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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David

Can cement board (Durock or Hardy Board) be laid over existing tile? Is this goofy or common practice?

Reply

Chuck

FE, thanks a bunch for writing and helping us. Do you use the same thinset under the backerboard as you use to actually set the tile(s), or are these different types of thinset/mortar for an important reason. I see above that you say just about any kind will do under the backerboard. I’m assuming it is because of your other statement that the purpose is not for bonding, but just to fill voids. Therefore, would I be safe to assume you care much more about the specific kind used for bonding the tile?

Reply

Roger

Hi Chuck,

Yes, I care much more about the thinset used for the tile. :D The stuff I normally use for tile is upwards of 30-40 dollars a bag, which is why I normally use the 12 dollar a bag stuff for under the backer. But you can always use the same under the backer if you want to.

Reply

Chuck

Thanks, Roger. I called Laticrete and they recommended 253 Gold for my 12×24 porcelain tiles over 1/2″ Durock in a laundry room. I hope the benefits are worth that price… “Gold” indeed.

Reply

Chad

Excellent article. I came by your site and info by chance. I live in a late 1800’s home and i am refurbishing a bathroom that i am going to place tile on the floor. current floor in 2 3/4″ thick. I have joists exposed as some repairs need made. Joists are 18″ apart. I have looked over lots of info here but have not seen if OSB is okay. Here are my thoughts…

If I start with 2 sheets of 3/4″ OSB(only first layer screwed into joists), then 1/2″ plywood, then 1/2″ backer board and finally 1/4″ tile.

another question is obviously the thinset will add to my height. should I reduce a layer of OSB or plywood to compensate for it?

Reply

Roger

Hi Chad,

OSB is fine. You need both layers. You can use 1/4″ backer on the floor rather than 1/2″, that can compensate for any height differences.

Reply

Tim

Best article I saw on floor tiling with cement board. One sub-step I would like answered, Is it proper to let the backer board screwed down into thinset to dry in place for the day and then come back next day with mesh tape and tile, or is that unnecessary and just do whole job at once?

Reply

Roger

Hi Tim,

You can do it right after, the next day, or as you set the tile. The timing of it doesn’t matter as long as it’s done.

Reply

Norm

Hi Tim. Roger stated in an earlier post that he prefers to mesh tape while laying tile. I highly suggest that you follow that advice. When I mesh taped a day or so in advance of laying the tile I noticed that the tape and thinset combo created a slight ridge (hump) that I had to account for when laying tile. I did my best flattening the taped seem & thinset but when it dried there was a detectible ridge.
When I mesh taped as I was laying tile and as Roger stated, the tape as you go process resulted in one even plain without any ridges.

Reply

Norm

Hi Roger. I am working by myself laying 3’ X 5’, 1/4″ HardyBackerBoard using applying Flexbond quickset with a 1/4″ notched trowel.
1.) How long can the board lay on top of the quickset before I have to apply the Buildex Backer-on 1 1/4″ screws?
2.) Do I have enough time to layout thinset for two or more boards before applying screws?
3.) Since it takes me about 20 minutes to screw down each board can I cover any leftover thinset to prevent it from drying out for 30 – 40 minutes?
3.) After 30 – 40 minutes of fastening the screws, Can I mix subsequent batches of thinset in the same 5 gallon bucket or do I have to thoroughly clean the bucket first before mixing the next batch?

Thanks Roger

Reply

Roger

Hi Norm,

1. It can lay on there until the thinset cures, if you want. But it needs to be flat and flush with adjoining boards while it cures.
2. Yes.
3. No, thinset does not cure because it’s exposed to air, it’s a chemical cure. It will cure at the bottom of a bucket of water. You need to pay attention to the working time of the thinset on the bag.
3 (2). You need to clean the bucket first, especially with quickset.

Reply

Luz

I am having tile installed in a bathroom and the hall way upstairs that when I took out the carpet has plywood, my contractor want to put the tile on the plywood, I Have read we need backer board but what happens to the toilet ,dose it have to be removed and reset on top of the tile?

Reply

Roger

Hi Luz,

Yes, it needs to be removed and tile is installed beneath it.

Reply

fred

Just letting you know this article was one of the best ones I have read in helping with my project great job.

Thanks,
FRED

Reply

Daniel

Hello,

Is it okay to use the liquid floor leveler before setting down your backer board?

Reply

Roger

Hi Daniel,

No, because you can’t screw through it without cracking it. You can use the thinset that should be installed beneath the backer to level it out if you need to. If you want to use the slc just install the tile right to the cured slc – no need for backerboard at all.

Reply

Jerry

Jerry

Hi Roger
I am tiling a floor with only 5/8″ plywood for the sub floor. I was planning on adding another 5/8″ plywood layer and then backer board or Schluter Ditra which is best for this job? We are using 12″by 24″ porcelain tile.

Thanks

Reply

Roger

Hi Jerry,

I prefer ditra, but either will work fine properly installed.

Reply

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