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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bub

Thanks Roger, I’m just floored (pun intended) by how many different answers i get from flooring professionals on my installation.
1. I have 3/4 plywood over 16 OC 2x10s spanned over 8 foot. so, I should put 1/2″ plywood for optimal subfloor i’m getting from you.
2. For the walk in shower, however, I have 3/4″ plywood with 2×8 joist at 12″ OC (with a couple of them actually being double joists) for an area of 4×8.

The question is, should I put more plywood over the shower floor or would it be acceptable to do a bunch of blocking to limit deflection? I haven’t seen blocking as a means of limiting deflection which may be a desirable option to limit floor heights (and to get rid of all the smaller pieces of joists!)

thanks again, love your column.

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Roger

Hey Bub (I’ve ALWAYS wanted to type that!),

1. Yes, you should use 1/2″ over it.
2. No need for it over the shower portion of the floor. Blocking will not limit your deflection in any significant manner.

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John Walls

Help me understand the operation of the gaps in the Hardi.

Paraphrasing: Leave a 1/8″ gap in Hardi seams to allow for potential inward expansion (actually shrinkage) of the wooden framing (framing expands/contracts, but Hardi doesn’t), but then tape and thinset the gaps to make the Hardi move as one large monolithic structure.

It seems that if you lock the Hardi all together so that it moves as a single unit, that the differential expansion function of the 1/8″ gap is lost.

Simply put, it seems that gaps are being made for expansion, but then filled with something that eliminates the expansion (contraction) feature. How is that any different than just butting the Hard edges together to begin with?

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Roger

The gaps in the corners and changes of plane are filled with silicone before the face of the backer is taped and mudded. The hardi (framing) can swell without putting pressure against the corner in a change of plane. The gaps on the same surface (plane) are filled with thinset as they are taped and mudded, this essentially turns those two sheets into one.

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John Walls

Roger, thanks for the response. I understand the function of the gaps in corners and changes of plane, and around the perimeter of a floor. I don’t understand the function of the gaps in the plane (in the field away from corners). Why not just butt those together?

Is it maybe to insure that the backerboard edges (in the field) all do fit together tightly—like maybe gapping it and then filling with thinset to compensate for any imperfections in laying out the backerboard, or imperfections in how square the room is?

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Rick

Hi Roger —

I am just a lowly home owner (with a dog not currently in flames), but I have done several floor tile installations and I’m happy to report I’ve always provided a nice, solid CBU substrate as you described in your article. Now, however, I am jumping into a somewhat more worrisome variation on this theme… an outdoor plywood & wood framed deck that will ultimately be surfaced with unglazed porcelain tile, and that I’d really like to survive in our Colorado weather environment for a long time! I have some questions on this topic of proper substrate, but first let provide a little rundown on the project for context:

My construction approach is two offset layers of exterior 3/4″ plywood, glued (with Titebond III) and screwed together over engineered lumber framing designed to L/720 deflection or better and set to a 3/16″ per foot slope away from the house. Some will insist on using 1/4″ slope, or not gluing the plywood layers together and avoiding joists with the second layer screws, etc. I pondered all that, but ultimately decided on a monolithic 1-1/2″ thick plywood layer and it is now built.

My next step, not yet installed, is a layer of 1/4″ CBU rated for outside use, installed with modified thin set using 1-1/4″ CBU screws. National Gypsum’s PermaBase Ultrabacker is the only brand I can find where the manufacturer supports using their product outside as part of a floor. Durock (USG) doesn’t, and of course Hardie is deathly afraid of the great outdoors (but I sure like it indoors!). Following this, the next layer is a 40 mil waterproof membrane made by the Noble Company called Noble Deck, attached with their EXT adhesive, and finally the finish layer of porcelain tile stuck down with modified thin set.

All that said, there are a number of details I’ll skip over (such as flashing, hand rails, etc), because the item I am currently pondering is getting the CBU layer done right. The deck is about 900 sq ft, about 50′ wide and varying in depth (it has a partial octagon shape, it fits around a bay at the back of the house, etc). Given the size and that it’s outside, the tile layer will have several control joints, and I’m thinking the CBU layer should have some as well. So here’s where I would really appreciate your advice: Would you put in soft joints in the CBU layer, probably by putting a poly caulk between groupings of CBU sheets instead of thin set? Assuming so, how many would you put in? Would you still tape over the soft joints? And with the waterproof decoupling membrane between this layer and the tile, how critical is it that a tile soft joint be perfectly aligned with a CBU soft joint? I’d like to use a pattern of 12” and 17” square tile where there would be no long straight line tile joint to align with a CBU joint, and I wonder how much it really matters if I have a “zig-zag” type of tile soft joint that bridges the CBU joint, again given there will be a decoupling membrane between the two?

I struggle a little (a lot?) with the best way to ensure that the CBU layer actually provides some isolation from the plywood expansion and contraction to ease the stress on the membrane… especially given all the screws that anchor the CBU panels to the plywood underneath. Any sage advice would be very, very welcome. Thanks!

Rick

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Roger

Hi Rick,

One way of helping isolate the the tile substrate from the movement below it (joists, base layer, etc.) is to only screw the first layer to the joists, second layer to the first, etc. Moot point, no?

Since you have what you have it makes no difference whether you put soft joints in the backer. The backer will not expand enough to separate anything, and it will still be screwed to the top layer of ply, which is screwed to the first layer, which is screwed to the joists. Whether or not you install soft joints, which compensates for lateral movement in that layer ONLY. Since each individual board is screwed (essentially) to your joists, your tile will move in the same manner your joists do. It makes no difference whether the individual boards can swell against each other or if they are one monolithic structure. That’s why you do not glue the ply, etc.

I would not install soft joints regardless. You will gain nothing. Provided you install proper soft joints in your tile layer you will have the sturdiest installation you can get at this point.

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Rick

Thanks, Roger, I appreciate your thoughts on this. It sounds like the way the backer board layer normally isolates tile from movement below is that it is constructed as a monolithic layer to the extent it can be, and individual plywood panels then expand and contract into the 1/8″ spacing left between each sheet. Apparently the lateral force on the screws attaching the backer board is absorbed by the flexibility of the wood as each individual plywood sheet gets a little larger or smaller, and the field of backer board does its best to remain motionless. I’m a little confused that some CBU manufacturers want 1/8” spacing between their panels (National Gypsum) and others want panels to loosely abut (Durock), but filling gaps with thin set and taping probably provides similar results.

If that’s the way it works, it would seem that breaking up the CBU layer might still have some benefit where you would have smaller fields of backer board (perhaps six panels in a 9’ x 10’ field) could still absorb some motion?

Perhaps the best approach for me at this point is to go to the extra effort of using soft joints between ALL the tiles (e.g., instead of grout), essentially making each tile independent of the others. It would be a lot more work, but an appropriate caulk (urethane perhaps) could also provide a more water resistant seal between the tiles to keep moisture intrusion to a minimum, which might reduce the likelihood of popping tiles during freeze/thaw cycles. Does this idea have any merit?

Since the weather is getting colder and the nights are already too cold to install thin set (most manufacturers want nothing lower that 50F for 72 hours, and we’re consistently below that now), I have little choice but to hibernate the project until spring. So for the next 7-8 months, I’m going to run a little experiment. Every morning and every evening, I’m going to take width (about 50′) and depth (about 25′) measurements of the deck surface to the nearest 1/16″ to see just what really happens over the course of seasonal changes. It’ll be interesting to see just how much dimensional change occurs, especially during sustained periods of high or low humidity where the plywood, I-joist and LVL moisture content might change significantly.

Many thanks,

Rick

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Roger

If you build your tile installation over backer which has been taped and mudded the entire unit, tile and backer, will all move as one in the same direction. If you do not tape and mud then it will not. Using a soft joint in EVERY tile gains you absolutely nothing, the tile will move according to the substrate to which it is bonded. There will be expansion of the tile which may cause portions to move independently, but one soft joint in the appropriate spot compensates for that.

You are attempting to create an installation application which already exists. There is no need to overcompensate for anything, the methods are there, utilized properly you will not have any problems.

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Riveter

We bought rtd plywood (actually my husband and his friend bought it). I was thinking there might be a better plywood that is better composed and constructed for humid environments in the south . I too like to over build and engineer. I’m unsure of the types and grades of plywood these days. Is a marine plywood applicable?
Rosie the Riveter

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Roger

AB rated plywood is what you want. I don’t even know what rtd plywood is. :)

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Steve_In_Denver

For informational purposes…RTD plywood:

1. Home depot sells something labeled as RTD plywood:
http://www.homedepot.com/p/Unbranded-23-32-in-x-4-ft-x-8-ft-RTD-Sheathing-166103/100041308

2. This fine homebuilding forum post suggest something about quality control in the gluing (they use RF to cure the glue, really?)
http://forums.finehomebuilding.com/breaktime/general-discussion/rtd-plywood

3. And this one seems to be the most authoritative:
http://www.conaxtechnologies.com/case_rtd_plywood.aspx

“The use of the in-process RTD sensors significantly reduced the delamination caused by insufficient heating of the plywood during the press cycle.”

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Roger

Thanks Steve!

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mike

would like to know best way to put a new floor over a stick down tiled floor. The floor was put down on a concrete floor and was once a outside porch on grade. The floor gets very cold in the winter as it was never insulated under the concrete. Should I use a laminate wood floor with the foam attached to the boards or can you recommend a better way to cover the floor. We don’t want carpet and should I put a vapour barrier over the old tiles . Look forward to your comments.
Thanks
Mike

Reply

Roger

Hi Mike,

You can put nearly any type of flooring over that you choose, it’s just a matter of the prep work needed. The laminate you mention can be placed directly over the existing tiles, so that may be the best option. If you wanted tile you’d need to remove the existing tile. The foam on the bottom of the laminate acts as a vapor barrier.

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Robert

Hi Roger,
Great article..! I have 1977 double wide manufactured home. I live in southern California. I want to put tile in the kitchen ( 149 sf ) and a small dining room ( 166 sf ). I have not pulled up the carpet or vinyl yet, but I am sure that there is plywood underneath. Not sure if MDF was in play back in 1977. After reading your article, could I use 1/4” plywood, thinset and then 1/4” cement board on top to create the stability you spoke about in your article….?????

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Roger

Hi Robert,

You likely have particle board under there, it was very common. If you have plywood you’re ahead of the game. :D If it is plywood you need to use an additional layer of 1/2″ plywood, NOT 1/4″, then your tile substrate (1/4″ backer is fine). If it is particle board you need to remove it and replace it with two layers of ply totaling 1 1/4″, then your substrate.

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Riveter

Hi Roger. Riveter was my mother’s nickname for me.
What is the best plywood to use as a sub-floor (using the Hardiebacker 1/2″ board over 2 layers of 3/4″ plywood, will use cement screws into backer board missing joists and apply them from center out). Do we screw the bottom layer plywood into the joists, then thinset between the plywood layers or just thinset between plywood and Hardiebacker? I’m confused. I know we don’t screw the Hardiebacker into the joists just the plywood.
So….let’s say it’s a four layer cake.
1st layer-Plywood screwed into (?) Joists
2nd layer – plywood ….(secured? with ? to the first layer plywood)
3rd layer – Hardiebacker set with thinset to fill voids between previous plywood and the Hardiebacker.
4th layer- Tile set with thinset.
Which layers require a 2/8 – 1/4 expansion void? Just the top plywood layer and the Hardiebacker board? And leaving that void unfilled and covered by base wall tile?
Thank you.

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Riveter

Forgot. …will be using redgard on backer before thinsetting tile. This is a floor outside a cast iron shower pan.
Just un-crated the pan today. I was expecting more of a lip or layered transition to the pan from the wall sides and there is effectively not even the smallest barrier from the pan to the floor on the side open to the bathroom. It just drops to the floor with nothing to hold a magnetized shower curtain at the bottom. Soooo, another shower door required? Was really looking forward to an easily cleanable shower curtain. :(
And the thought of backer board , redgard, thinset and tile for the surround just became more challenging. I guess cutting a groove for the bottom on the back of the backer bottom. Darn! Your thoughts, Roger. Please. Thank you.

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Roger

What type of cast iron pan did you get? Do you have a link to it? I’m not real sure what you’re asking me here.

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Riveter

Hi Elf,
We are installing two layers of 3/4″ RTD plywood from HD. I hope this is the correct plywood. (It’s very hard to find exactly the application meant for this plywood) I searched for “marine plywood” to no avail, but that was probably not what is recommended anyway. At any rate, just wondering if we can screw the FIRST layer of plywood to the joists, then what goes between the first and second plywood layer? We will screw the hardieboard layer to the top plywood and miss the joists after thinset applied in that layer, plus leave a tiny space around the perimeter of the top sandwich of plywood and Hardiboard. Applying the redgard to the Hardieboard before tiling.
So if we screw the bottom plywood to the joists, what keeps the second plywood secured with the first? Are all layers spaced 1/8 of an inch from the wall?
Apologies for my disorganization. It is part of my permanent challenges.
Thank you.

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Roger

Nothing goes between the first and second layer of plywood. The second layer is only screwed to the first layer, not into the joists. 1/8″ perimeter around all layers above the bottom layer of plywood. Screws keep the top layer secured to the bottom layer. :D

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Riveter

It’s a Kohler Salient, 30×60. Since you can’t see any at the store, you’re buying a “pig in a poke” by ordering online. Just uncrated it a two days ago and was not happy with the engineering of it.
So…we are shimming the studs for attaching the Hardieboard and we will leave 1/8″ distance between the Hardieboard and shower pan (so it “hovers” over the rim?) Can we redgard under the edge of the Hardieboard? Do we silcone the underside edge of the tile on that bottom edge? This on the three walls of the alcove for the pan.
It looks like we will be getting a door since the pan is so shallow.

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Roger

You need to silicone the gap between the hardi and base, and paint the redgard over that. You need a waterproof seal between your hardi and your pan, that will do it.

Reply

Roger

First layer into joists.
Second layer into the first layer only.
Third backer with thinset.
Fourth Tile with thinset.

All layers need an expansion gap between sheets, the top (backer) gets taped and mudded.

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Riveter

Thank you! I just replied before seeing this. :) We’re on it.

Reply

Roger

Oh, okay. :D

Reply

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