.125″. I read your articles about level floors & understand.

What I plan on doing is lay thinset under the backer board as you recommended to Brad (screw it down after it sets). What kind of tape do I use over the floorboard gaps so the thinset don’t run into the basement? The floor is solid and the boards won’t wiggle after I screw them down. Or should I use a membrane under the entire floor?

Also, on top of backer board will be a Nuheat mat, then tile.

Thank you

Reply

Cynthia

First I’m so thankful I found you! I was telling my son I needed to put mortar — umm thinset under the backerboard and he laughed and said, “No mom, you don’t put anything under the backerboard, you grout the tile after you have it in place.” Well, I quickly printed off your instructions and showed him what I’m doing because I want it to be right. But, now I’m getting worried.

I’m reading these questions and starting to freak out! I’m attempting to tile 316 sq ft. by myself because I can’t afford to have someone here for hours and hours charging me huge dollars. I’ve never done this before, but I’m giving it a go. I’ve laid and thinset all of the backerboard except in the kitchen. When I look at your pictures of thinset before the backerboard is laid, it looks so nice and, well, even. The first glop I put down was wayyy too much, and being concerned that it would dry faster than I could spread it with the trowel, I had to use my hands to push it down to the end of the area I was trying to thinset. It didn’t look very even to me and then dragging the pointy end of the trowel through it just made it look like there was even less thinset. I had a little thinset sneak through the sides and up between the next piece — so I used my little detail sander to get rid of all those little blobs. Do I have to be concerned if the backerboard doesn’t have the same amount of thinset under each piece? (One piece by the steps popped off yesterday. :eek: ) I only mixed a bit at a time, after the first fiasco. I don’t have a dog that will burst into flames, but I do have two cats and that might smell worse than say, a burning dog. Eeek.

My next and probably more important question is this: I have arthritis in both of my thumb joints and somewhat my wrists. I thought I could use a screw gun to put the 30 screws per 3′ x 5′ piece in. I learned today that a screw gun won’t do the trick as the screws aren’t coated and the coated screws don’t come in sheets that would feed into the screw gun. What will be the most efficient and hopefully least painful way for me to screw those babies to the floor? Thank you!! :corn:

Reply

Barb Brunswick

Hi there! We had just got tile with grout and noticed cracks in much of grout in two weeks? The professional men are coming Monday to regrout? I am afraid that we will have trouble in future since only putting more grout on top of old grout? Can u please enlighten me? Thank u so much for your time!

Reply

Roger

Hi Barb,

I’m pretty sure that by now it’s been regrouted, and probably cracked again? If so it’s an issue with the substrate beneath the tile not with the grout.

Reply

Brad

Hi Roger

I found your design manual and website very helpful. I have our shower area ready for tile without having to use any beer to extinguish our dogs!

I need to prep our floor for tile. We have a log home with 6×8 floor joists 24″ on center with 2×6 tongue and groove spruce subflooring (very solid, no deflection issues). The tongue & groove flooring is not flat, it is like a washboard, with high points in the center and low points at the joints (cupping downward). The total elevation changes from peak to valley is 1/4 inch or less.
I read your article How to Properly Install Backerboard for Floor Tile, which doesn’t address tongue and groove subflooring.
My thought was to use thinset between tongue & groove and hardiebacker to fill in all gaps flattening out the floor. I would screw down the hardiebacker in the center of the tongue and groove planks. Over the hardiebacker would be infloor heating wire covered by self leveling cement and Ditra.
Would this be acceptable? Any suggestions or advice would be appreciated, I don’t want to waste good beer extinguishing our beagles!

Thanks!

Reply

Roger

Hi Brad,

It would be acceptable, but a better way to do it would be to use ditra-heat mat rather than slc then ditra. Considerably cheaper as well. You can use other manufacturer’s cable in the heat mat. As far as flattening the floor, I would use a larger notched trowel, lay the backer in there nice and flat without screwing it down, wait until the thinset cures the next day, then screw it down. This allows a solid, flat base without the screws pushing it down into the lower areas while the thinset is still wet.

Reply

Jeremy

I am getting ready to prep the floor for tile. There is a 1 3/4″ thick set type plaster that covers the plywood sub floor. I had one tile guy say he was going to lay the new backer board over that material. Is the proper way to bust all that material out and take it all the way to a clean plywood serface first?

Reply

Roger

Hi Jeremy,

Yes, that is the proper method.

Reply

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.

Reply

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?

Reply

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?

Reply

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.

Reply

Roger

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

Reply

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.

Reply

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.

Reply

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??

Reply

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…)

Reply

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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