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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Todd

My situation is I have a mortar bed in a bathroom upstairs, and would like to install ceramic or porcelain tile floor upon it. When I demo’d the floor where it had ceramic tiles upon an older tile floor. I started removing both floors and had the material sent out for testing because of an asbestos scare, and everything came back ok. After pulling all the material, I noticed the base was a mortar bed. There is a portion of the mortar bed that was filled in with thinset, which was cracked and had to be removed with a hammer drill. I began to fill in this void with floor leveler, and it seems to be working well. What I wanted to ask is can I tile on top of the old mortar bed if it is repaired and intact? Or can I install new backerboard on top of the mortar bed, to provide a better surface to start with? Please advise.

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Carl

Roger
I have a 10 year old kitchen with 12 inch tiles. Numerous tiles have cracked. I went to demo and the tiles basically came up by hand. Not sure if the thin s et failed or the backer board? I have never seen the kind of backer board they used, it’s national gypsum board and was nailed to osb. Being that so many tile failed to adhere to backer board (even tile under the refrigerator failed). Should I be worried about deflection? Could have tile failed because of backer board? I am putting 20 inch tile down, but wondering if I need to reinforce floor.
Love your articles, always follow your advice.

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Allen

Roger,
Thanks for the great and easy to understand info. Getting ready to remove two layers of vinyl over probably asbestos tile (House was built in mid 60’s. Know to NOT disturb the potential asbestos tile. (In the roofing business so I know asbestos) but is there any particular thinnest I should use for the backer board with this type tile? Thanks.

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Roger

Hi Allen,

Any type of thinset at all will work. It doesn’t need to bond, it just needs to fill gaps.

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Jordan

You cover backerboard screws going into a joist, but I have a slightly different scenario. 1959 construction has provided me with some giant beams under the house, across which they laid 2×6 tongue and groove car decking. I firmed everything up from below with a matrix of 2x8s, then replaced the subfloor with 5/8 exterior grade plywood.

Do I need to avoid the backerboard screws penetrating the car decking, or is that not of great concern?

Thanks for a great article.

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Roger

Hi Jordan,

That won’t be an issue at all.

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matt

Hello Roger,
I have 1/16″ sheet vinyl adhered to 5/8″ plywood. The floor seems to be in good shape, well adhered, and feels solid. I am wondering if it would be OK to thinset over the sheet vinyl and then screw in my hardieBacker? I am prepping for a porcelain tile kitchen floor. Thanks

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Roger

Hi Matt,

Yes, that will be fine provided it’s not cushioned vinyl. If it is it would have about 1/16″ or larger foam backing.

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Bob Stanzione

Roger, thanks for sharing your wisdom, expertise, and most importantly….your sense of humor. Well done. The dog is already out on the porch.

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Pat

First…. Thanks So much Roger for your help…. I have a very small tile area in an old house bathroom gut, I need to make up a 7/8″ difference in level over 5 feet… should I make it up halfway under the hardie backer & half over it with thicker mortar ?

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Roger

Hi Pat,

No, not with that much space. Your best bet is a self-leveling cement. You can also do it with deck mud.

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Greg

By far one of the best articles I have read online about how to prepare your floors for tiling. I did a bathroom last year with backer board and followed pretty much all of this advice. The only thing I didn’t do was the thinset between the subfloor and backer board. It was a small area and It is still pretty solid after a year, but I wish I had seen this before I did that bathroom. I am getting ready to do another one this weekend and I WILL be putting the thinset between the subfloor and backerboard on this installation. Thanks again Roger!

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Bob

When you put the thin set over the plywood, do you use a notched trowl? I could not tell by the picture

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Roger

Hi Bob,

Yes, normally a 1/4″ or 3/16″, depending on how wonky the floor is.

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Lin

Thanks Roger, for one of the best summations of backerboard installation I’ve come across. Simple question : How soon after installing my backerboard can I lay my tile? Would there be a problem with light traffic and a week interval before tile application?

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Roger

Hi Lin,

You can install immediately if you want. You can also wait as long as you need to – there is no real problem with any type of timeline.

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Ed

Ok, so all you lucky folks that spent thousands of dollars not to mention hundreds of hours letting anyone touch a floor PERIOD without following Roger’s great advice is just not worth getting rid of good hard earned money and between all the animals here that the fire department get from Roger we all need to save bad tile jobs and the wasted money for redone jobs and donate to the local vet’s apparently. So first off. I am a professional floor and tile contractor in VA, MD, DC and everyplace has different “Minnimum Codes” listen! Don’t focus on anything minimum! I would like to be blunt to save folks! (Sorry) but don’t use anything wood as a substrate period. Why? Because even know it’s allowed in most places ive rescued, redone, replaced, and ummmm…..ripped out entire FRESH installations because proper research was not done by the homeowner! When this site, article, questions asked, and questions answered the world continues to turn! What does that mean? It means that by the time Roger or myself alike can respond, assist, help period it’s very possible what we’re saying is obsolete! That means it doesn’t necessarily apply anymore! We have this great tool that we didn’t have a few years back called the Internet! Use it, research, I bet even Roger has YouTube videos! I do not lol. But even some of the responses I’ve read have totally different technologies now in almost Nov 2016! And yep! Some suggestions here are actually not the best methods any longer because someone invented something or made improvements that don’t require you to do things the same! Main point! Sorry for veering off! Yes I’m passionate and a perfectionist sorry! If Any TILE is your final floor then you should never use without my FAV due to shear strength and doesn’t flake apart “Hardibacker” and this article is correct but an update or two! (No disrespect) there is spedicalky joint tape designed for the seams now even your exact substrate so you can keep buying your fav company substrate! I like to do my seams and tape in advance of tile only because I want each layer to cure properly and again yes I am a bit obsessed with each layer and proper airflow for setup. Lastly I don’t think I saw a single tip, question, or response about using the most important tool for any project “Especially” tile and substrate floors and that’s the use of a laser level 2-3 lines perferred! But wait! A laser line does not burn lines for you “Yet” when you have your “Square” and leveled floors, tiles whatever you’ll need to pop many chalk lines for reference! This is huge! Anyone? Bueler? Because your using thinset and like glues that move! How can you tell? Hmmmm…..one end of my board is on line the other is gone! Maybe my Hardibacker shifted! Something else chalk lines are great for as featured above? Marking your substrate floor pieces, tiles etc so you know where to put thinset and where you can wait so your not tracking thinset on someone’s Persian rug (yep dogs on fire too Roger)…..I can tell you if you Google 10 tile videos and prep videos you’ll get differences here and there but what you should pay attention too is what is the same in all of them! That’s the part where you should feel
Comfortable with decisions you make! Remember even as I write this I’m already obsolete with my practices and tips because the next day something else has taken place of something said an now you don’t even need screws lol…..good stuff here everyone. So sorry for those that got stuck with bad bad contractors! Do your research and question your contractors frequently! If they’re great they will be proud to tell you the proper way and even point you to lasting examples! If they were CHEAP and you can’t seem to understand why they won’t come clean! Grab the damn dog on fire or not and run like hell! Cause you’ve been had! Get references. Or if they point you to a showroom even better like DalTile where they will tell you “Yep! Roger’s the best around” only person or company I would reccomend”……..morale of the story! If your gut hurts and the dogs on fire you’ve got the wrong folks doing your work. And please! For those of you not able to pay a professional and cannot Phisically handle this type of work don’t! Don’t pay a dime for cheap labor! Keep your money! Take your time and research! There are good folks on this planet still that are very professional and not always out for the allmighty $$$$$. I do well and when I’m ahead sometimes I do work for chairity or very little cause everyone deserves good work regardless if they can afford it or not. Be patient, research, don’t be in a hurry to find the perfect contractor if possible. You’ll know when you have cause the dog will be fast asleep!!!! Take care!

Ed

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TES

Please respond ASAP. Finally hired someone to come in and put porcelain tile down. We have OSB for subflooring and they rented a big sander and sanded the osb down. We discussed in detail with them the steps they were going to take to prepare the floor. Just discovered that they have screwed down cement backer board using DRYWALL screws and there is NO thinset at all down. They are telling my husband that “someone” from the tile store told them NOT to thinset it down because we were concerned with moisture issues. We are on a crawl space in a very humid area. This is NOT what we discussed with them. They told us that they would be using “ultraflex” thinset between the cement backer board and OSB.

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Greg

I would fire them and find a different installer. At the very least drywall screws aren’t acceptable for this installation.

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Roger

Greg’s right. And the lack of thinset beneath the backer, ESPECIALLY after sanding down the osb (wtf???), will likely lead to a failure down the line. Thinset beneath backer isn’t going to create or add to ANY type of ‘moisture’ problem, they’re feeding you a line of BS.

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Ed

So I’m sure in a month you’ve got this resolved! What nobody mentioned here (I think) is what the beautifully sanded (or not sanded) OSB will do to your thinset after sometime your “Any Wood Subtrate” will at some point slowly suck the moisture out of your thinset turning it to nothing but wasted “Dust in the wind” some may not agree but I’ve seen thousands of installations and examined “Code Plywood” subfloors suck the mosture right outta the thinset. So in soon to be 2017 there is no damn reason to NOT use a tile rated NOT wood made subfloor! Period! None! Again HD and Even Lowes has plenty for all of the floors in the world! Lol! Use it! Put it down as Roger and team call out and even then still google or look and see if we are outdated in our suggestions!!!! Bet you we are! Myself included! (reason I carry 2 computers and iPhone with internet access 100% of the time) happy flooring! OSB ouch my stomach hurts thinking about that…..

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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:

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Paul

Hi Cynthia,

I unfortunately don’t have enough background to answer your first question, but as for the secures, I has used an impact driver with much success in the past. It’s very loud, so be warned, but it will get the job done and won’t burn up batteries like a drill/driver would.

I got a Ryobi drill/impact driver combo from a big box store for <$100 and have done many projects without any issues come up.

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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!

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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.

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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!

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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.

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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?

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Roger

Hi Jeremy,

Yes, that is the proper method.

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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.

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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?

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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?

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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.

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Roger

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

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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.

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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.

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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??

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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…)

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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??

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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.

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