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
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 - 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
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
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.
Each 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
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
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.
Hey Roger,
Have you had any luck removing ceramic tile off of Durock and then re-tiling that space on the existing Durock. If so what method did you use to get the tile up?
Thanks Brad
Hey Brad,
The only time I’ve taken tile off of durock (or hardi) was tile that I’ve installed. The backer came off with it. When I remove showers to replace them I take the whole wall out at a time, backer and all. You can try to pound a drywall knife behind the tile to separate the tile from thinset, that works sometimes.
Thanks a bunch Roger,
Ok, so here is what I’m considering. The existing tile is 12X12 ceramic over 1/2 inch durock over 1X8 planking on the bias. It is all very solid, very little deflection, no cracks and I’d rather not tear it out. The homeowners want to go down with either hexagonal or octagonal tile in 12X12 sheets for a period look. I’m considering tiling over the existing tile. I can deal with the height with a carpet shim. So I have a few questions.
1. Do I need to abrade the existing tile and will a grinder with sanding disk do the job?
2. What thinset should I use? (Both Mapei and Custom BP allow for tiling over tile.)
3. Do I need to fill the grout lines with thinset before setting tile?
Thanks so much for your help.
Brad
Hey Brad,
1. Yes. I don’t know that the sanding disk would hold up, a diamond cup wheel would be better.
2. Between the two I would choose the mapei.
3. Yes.
Roger,
I am putting down 1/4″ backerboard on my bathroom floor. I think I made a mistake and forgot to screw in my backerboard screws while the thinset was still wet. I did walk on the backerboard to make sure it came in contact with all of the thinset and it is very solid. I have not taped and mudded my joints yet. I plan to do that this evening. Would you recommend I still put screws into the backerboard at this point before I do the joints?
Hey Josh,
Yes, you still need to screw it down. It’s fine to do it after the thinset cures.
So installing felt paper relieves the problem with the deflection of the 1×8? Really, 1/4″ Backerboard? This seems like a recipe for broken tiles to me. On one hand you say that 3/4 subfloor is minimum and 1 1/4 is preferred and the other you agree with felt paper and 1/4″ backerboard.
I agreed that you can do what you’ve described, I never stated that it is a proper installation. If you are dead-set on retaining the height with the surrounding flooring I’ve come to the conclusion after four years of answering questions for people that nothing I say will convince them that the tile needs that much beneath it.
Yes, 1 1/4″ (it’s actually 1 1/8″ technically) is the minimum proper substrate thickness over a properly constructed joist system. The tile doesn’t care what height the rest of your floor is. You stated you did not want to replace the pine -then don’t. I can’t stop you from here. I can tell you to until I’m blue in the face, but I’ve found that doing that simply wastes my time.
The only “one hand and the other hand” with these points are these: On the one hand there is the proper way to do it and on the other there are people’s unrealistic expectations with the amount of work they’re willing to put in. You want it done properly – tear out the pine, shore up your joists, install 1 1/8″ minimum plywood, install a proper tile backing substrate (1/4″ is fine over a properly layered flooring substrate), install your tile and work out some sort of transition to compensate for the height difference.
Sorry Larry, I just realized that it was you that asked that – not Doug. When people state right up front essentially what they are or are not willing to do I try not to waste time trying to convince them to do it properly – it gets me nothing but frustrated. It’s quite clear that he’s aware of the requirements for a properly build floor, yet still wants to install over the pine.
Everyone thinks that everything needs to be at the same level, always. It simply doesn’t work out that way most of the time, tile requires more foundation than any other flooring, it’s going to be higher.
I understand your frustration, but I am trying to arrive at the right decision. I have 2×6 joists – if I want to arrive at approximately the same height as the wood with a 1 1/8 subfloor, 1/2″ backer and 3/8″ of tile then I have to cut 1 1/2″ out of the joists and then sister them with something larger. What I need is a compromise that will not end in broken tiles and that bit about felt paper and 1/4″ backer doesn’t help at all.
Replacing the pine with the 3/4″ like we discussed before will give you a fairly solid substrate, installing 1/2 backer over that with thinset beneath it will give you height you need as well as an adequate substrate which will support normal residential traffic.
The only reason he wants to use felt paper is to prevent the thinset from running down through the plank flooring, it has nothing at all to do with the stability of the floor. It’s two different installations. You’ve already stated that you are willing to replace the plank flooring, in which case you can build an adequate enough floor to last. If you want the minimal height you need to make a compromise somewhere, the lack of a second layer of ply is the compromise. It is not, however, dire enough in most installations to lead to an installation which will not last. I’ve installed thousands of feet of tile over only 3/4″ ply substrate, it works in most cases. You are willing to do that, you’re fine.
Thank you for the clarification.
To all subsequent questions from readers regarding 1×8 pine subfloor: Cut out the pine and replace it with plywood. I installed 1/2″ durock in thinset over 1×8 pine and the durock cracked along the edges of the subfloor. The 1×8 is clearly not sound enough to support a tile floor.
Hey Roger,
I discovered this wonderful site today as a was researching installing a tile floor. I have a situation similar to a previous post; that is, 1 x 8 pine subfloor on the diagonal with 1/4 gap between the boards. I want to keep my tile floor to a total thickness of 1 1/2″ to match existing floors elsewhere. I really do not want to pull up the 1x subfloor and replace with plywood as you suggested earlier, yet parging this subfloor with thinset would be difficult due to the gaps between the boards. Can I staple felt paper over the 1x subfloor and then proceed with the thinset parge, 1/4″ backerboard screwed to the subfloor, then thinset & tile thereby providing a total thickness above the joists of 1 1/2″?
It seems this would also serve to separate the tile installation from lateral movements in the structural floor. What do you think? And if you do not recommend this, can I just parge the back of the backerboard and then screw it down to my 1x subfloor?
Thanks,
Doug DT
Hey Doug,
Yes, you can install felt paper first. You need to install the thinset on the floor rather than the backer. If you don’t get thinset into the low spots it defeats the purpose, putting it on the back of the backerboard may not make full contact with the floor in certain areas. It won’t help much with lateral movement – you’re still screwing it down.
Ok great! So my understanding is that I can staple down a 15# felt paper on top of my 1 x 8 subfloor, then 3/16″ thinset troweled on to the felt paper and then 1/4″ backer board set in this thinset and screwed to the subfloor. This is good news as I did not want to remove the existing subfloor, nor add a layer of plywood on top of it!
And just to clarify, I have real 2 x 12 joists 16″ o.c. in an 11′ span. So my deflection is ZERO!
Thanks for the advice!
Hi Roger,
Your comments and articles are thoroughly informative and though I found you in search of a solution to a problem, I’m happy I did.
We’ve got a 50’s house with oak floors through out the living area’s with the exception of the kitchen, laundry and now the foyer and have planned to install ceramic tile. The subfloor is 1×8 pine on a diagonal to the joists with 1/8-1/4 gaps between. The joists are 2×6, 16″ OC with 5′ and 8′ spans in kitchen and 8′ span in the foyer. After removing the wood floor in the foyer I installed durock on thinset and screwed it to the subfloor. The project moved slowly and now diagonal cracks have appeared in the durock. I read your article “Why is my grout cracking?” and “How to Install Cement Backerboard for Floor Tile” and understand that the durock does not add deflection stability to the floor. Which subsequently means that the cracks will appear in the tile, yes?. I looked for a link on the deflection problem and couldn’t find one. My question is: What is the required spacing and sizing of the floor joists and subfloor to properly support the durock and tile floor? and what would be the least invasive and best solution to fixing this deflection problem?… I’m having visions of cutting out the subfloor and adding joists!
Hey Larry,
The easiest is to simply add a layer of 5/8″ or 3/4 plywood, then backerboard then tile. Yes, if your backer is cracking it will transfer up through the tile eventually. The problem isn’t so much the joists, it’s the pine. You don’t have a solid continuous layer of support, each board can move independently when it’s stepped on. If you step on the end of one of those boards it will move the entire board, not necessarily either of the two on each side. Differential movement leads to unnatural stresses which must be relieved somewhere. The additional plywood will take care of that by dissipating the movement over the entire sheet and not allowing an individual stress point in your floor.
Thanks for the speedy response! Of course the problem with adding plywood on top of the 1X is that the tile will finish out well above the existing hardwoods. So if the 2×6 joists give enough support, I can cut out the 1x subfloor and replace it with a plywood or OSB subfloor and durock on top without the need to sister additional joists?
Yup, you need a minimum of 3/4″. 1 1/4 would be ideal, of course, but lead to the same problem.
Roger,
It’s funny – tile projects for people who don’t do them everyday (me) turn out to be this perpetual question machine. I’m convinced that tile projects are a distant cousin of the Lernaean Hydra.
That said, here’s the latest question that’s popped up:
My contractor – the guy who built the shower walls – put two niches in at my request. The sides and top, he built with backerboard, but the back (the part that faces you when you’re in the shower) – he installed the niche such that the back of it is the drywall on the far side of the wall (which is actually the drywall for the master bedroom wall).
After reading many posts here, it appears that I can’t just put two coats of Redgard on that drywall and install the tile, so what do I have to do?
If your answer includes backerboard, two sub-questions:
1. Can I just use 1/4″ backerboard rather than the 1/2″ the contractor used everywhere else (to keep the depth of the niche as deep as possible)?
2. How do I attach the backerboard, since there’s nothing structural to attach it to (the sides and top are attached to framing, but obviously there’s no framing along the back)?
I just want to try to do this the right way so I only have to do it once. And so I don’t end up with a huge water stain on my master bedroom wall, of course.
Thanks,
Brendan
I forgot to mention – in case it makes a difference – that the drywall section in question is greenboard, but the green side is facing the bedroom, not the shower (duh, right?).
Same answer.
Hi Brendan,
No, you can not just install redgard on the drywall. Yes, you should use backerboard. Yes you can use 1/4″. Just stick it on there with thinset, it’ll stay just fine.
Roger that. Thanks!
Roger: reading about backerboard and the gap with thinset then tape,
I installed the backerboard in the shower horizontally then filled the gaps with “real” silicone then used the alkline resistant mesh tape…..is this acceptable or do I need to take it out, thinset it, then re tape it ? I still have some time before the tile arrives, so I appreciate your expert advice. Dont know what we would do without someone like you helping us out.
Hey Chuck,
The silicone needs to be removed. The purpose of the tape is to tie the individual boards together so they don’t move in separate directions. The thinset needs to be locked in there.
Very informative and fun to read. You answered all my questions and now my substrate is rock solid. Thank you!
Hello Roger:
Excellent article. I am remodeling my bathroom, the sub-floor is 3/4 tongue and groove boards. They run at a 45 to the joist, and are not tight(1/8 to a 1/4″ gap between them. Can I thinset to these boards and lay my cement board. Unfortunately I placed one backer board under my tub already that jets out 6″ or so(prior to reading this). I dont think I can lay any kind of underlayment prior to the cement board because it would be a different height than the existing one. Any imput or improvisations would be helpful.
Thanks
Hey Chris,
Not much you can do at this point. Just install the rest of them with thinset to make sure they are supported well.
Is 1/4″ cement board (DuRoc) acceptable if it is applied to a layer of solid sheathing on the WALLS of a tub surround? I understand that if applied directly to the studs then 1/2″ is required. I just wasn’t sure if it were a matter of the flex in the 1/4″ or if the actual strength of the 1/4″ board is not sufficient to hold the tiles being applied. Thank you.
Hi Sherry,
Only if the layer of solid sheathing is another layer of backerboard. There cannot be any type of plywood or drywall behind the backer – it’s not waterproof. It’s the flex of the 1/4″ which makes it unsuitable. Why not use 1/2″?
The reason is because we have to “shim” the wall. The frame in for the tub was done about 3/4″ too big and now we have to shim each stud to the appropriate thickness so the concrete board will just fit over the lip of the tub. Was just wondering why not just apply plywood over the entire wall……………and then apply the concrete board. Your answer made me think. We have solid plywood UNDER the concrete board on the floor………… so why isn’t it acceptable for the walls?
Because your floor does not have more water dumped onto it per year than the roof of your house. Your shower does. Installing a substrate and tile onto a shower wall is a completely different process than on a floor.
You have made no mention of either a vapor barrier or topical membrane – cement backerboard is not waterproof. If you place it over plywood water will soak through the backer and into the plywood, once that happens it will soak into your wall framing. That’s going to create problems.
You need to use at least 1/2″ backer on the wall and have either a vapor barrier behind it or over it to protect the studs from water.
Yes…….. we are planning to waterproof the concrete board with RedGard before applying the tile……and its my understanding that we shouldn’t use a vapor barrier BEHIND the concrete board and the RedGard., one or the other but not both, right?
You are absolutely correct!
Hi Roger,
Absolutely fantastic site! We all appreciate the hard work that you put into writing the articles and helping us with our tile questions.
I’m planning a master bath remodel to remove the tub and make a curbless (accessible) shower with the entire floor tiled. As I’ve already remodeled a powder room and a kitchen, I know the floor construction is 5/8 plywood underlay over 1/2 plywood subfloor (1970’s construction). The bedroom wing construction is 2×10 16″ OC with a 12′ span. The master bath is 5×10.5 in an outside corner with the long dimension perpendicular to the floor joists.
The installation instructions for Hardiebacker 500 call for a minimum 5/8 subfloor but I plan to install it over the 1/2 subfloor with thinset and screws to keep the tile floor level with the bedroom carpet. What is your opinion about this?
Thanks again!
Hi Larry,
It should be fine. Using ditra over the 1 1/8″ double layer that you have now would be ideal, but 1/2″ hardi over the 1/2″ will work provided you make sure to install thinset beneath it.
I’m a new homeowner, and just getting into all the DIY’ing, this site is amazing for me! I have a question though:
My wife and I are preparing to tile our kitchen/hall (~150sf) with a pattern of 12×12 & 6×6 tiles. We ripped up the two layers of carpet in the kitchen (ugh… carpet… kitchen… puppies on fire?), and found another layer of vinyl/linoleum attached to the plywood. It will not come up. The rest of the subflooring looks ok, 3/4″ plywood over the subfloor. We’re going to do the cement board underlayment, but will the mystery layer cause any issues with the bottom layer of thinset?
Thanks!
Hi Eric,
It depends on what the mystery layer is and whether or not it will compress when walked on. If it does then it needs to come out. If it doesn’t you can install thinset then your backer and screw it down really well. The thinset does not need to stick to it – it’s only there to fully support your backer in all areas without any voids.
As near as I can tell there is no compression to the layer. The layer is really no more than an 1/16″, directly glued (entire thing, not just the edges) to the plywood. It doesn’t budge, and won’t come up without taking chunks out of the plywood beneath it.
Thanks for the reply!
Hi,
Okay, so I thought I was done tiling; however, when the sink was installed, the vanity drawers – which butt up against the wall – can open only a few inches before running into the door trim (duh!). So my idea was to offset the sink 1″ and run a thin strip of tile around the sink to make it perty (rhymes with kerdi, shoulda been a poet eh?) There’s lots of 1″ tiles already installed, so aesthetically it actually works.
I attached some 1 1/2″ plywood to the walls, and have some leftover hardiboard, but not sure how to make it work. Is there an adhesive I can use to attach the hardi to the plywood instead of screwing it in? There’s no way to tape the seams, is there another way to go about doing this? Is it even a consideration when running just a strip of tile?
Thanks a bunch!
cat
Hey Cat,
Considering you have a 1 inch tile over a 1 inch strip you don’t really need to tape and mud the seam. Just keep in mind that if (when) it does move it may crack the grout line over that seam. You can attach backer to the plywood with just thinset – on a countertop it’ll be fine (unless you walk on your countertop a lot
).
Excellent, thank you. Nope, don’t walk on the counter too much, but may have to have a few words with the cats.
Roger,
You said “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.”
I keep getting told (by various ‘experts’ at the tile stores) that I should put thinset in the gaps between the backerboard and *then* put the mesh tape on top of that. But the way you said to do it implies that the mesh tape goes directly on top of the backerboard gap, and then you put thinset over that.
Seems to me that there is a confluence of inconsistency here.
Their way seems inconsistent because the whole reason for the gap between backerboards was to allow for movement. If you put thinset in there, it fills the gap. No gap = no allowance for movement.
Your (stated) way seems inconsistent because if you don’t put thinset in the backerboard gap, you essentially leave a small air pocket, which in this very article, you point out equates to spontaneous dog combustion.
I’m more apt to believe you than the ‘experts’ at the tile store, but could you please address the air gap inconsistency of not putting thinset in the backerboard gap?
Pooches gracias!
Sorry Brendan,
They’re correct. I need to change my description. The thinset and tape locks the individual boards together and creates consistent in-plane movement. The perimeter gap around the room compensates for excess movement in the backer. The gaps in the subfloor (plywood) should be filled with silicone to prevent getting thinset into them as you install your backer – that compensates for movement there, but you want a solid plane with the backer and gaps around the perimeter for movement in that layer.
I probably wrote it that way because I force thinset through the mesh tape as I install it – I do it all in one step. I did not mean that no thinset should be in those gaps. I’ll change that here shortly, thanks for pointing it out.
Ah…now I get it.
Wasn’t my intent to point out an error, per se. Just trying to figure out the right way to do it. Thanks for clarifying!
Oh, no worries. Just want to make sure I’m clear about stuff when I write it. If it’s not clear for one person it likely isn’t for many more. I appreciate when people point out anything that may be less than clear. It’s stuff I do every day, so I don’t think about the intricacies of it many times.
Hello again Roger, I downloaded the Traditional Waterproofing Showers with a pre-formed base. I wish I had seen your Library before I started. Not enough beer or too much, not sure! But any way I ordered Durock’s waterproof membrane. to put on Durock’s 1/2 inch cement board. Everything that I have read on their website and that they sent with the membrane and adhesive does not explain how to do the corners. I’m only doing two walls and about 1/4 (the height of the tub surround). Should I cover each wall separately before I silicone the corners, or, run a continuous section (36″ wide”) on all the walls?
Another statement then a question if I may! I let each cement board rest on the pre-formed shower base and did not leave a gap between the first two sheets that I stacked. I caught my mistake there and left room between the others. Should I take those down and start over or cut off a little of one of the sheets? Can the sheets of cement board be reused if I take them down? Thanks 4 the HELP! I also enjoyed 
reading the book! Truly…I’m weird like that!
Hi Doug,
Got some bad news for you – provided I’m understanding you correctly. Durock’s waterproof membrane is not made for use in a shower – it is a wateproof crack isolation membrane for floors (as far as I know). I have never seen it sold or approved as a waterproof shower membrane. That said, I don’t know that it wouldn’t work, I just would not feel comfortable telling you that it absolutely would, then have it fail. I honestly have no idea. If you choose to use it in that application you can fold corners and overlap them two inches like any other sheet membrane. I don’t know that the adhesive would make it waterproof, though.
Course I could be completely wrong – I don’t know everything.
Your backer should be fine. and yes, you can reuse backer if you need to take it down.
Me again… Thanks for the reply. I think Durock’s membrane may be new or new and improved like your grout! lol Here is a link to their installation video but it does not explain the corners of the shower. I called USG and some operator said that I could install it any way that I wanted ! Great help!
http://www.usg.com/durock-tile-membrane.html#tab-videos
Well I guess I will install The way that I want since I bought it I guess its mine. Thanks again!
Crap Doug, I was thinking of a completely different product. Sorry.
Take your utility knife and cut from the corner straight out – at a 45 degree angle from each wall so the flaps can overlap, then use the adhesive to glue them together.
The membrane came with 4 fast and easy
instructions! We got it on the wall…hope it stays. Maybe it was fast and easy for the Handiman in their video. Thanks a bunch. I’m sure I will have a couple more questions later!
Roger,
My wife wants to put in a tile kitchen floor that currrently is vinyl over 1/4 inch USG Fiberock Aqual Tough. Upon first look, an installer thought it was vinyl over luan and he said it all would have to be taken out. I took out the vinyl and discovered a very nice, level Fiberock backerboard, not luan, under it. My question is, do I really need to pull up that Fiberock or can I use it as the backerboard for the tile installation? I also would like it to stay because it is only 1/4 inch: I’m concerned about the difference in heights when I add the proposed 1/2 backerboard with the 1/4 tile on top of that. That would mean the new floor would be at least 1/2 inch higher than the current flooring is. I say at least because I’m assuming the ‘thinset’ will also add height.
Roger,
Just noticed that the existing fiberock backerboard is not ‘backerboard’: it is labeled ‘underlayment’. Would that make a difference in your advice?
Hey Larry,
All fiberock is labeled as underlayment. 1/4″ is fine on a floor installation provided your flooring substrate is sturdy enough to support your tile installation.
Roger,
First, thanks for the comeback. A second question, do I have to lay all the tile in a room down at the same time? That is, assuming it’s thinset I put the tile down on, can I stage the installation or, because the first thinset will be hard by the time I put down the second, will that cause a problem. It’s a big area and I don’t think I can do it all in one day.
Nope, no problem at all. If I had to do it all in one day those 1500 square foot jobs would kill me!
Is it ok to screw the screws in right after you lay the backerboard over the thinset? or do you need to wait the 4-6 hours?
Hi Erika,
It is okay, and proper, to install the screws right after you lay the backer in the thinset. Do it all at once.
Roger
Great site! Preparing my son’s kitchen floor for ceramic tile, I’m getting ready to put 1/2″ backer board over 3/4″ tongue and groove plywood (it’s actually chipboard not plywood). After reading all of the above, I’m concerned it may not be strong enough. The floor joists are 2×10’s, 16″ on center, spanning 11 feet. How can I tell if there is too much bounce? It seems very solid. How do I measure deflection, if that’s the right term? Please help!
John
Hey John,
The deflection of your joists is 687, which is plenty for ceramic or porcelain tile. When you say chipboard, do you mean osb or particle board? Osb is fine with thinset and backer over it – particle board is not.
Roger, I have tried researching but could not find answers to “I guess elementary” questions! We gutted our Master Bathroom. We are installing porcelain tile floors, drop in tub, and a shower. When framing the tub I used 3/4″ advantech and 1/4″ inch backerboard on a 2×4 frame. 2 questions… 1. Should I tile the floor first? 2. To make the tile on top of the tub overlap the the Knee wall tile should I install the 3/4″ AT on the Knee wall then let the 3/4″ AT on top overlap it. Then do the same for the 1/4″ Backerboard and tile? Sorry for the long questions! thanks for your time and help in advance!
Doug
Hey Doug,
You can install them in any order you wish. If you want the wall tile to overlap the floor tile then yes, the floor needs to go in first. From an installation standpoint it makes no difference, but I think it looks better that way.
The order of the AT doesn’t make any difference, the backer should overlap in the order you described.
Hi Roger – trying to figure how/what to use to transition between new porcelain tile floor in kitchen and wall-to-wall carpet that butts up against kitchen at two entrances. Currently, carpet has a flat, golden metal strip nailed over the carpet right at the entrance to the old linoleum kitchen floor. I’m replacing linoleum with tile and 1/4 hard backer board over wood underlayment (pretty flat) so the kitchen floor level will rise a bit. Not sure what to use – any suggestions?
Hey Dave,
Those same types of strips can be used if you want. I prefer the metal edging like Schluter has for the tile edge, the carpet is just tucked to that. You can also get some carpet shims – they’re actually just particle board planed down to a ramp – and install that right at the edge of the tile on the carpet side, install your tack strip to that, then your carpet tucks to the tile edge or schluter edge. It just creates a little ramp up to the tile edge so they sit flush.
Gi roger.
i am putting 1/4″ hardiy board concreat floor that is level just using as hieght to match tile meeting wood floor what kind of screws would work for that .wouldn’t the screwes mess up cement floors?oh and should i use what i have under hardy is mapie ultr set 11
Thanks
carol dawson
Hi Carol,
No screws will work. Cement backerboard should not be installed over concrete – there’s no way to reliably attach it. If you need 1/4″ height addition you can use either DitraXL or SLC. Backerboard won’t work for your installation. You can also get a medium-bed mortar that can be built up to 1/4″ and install your tile with it rather than a regular thinset.
thankyu roger i was about to make big mistake all i want in to make smooth transition frommy wood floor 3/4″ from tile.By the way I layed wood floor with the bostic single step you recomended lookes great and is awesome I will go back look at ditra
love lovelove your site
carol dawson
Hey Roger,
This is a great site, thanks so much for sharing your knowledge for weekend warriors like myself! I’ve picked up my CBU and was just going to place it over a new piece of 1/2″ plywood to form the base for my tiles. However, after reading your post its pretty much a guarantee I will screw the 1 1/4″ cement board screw through both materials into my floor boards. To solve this I’d like to add an option like Ditra, that will give me piece of mind if the floor shifts…..do you have any recommendations on a cost effective alternative to Ditra? It is over $150 a roll at Home Depot and I would not even use half of that roll for my 30 sq. foot bathroom. Any suggestions would be greatly appreciated!
Also sorry to post this here, but when putting CBU on my shower walls can I just put fold the mesh tape into the join and apply mortar or do you recommend a different method for sealing the corners?
Thanks!
Dustin
Damn! Ignore that first question please Roger, putting the Ditra on top will not fix the thickness issue of the subfloor and backer board, I may just have to go with 1 1/8″ plywood as my subfloor OR just take the cement backer board back to the store and just put Ditra directly on a different sub floor option. Sorry to waste your time, after I read my post over again I realized the first question was flawed.
You didn’t waste my time – someone else may have the same question or train of thought. Ditra works just fine over just plywood, no need for the backer. My flawed answers usually pair better with flawed questions anyway.
Hey Dustin,
Spider-mat by custom should be available at either home depot or lowes, or you can get prove-mat online for less money as well. They both work just fine on smaller installations.
I prefer silicone in the corners rather than tape and mud. Different planes (walls) will move in different directions, the more you can compensate for that movement the less the walls will be able to dictate where that pressure goes (into the back of your tile). Silicone allows the movement into the corner silicone bead and keeps thinset out of it, which will eventually crack out.
Thanks for the quick response Roger! I’ll check into those underlay options at Lowes this week.
About my subfloor, do you consider that first row of angle boards on the joists to be part of the subfloor? If I take those into account then I don’t need the thicker plywood to fasten the CBU too…I was thinking I didn’t want the CBU screws going into those angle boards, but in re-reading the article you speak of not putting the screws into the actual joists. Can I get away with counting those angle boards as part of my 1 1/4″ substrate for putting the tile on? I will still need some form of plywood to adhere the CBU too, but if I can get away with a thinner sheet then it won’t mess up my door or hallway flooring transition…since my tile is going to inevitably be higher than the previous linoleum floor.
Thanks for answering my shower corner question to, but just to clarify, I can join the cement boards in a shower wall corner with silicone rather than thin set mortar? I like the play that would give, but everywhere I’ve read the joins had to be mortar…just want to confirm.
Thanks again for your help, I really appreciate it!
Dustin
Hey Roger,
I was just looking over your topics regarding Redgard, the guy at Lowes the other day recommended that and I think I’ll give it a try. I’m insulating my exterior wall tonight or tomorrow and was going to put up a plastic vapor barrier, then screw the cement board on.
But, after seeing you stress this could create a mold situation by sandwiching the cement board between vapor barrier and Redgard I will now just use the Redgard coating once I’ve prepped the cement board…does that sound correct? Glad I never started cutting that vapor barrier…its probably heading back to the store shelf!
Cheers,
Dustin
Hey Dustin,
You are correct – no vapor barrier if you’re using redgard.
Yes, that is part of your subfloor. Anything installed onto the joists is subfloor.
Yes, silicone in the corner. If your building a mud shower then it would be mudded. It can be mudded regardless, I just prefer to compensate for movement where I can – that is one area I can. Doesn’t mean nothing else will work or isn’t correct, just means that’s the way I do it.
Excellent, thanks Roger! So for that silicone in the corner just fill my 1/8″ gap with a nice bead and then once it cures throw the Redgard on top?
Thats my last question, then I’ll be starting this weekend and your help has been a big plus, thanks again! Also looked at your niche articles…thats exactly what I was trying to figure out how to build and your descriptions will make that task much, much easier. Great site!
Cheers,
Dustin
Yup, just fill it up and let it cure. The redgard will not stick to the silicone very well! All it really does is fill any gaps the silicone may have missed. Just go right over it like you are the rest of the wall.
Roger, you are seriously the man! With the help of your reply this am, I have my shower walls properly hung, taped etc. I also took your advice and bought me some red guard. Hells to the yes. Will be using this post for laying my cbu on the floor tomorrow and your shower niche tips on Saturday.
Quick question. What size trowel am I looking at for the cbu to the subfloor?
Thanks
B
Depends on how badly out of whack your subfloor is. If it is fairly flat and you aren’t trying to flatten anything you can normally use a 3/16″ x 3/16″ – that’s normally the one I use to install most tile as well.
Roger,
I found your site yesterday (found it twice actually; more on that later) and read through it for a couple of hours, and I think I might be hooked!
I first found this thread while trying to figure out if I needed a vapor barrier under my brick paver floor in my pier-and-beam-over-dirt-crawlspace home (I think I might be overhyphenated this morning) and if needed, how to install it. Reading this thread, I became concerned with the subfloor structure I was lead to believe I could use, and went to the paver manufacturer’s website (PortStone – look ’em up, it’s a nice product) to see if they had any recommendations on subfloor deflection ratios. Short answer: no. But I did find this: “We do not recommend the use of cement backerboard when installing PortStone over a wooden subfloor. We recommend using an uncoupling membrane such as Ditra or ProVa-Flex.” Preferably provaflex, apparently, as this is what they sell.
Well, I know about Ditra, but what is this provaflex… stuff? So guess whose blog is the #1 result for “provaflex”. 
Moving on to my structure, subfloor, and what I was led to believe I could use: I have 2×8 joists 16″oc over a 9′ span. On top of this is diagonal 5/8″x6″ lap board (both edges rabbited, not the cove cut stuff used for siding) and then 1/2″ ply pieced in after the walls were up. I was very concerned about keeping the new paver install fairly even with the engineered hardwood I just installed in adjacent rooms, and was told by the tile store owner / certified tile inspector / expert witness / guy I bought the pavers from (whew) that I could take up the 1/2″ ply, replace it with 1/2″ PermaBase (properly installed) and all would be good. Now, having read through your site some, it’s sinking in
that 5/8″ board under 1/2″ CBU may not be a sufficient installation.
Now I don’t know what to do. Do I give up on having an even transition from wood to brick? Do I use the CBU I already bought and hauled to the house with the wife’s minivan?
Do I go with Ditra? Notice I didn’t list that “P” word as one of my options.
Hey Daniel,
I would either leave the 1/2″ or replace it with new 1/2″ (if it’s in bad shape) then go right over that with ditra. The 5/8″ plus the 1/2″ is the correct substrate beneath your ditra. Your joist deflection is at 620 – considerably more than the required 360 so you’re good to go.
Roger,
Thanks for your same-day reply. I don’t think I’ve ever seen a site so informative, helpful, and consistently maintained by an individual. I admire your dedication.
One final issue now that you’ve shifted my gears:
The 1/2″ is in good physical condition, but some moron rolled paint on half of it. Will it be necessary to roughen it with a sander (or sand it up completely) to ensure a good bond with the ditra? Or can I risk it so long as I douse the dog?
Hey Daniel,
Would you happen to know this moron personally?
It needs to be at least roughed up with a sander to create a substrate the thinset can bond to well. Ideally it would all be removed, nothing’s ideal eh? Douse your dog anyway.
Roger,
Thanks for all of your advice above. I’ve finally gotten my project started and now have my ditra set.
I know YOU would never do this, as you are a person who abides by all rules… but based on what I have read on some jackass rulebreaker’s website
I’ve skimmed the ditra with modified. IF you were a person who would do such a thing, how long would you wait for the skim coat to cure before setting tile, and would you use modified or unmodified to do so?
Thanks again.
Hey Daniel,
A minimum of 12 hours before you go over that cured thinset. You can use either modified or unmodified at that point. The requirement for unmodified is due to the need of the polymers in the modified needing air to cure. Since it cured in the cavities without tile covering them you now have a solid, completely cured substrate on which to install your tile.
Hello Roger, when using cbu in a shower stall that you are waterproofing with Redgard, how do you prepare the corners that run from floor to ceiling? I know that Redgard doesnt stick to silicone. Do you use mesh for the Redgard to “build up” on or what? Also, same question for the niche. Thank you for your time and your help!
Hey Scotty,
You silicone them. While redgard will not adhere well to it you need to allow for movement in the different planes – silicone does that. As you paint over it with the redgard it will fill in any tiny portions you may have missed.