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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myriam

Hi Roger, what about using that self leveling stuff so the floor will be flat? (I think my floor is fairly flat, but not completely, so I’m afraid to install the tiles on top of the thin set . . .)
Thanks for a great blog :-)

Reply

Roger

Hi Myriam,

SLC works very well.

Reply

Angel

Hello. Can I use two by tens instead of ply wood for my sub floor?

Reply

Roger

Hi Angel,

No, you need the monolithic structure of a sheet of plywood. Every seam is a weak spot, why would you want 5 seams where there doesn’t need to be five seams?

Reply

Kathryn Eccleston

I decided that I should put the dogs out BEFORE I write this :-D I do lots of DIY (out of necessity, not because I like it). This article is by far the best DIY one that I have read. I love that you give REASONS for why each step is important. Your directions are very clear, concise and the humor is much appreciated. Thank you so much. Now off to buy backer screws and tape. I totally forgot that I needed them. :bonk:

Reply

MeganC.

Roger, great article! I have done this method in our smaller upstairs bathroom, but now I’m getting to tile our main floor mudroom that is a concrete slab with in floor heat. It’s about 7′ by 20′. For the wood floor going in next to the mudroom we’re putting in a floating subfloor of two layers of 1/2″ plywood and the flooring is 3/4″ thick, so the area in tiling needs to come up quite a bit to be level with the rest of the floor.
What do you recommend? Can we put in a plywood sub floor, then thin-set cement board to that and screw it down then put in tile? I think our tile is 8×8″.
Other options i have read say to use self leveling ($$$$) or deck mud, but that looks complicated for how big the area is and I feel like there should be an easier way. What would you do?

Reply

Roger

Hi Megan,

I would use deck mud. I don’t know how you would reliably fasten plywood over the concrete, I wouldn’t trust it under a tile installation. You can also double-up kerdiXL?

Reply

Amy

Hi Roger,
Getting ready to tackle a small bathroom redo. Thank you so much for sharing all this info. I think we might be able to actually pull it off now. Thanks for the thinset tip….had no idea that was needed. I don’t have a dog so, guess I’ll have to put the cat out just in case…..haha. Gonna use all ya tips. Only question I really have, is there a certain kind of thinset that you use or does it even matter?

Reply

Roger

Hi Amy,

I use a lot of different ones. Mostly versabond, just because I can get it at home depot (it is a very good thinset), as well as laticrete 253 and mapei ultraflex 2. Those are my base goto thinsets.

Reply

Jeff

Hi Roger,

I installed cement board and on the edge of my tub on the floor there is a slight gap. Will the mortar and mesh tape adhere to the wood? My plan is to put the mesh tape over the gap and float the floor with mortar to make level before installing tile. Please let me know.

Thanks!
Jeff

Reply

Jeff

Let me clarify, there is a small gap between the wood and cement board. Will mesh tape and mortar work on the seam even though its wood and cement board?

Reply

Roger

Hi Jeff,

It doesn’t really need to stick to the wood, does it? If I understand correctly you’re just trying to extend the substrate, the mortar will fill that gap and keep the tape flush while it cures, once it’s cured the tape will be stiff enough to extent that without the mortar needing to actually bond to anything.

Reply

Jeff

Sounds good. Thanks for the quick reply. Very helpful

Reply

Laura Carol Forster

Hi Roger,

Great article. I am tackling a second story tile installation. I am new to this but I do have some guidance from an extremely experienced friend. That said, some people are mentioning vinyl floors. Are they referring to the existing roll vinyl that’s on their floors now or an application before the thinset? I’m asking because I have a vinyl floor in my bathroom currently. I was going to pull it up. Would you do That? Also, my closet joins up to my bathroom. My initial plan was to put hard wood floors in the closet that match the rest of my house. How would you handle that transition? 1/4 inch backer board and then just a goodgood transition board for the doorway once the wood floors are down? Anything I can do level the 2 floors up, should I tile the closet as well or should I not worry about that?

Reply

Roger

Hi Laura,

They are referring to existing vinyl on the floor. I would tear it up, it normally has luan under it, which is an extremely thin vinyl underlayment that has no business anywhere near a tile installation. It depends on the thickness of your wood, if it’s 3/4″ then 1/4″ backer and tile with a 3/8 trowel normally levels it right up, as does ditraXL and tile. A good transition strip will work just fine, make sure to leave 1/8″ – 1/4″ between your tile and wood.

I would probably tile the closet – but I’m biased…

Reply

Brian Toms

Roger, I’m also installing the hardie backer over a vinyl floor. What size trowel should I use for the thin set under it? And are you saying that you can use any cheap thinset under that?

Reply

Brian Toms

Also what size trowel would you use when laying an 18 inch by 18 inch tile?

Reply

Roger

3/8″ or 1/2″.

Reply

Roger

Hi Brian,

1/4″ and yes, any thinset.

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John

Hey Roger, Thank you for a very helpful web page. I am now confident to tile my kitchen floor. One question for you. one corner, of my kitchen , is not level with rest of floor. i plan on using some self level product to correct. would you put self leveler before cement board or after cement board?
Thanks for your help.

Reply

Roger

Hi John,

You need to put it after the backer, you can’t screw through it. The tile can be bonded directly over it.

Reply

John

Hi Dan,

I intend to install small octagonal tiles over a 1/4″ backer board in my 9′ by 9′ upstairs bathroom. The house was built in 1906 and has tongue and groove fir floors throughout. There is no sub-floor, no hardwood, no tile, just a little bit of dirt in the grooves between boards that have a thin layer of old oil base green paint somewhat worn off. I’d like to minimize the step up at the doorway from the hall, that’s why I’m thinking the 1/4″ backer.

Would you add or change anything in your well written and detailed directions above for my particular case?

Additionally, following installation of the new rough plumbing, there are some serious holes around the new copper pipes. The toilet flange moved 4″ closer to the wall; the claw foot tub pipes have extra space around them (1.5 to 2 inches). How would you recommend filling these floor holes?

Thanks!

Reply

Roger

Hi John,

Not sure who the hell Dan is, but I’ll help… :D

I would do everything exactly the same for your installation. Get some spray foam to fill those holes, then put your backer right over it. Actually, overfill them, let it cure, then trim it down flush. Works great.

Reply

Dan D

I have screwed 1/4 in backerboard directly to a level vinyl floor with a true 1 inch thick plywood subfloor with no thin set mortar under the backerboard. The jobs I’ve done like this are over 12 years old. There has been no grout or tile cracking at all.
Maybe I’ve been lucky. I used 12 X 12 tile. The bathrooms were less than 100 square foot. I am now starting a larger job about 450 square foot ; kitchen, bathroom, and laundry room with 18 X 18 tile over a larger space than in the past. The subfloor is a true 1 inch thick plywood. Is the thinset really necessary under the backerboard? If I use thin set under the backerboard can I put it directly on the vinyl? Will it dry properly sandwiched between vinyl and backerboard? Or, do I have to lay luan on the vinyl then thinset, and then backerboard? Final question, will the vibrations from a washer and dryer be a problem with tile. Wish I was going to work on a concrete slab. Thanks

Reply

Roger

Hi Dan,

I would use thinset beneath it. It will fully cure just fine. You can go directly over the vinyl, it doesn’t need to bond to anything, it is simply there to eliminate ANY voids beneath your backer. If properly installed (with the thinset) a washer and dryer will be absolutely no problem at all.

Reply

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