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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Kay

Hi Roger:

My contractor prepped my fireplace for tiling and left the hearth with a cement board base. However, I would need to build it up by another half inch so that my tiles are flush with the rest of the flooring. How would you recommend I do that?self leveling concrete?

Reply

Roger

Hi Kay,

Self leveling cement will work, another layer of 1/2″ cement board would be cheaper and easier. It can go right over that existing layer.

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jack

Hi Roger! Wish I’d found you a few weeks ago! The family room addition is on a slab foundation covered with linoleum tiles, which were remove, and under that are possibly asbestos laced vinyl flooring. They are very secure. Can I put backerboard on top?

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Roger

Hi Jack,

Not over a slab. But if they’re secure you can go over it with something like greenskin (just google that) and tile directly to it. Ardex also makes a glue to install ditra with that can be used over that.

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Ryan

Hi Roger,

Are these instructions applicable to studded counter walls too? I am planning on putting a subway tile on a studded counter banquet. Is it recommended to use a layer of plywood, thinset, and a tile backer for this assembly too?

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Roger

Hi Ryan,

Just screw the backer (1/2″) directly to the studs and tile away. Nothing else is needed.

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Bill

Can backerboard be placed over a linoleum floor?

Reply

Roger

Hi Bill,

Yes.

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Richard

Hello Roger,

I’m planning on using a linear drain for my shower and I’m starting with a ~5/8″ plywood subfloor with joists 16″O.C. I hope to do something that doesn’t get an honorable mention on your “Flawed” page.

I have a 3ft. x 5ft. space that I want to slope towards the drain along the 5ft. side. I consider myself to be “handy” but creating that slope seems difficult… at least for me. Is it advisable to shim the backerboard at regular intervals to slope the entire board towards the drain? I can then fill in the voids with thinset, but that won’t be very “thin” at some points.

If that’s not a good idea, any tips or links or references etc on how to do that slope without making a mess would be sincerely appreciated. It seems to be much harder than it looks.

Thank you for your time, effort and humor you put into your site!

Richard

Reply

Jeff

Richard, You don’t want to use backerboard for the shower floor. Big mistake… You want to create a sloped floor using a mortor bed and rubber membrane.. There’s some really good videos on you tube how to do this.

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Roger

Hi Richard,

I’m not getting what you’re attempting to do with the backer? If the drain is along the BACK wall along the 5 ft area then your shower entrance needs to be 1 1/2″ above the subfloor of the shower (which should be flat). You then create the slope from 1 1/2″ at the entrance to 3/4″ (assuming your drain body is 3/4″ high) at the drain.

If you are placing the drain along the front, you simply set the top of the drain body flush with the bathroom subfloor and create your slope from there to 3/4″ higher in the back to the wall (1/4″ / foot).

Both of those methods are done with deck mud creating your slope – not backerboard.

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Kristen

Hi Roger,

Thank you for making all of your awesome know-how available to us all! I am rehabbing an old home and adding a full bath on the first floor. I just put down durock over ply, taped and thinset the seams, but was wondering if I should also use redguard over the floor prior to laying tile. We will have a bathtub/shower combo, so it won’t be overly wet, but since I’m here and can do it now, I was wondering if I should.

Thanks!

Reply

Roger

Hi Kristen,

It won’t hurt, but it’s not really necessary.

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Greg

Hey Roger,

In a 9’x8 foot bathroom? I have 3/4″ T&G OSB over old lumber floor joists that are spaced 19.5(kinda…there’s just none above 19.5.). I’ve sister joisted everyone of the floor joist, the entire span, and leveled the floor as much as possible. However there’s still a considerable dip, from end to end in floor(almost a half inch at worst spot in the middle). I’m installing ditra heat and I was told I could use laticrete 254 to fill the dip as long as I use wire lath on the OSB first. I’m wondering if you agree with that or I you think I should use some type of actual floor leveler, and what should I use since its OSB?

Thanx Greg

Reply

Roger

Hi Greg,

Absolutely not! Who the hell told you that? You can not build up thinset that thick, it will crack and fail – no matter what thinset it is. You should prime the osb and use a self-leveling cement if it’s that far out of whack.

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Kolja

Hi Roger,
I am about to do my second bathroom remodel. I did my first remodel a few months ago and learned plumbing, framing, tiling, and some new shades of the emotions of rage and frustration in the process. One particular issue we had was that we had, as instructed, taped and thinsetted the seams of our hardiboard. But as we had that dry and went to apply the thinset to the hardiboard the seams were about 1/8 inch higher than the rest of the board, so when we troweled along we had some hills and ended up resetting a bunch of tile while re-distributing thinset just because things weren’t straight. (Unfortunately we were also dumb enough to pick marble tile with a 1/32 grout line for our first tiling attempt)

Any tips for making the seams flatter or how to deal with the no-longer flat hardiboard with mudded seams?
Thanks,

Kolja

Reply

Roger

Hi Kolja,

Get a slightly bigger trowel and you can compensate for that unevenness as you set the tile.

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Andrew

Hey Roger,
As I type this, I have about 20 open tabs from various links and sections of your site. Why? Well, I find your knowledge to be invaluable, and I’m in the demolition phase of remodeling my small ’50s master bathroom. It currently looks like a war zone with piles upon piles of broken yellow ceramic tile, thick (and I mean THICK) chunks of mortar, and scraps of the wire lath, covering the floor—I guess they thought whoever owned the house 65 years later would still love that hideous tile!

Anyway, I’m sure I’ll have several questions for you in the near future, but my first one involves the subfloor. Instead of a plywood subfloor, the house has 1x8s run on a diagonal to the floor joists, with about an 1/4″ gap between each 1×8. How do you recommend properly preparing this substrate for floor tile (material and thickness specs appreciated)? Also, in what situations do you advise using an uncoupling membrane, like Schluter-Ditra, in addition to a backerboard?

Thanks,
Andrew

Reply

Roger

Hi Andrew,

I would put 3/4″ ply over those boards for the floor, then ditra or backerboard. You can normally go with either one or the other, you rarely need both backer and ditra on a floor. Installing heating elements in the floor is about the only time I do that, and I only do it then about half the time depending on the joist structure.

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Andrew

Thanks Roger,

The sheet vinyl, luan, old ’50s pink asbestos tile and tar paper has been removed. I discovered some original 3/4″ ply already nailed to the 1×8 plank subfloor, but it’s covered with stinky glue (read they used to put asbestos in the adhesives too) and has some old water damage, so I’ll be replacing it.

Most importantly, I’ve decided on 6×12 slate for the bathroom floor, 1″ hex for the shower floor, and 3×6 subway for the shower walls. Will the slate require a different type of thinset (medium bed or unmodified perhaps?) than the ceramic and porcelain shower tiles? Are there any special preparations, cleaning, soaking, etc. that should be done to the slate prior to setting? Is a backerboard or uncoupling membrane better for slate? Do you recommend sealing the tiles prior to setting them, or only prior to grouting, and again a second time after grouting? I’ve done porcelain and ceramic tile, but natural stone, and slate in particular, is new to me.

Thanks again for your willingness to share your knowledge and experience!

Sincerely,
Andrew

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bob

I’ve found the hex tiles very difficult to keep aligned….with the imperfect sloping pan. not sure if they make a spacer for the hex tiles. on my second hex job I ripped them out and then used 2″ squares in the very large curbless shower pan i built…..much much easier and better install. I was able to keep my grout lines straight by using lots of spacers. Instead of slate I chose a charcoal porcelain I got from lowes, it came in 12 by 24 and 2 by 2, and price was decent and was in stock everywhere. so the whole floor matches and porcelain has many benefits over slate…….just my experience, wonder what Roger thinks. bob

Reply

Roger

Hi Bob,

I would never use slate in a shower floor unless it was extremely good slate. Hex tiles can be aligned well, it just takes a lot of time. Same spacers as regular tile, just more of them. :D Porcelain is always a better choice in a shower.

Reply

Roger

Hi Andrew,

It is always best to use some type of uncoupling membrane beneath natural stone. With slate you want a medium-bed mortar, the others can use regular modified mortar. No need to soak, I would seal before grouting, then again after.

Reply

Andrew

Thanks Roger.

You’d recommend medium-bed mortar even on a 6″x12″ slate? I guess I thought that type mortar was meant for larger and heavier tiles.

It’s funny you mention porcelain in the shower. My wife and I definitely like the look of the 1″ hex, but she’s recently fell in love with a 1″ white marble hex. Any special considerations with this tile used on a shower floor? Medium-bed mortar as well? Does that come in white, or only gray? I like to consider the long-term practicality of tile selection as much as aesthetics.

Thanks, as always!

Andrew

Reply

Roger

Medium-bed is made for larger format tile. That is it’s purpose. The suitability of marble in your shower floor will depend on the particular marble – how porous it is, etc. Just set it with regular thinset. Yes, medium bed is available in white as well, but you don’t need it for the marble hex.

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mike

Hi Roger,
would like to know about putting down tiles in a basement floor that is uneven all over in each room. was thinking of using a skim coat and applying cement board to level it instead of using the leveling cement. Also will be heating the floors and laying a 18 X 36 travertine tile down. Any advice on this situation .
Thanks
Mike

Reply

Roger

Hi Mike,

You need to either use slc or a membrane like ditra. Backerboard is not for use over concrete.

Reply

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