I know there are some people out there (not my regular readers like you!) that read what I write and think to themselves ‘okay, but I’m sure that won’t happen with my installation’. So periodically I’m gonna post things like this that show exactly what happens when things aren’t built correctly. And yes, it will happen to yours, too, if the proper steps aren’t taken. If you care to see more train wrecks you can check out my ‘flawed‘ page wherein I post photos of absolutely horrible tile installations which I’ve torn out and replaced.
See that crack in the tile right there? (The line down the center is not a grout line – it’s a crack. You can click on it for a larger version) That bathroom floor is less than eight months old. It was installed with hardibacker over the subfloor and thinset. At least that part is correct, but that was about it. There was no thinset beneath the hardi and the seams between the sheets were not taped and thinsetted. To a lesser extent the correct screws were not used in the hardi – they committed the cardinal sin of using drywall screws in the backerboard. Yeah. Wrong.
So, back to the crack. (Never thought I’d ever type that!) As soon as I walked in and saw it I knew exactly what was wrong and I knew why. The crack was in an absolutely straight line – a dead giveaway that the crack is likely over a seam in the backerboard which wasn’t taped. If you read my post about how to correctly install flooring backerboards you will see that there needs to be thinset below the boards, the seams need to be taped, and the proper screws need to be used. None of which was done.
And here’s what was beneath it: 
If you click on that photo (I dare you!) you can see the crack follows the seam of the backerboard exactly. Without the tape on the seams the individual boards may move in different directions and, without the support beneath from thinset, they will move independently and eventually crack your tile or, more commonly, your grout lines first.
When you tape and thinset your backerboard seams it will lock the two separate sheets together and any movements in the substrate (seasonal micro-movements, completely normal) will all move as one and in the same direction. This won’t cause any stress on your tile.
I simply pulled up all those cracked pieces and chipped the old thinset out of there, installed proper screws along the seam, taped and mudded the seam (when I say ‘mudded’ the seams I mean thinset) and reset new tiles and grouted it up. Once that grout cures it will lighten and it will look brand new.
So all these little things like ‘tape and mud your backerboard seams’ that I throw out there may seem like it’s just overkill or taking extra precautions which aren’t really necessary – well, they are necessary. And this is why. This will also happen on a shower wall if your seams are not taped and mudded. If the boards move differently it causes uneven stress on your tile – it needs to release somewhere. Ninety seconds worth of work to tape the seam to begin with would have prevented this – just do it! (damnit)
If preventing cracked tile isn’t enough motivation for you maybe this will: all of my regular readers know what happens if your tile or grout crack – your dog will burst into flames! So if you don’t do it for your tile, do it for your pooch. Not only are flaming dogs bad for, well, the dogs, but they tend to run around and catch other stuff on fire too! You don’t want that, do you?
TAPE YOUR SEAMS! 
I am installing ceramic tile over VCT on the 2nd floor of a commercial building with a steel frame. Subfloor is 3/4″ plywood. Do I need to apply thinset below the backerboard for this application?
Hey Stuart,
Yes. The thinset is not there to bond anything, it’s there to fill any voids that may exist under a 15 square foot sheet of backer. Once cured your entire backer is fully supported without any movement.
Is there a certain style of thinset to use? Multi-flex or anything like that, or just standard thinset?
Thanks for your reply.
You can use any thinset at all for application beneath backerboards. Makes no difference.
Roger,
I’m installing a tile floor in a laundry room which has VCT tile currently. I can see black cutback adhesive under it at the leading edge where the flooring in the hallway was. Should I install 1/2″ Durock over the VCT, or remove the VCT first, leaving the cutback adhesive as my next obstacle? The subfloor is 3/4″ plywood over pine planking with 2X12 joists 16″ O.C. Not sure how to prep VCT for thinset prior to laying the backerboard. It’s my in-laws’ house (read-not getting paid), so I’ll never hear the end of it if I screw it up! Thanks for your great site, I’ve used it a couple times to bail my ass out!
Hey Jeff,
The easiest will be to simply go over the vct. You don’t need to do anything to the vct, the thinset doesn’t need to bond to it. It’s only there to fill any voids and make them solid. Just install a layer of thinset and screw down your backerboard over it.
In-laws – yeah, been there, done that.
Thanks for the info, Roger!
Roger,
Can I tape and mud the seems on my backer board as I tile or do I need to do this first and let it cure before doing tile? How long to cure before tiling?
Hey Noah,
You can do it either way. If you do it first it needs to cure for about 12 hours before tile installation.
Hi, I’m working on my first tile job. It’s a 5’x6′ half bath. I put down hardibacker over thinset this weekend. I tried to remain aware of how much thinset (used a 1/4 in notched trowel) to keep the board edges level; however, I’m noticing now that I have about a 1/16 difference in height in a few places. I haven’t thinset/taped the edges yet. Can I just level the difference out with thinset, let it dry, and then tile?
Also the hardibacker screws didn’t go all the way flush. There is a slight bump. I’ve tried backing them out and back in, but it’s still now completely flush. Will the thinset even it out when I tile?
Thank you!!
Hey Jeff,
You can just even all that out as you set your tile. With a 3/8″ or larger trowel you’ll have a good 1/16″ + of wiggle room to cover anything you need to.
i have a particle board subfloor and i want to lay ceramic tile on it and i already put cement backer board on it without using thinset underneath but i did tape the seam and applied thinset. can i go ahead and lay the ceramic tile in this kind of situation?
Hi Sunday,
Not if you want it to last. Particle board has no business beneath any type of tile installation at all. I needs to be replaced with regular plywood. You can sometimes get away without thinset beneath the backer, but not often. The good news is that since you didn’t put thinset beneath the backer you can remove it and reuse it.
Redoing an old bathroom floor and The closet flange is nailed to the plywood. Per some googling it doesn’t appear that this will be up to code once the backer and tile are in place… To what do I (re)fasten the flange? Or should I just leave it and get a huge wax ring?
Hey Nick,
You can remove the flange, install your tile and either cut slots where the screws go or drill through the tile and install the flange on top of the tile. Or you can get a huge wax ring. The first option is better, and proper.
Thanks. I appreciate the help and all the work you do on this site.
I’ve got the hardi-backer board set onto the floor with thinset and I am trying to srew it down to the plywood sub-floor but the screws won’t counter-sink into the backer board; I double checked that we’re not screwing into a joist. They’re all sitting up slightly less than the height of the screw head, which is understandably making me nervous.
Hi Nick,
Are you using backerboard screws or regular deck or drywall screws? Backerboard screws have teeth beneath the head which will dig into the board and countersink them. If you are using the backer screws then your drill may not have enough power to countersink them.
I had a feeling that might be the problem…thanks for the response. Quick follow-up question: my wife and I laid the backer board before I left on a work trip, will it be a problem to come back to these in a couple of days with a stronger drill to countersink them after the thinset hardens?
Thanks again for all your advice.
Nope, shouldn’t be a problem at all.
Hey Elf,
Laying the thinset under my hardie backer boards today…how long do I need to let the mortar cure before being able to walk on/begin laying my tile? Also, do I tape and mud the seams right or way or is it best to do that as I tile?
Awesome. Thanks for the help.
Sensei,
The kid at the bigbox store sold me 5/8-inch particleboard (since they no longer carry 5/8-inch plywood) to match the 50-some-year-old plywood subfloor in the bathroom I’m retiling (shower tiling job turned out great, thanks to your guidance here).
I tore out some of the old, waterlogged/rotted subfloor, but left the majority, which was dry, and which is why I’m hellbent on installing new 5/8-inch stuff–to match the thickness of the installed/remaining subfloor.
But I got this tiny little elf-like voice in the back of my head that keeps suggesting that particleboard may not be up to the task. (I plan to install half-inch Hardibacker–PROPERLY, thanks to your guidelines–over everything.) Should I keep going with my particles, or did I do the right thing by pausing mid-job to seek your wisdom?
As always, you remain,
My great and powerful tile ninja
Hey MK,
You were absolutely correct! Particle board belongs nowhere near a tile installation, especially beneath it. 5/8″ plywood is actually 19/32″. It’s sold as that rather than 5/8″ You should be able to find that nearly anywhere that sells plywood.
Floor Elf Rocks!
Floor Elf Rocks!
I KNEW it!
You da man, Floor Elf.
Thanks!
Forgive me, Father…
…It’s been two years since my last inquiry and I STILL haven’t gotten around to finishing my bathroom floor. (If you started a second site called The Electrician Elf then maybe I’d be done by now, but that’s just me deflecting blame anywhere I can.)
In any case, some wiring has FINALLY happened, and I’m back to the world of thinset, and modified vs. unmodified, and sanded vs. unsanded.
i finally installed new plywood over the exposed floor joists. Upon checking the floor with a level (NOT to verify whether the floor itself was level, but rather to check whether it was flat, per my sensei), I noted a distinct bowed depression mid-floor, maybe 1/16 of an inch, so I rolled on some latex primer over the plywood then filed the divot with some floor leveler.
(So far so good?)
My next plan is to layer on thinset prior to affixing DUROCK over the plywood/leveling compound, but I have a question about the waste water drain where the toilet flange will go.
I understand that modern flanges should be affixed to the floor, but does this mean they should be affixed directly to the plywood surface, to the DUROCK surface, or to the actual tiled floor once it’s installed, grouted, sealed, etc.?
Appreciate your guidance, as always.
M “The guy who long-ago suggested you refer to your content here as ‘elf-help’ books” K
You remain
Hi MK,
I thought you were just on a two year drunken sabbatical! Little disappointed I wasn’t invited.
Ideally the flange should be affixed to the tile surface. More often than not (in most remodels) they are affixed to the substrate surface. Either works, but given the choice put it on top of your tile.
Hi,
We had a tile guy install tiles in a bathroom for us. He installed 1/2 inch denshield and “screwed the heck out of it”. He did not put thinset under it from what I can tell. Of course, less than 2 months later we have a crack from one side of the bath to the other. I believe it is where he seamed the denshield (it would be an easy and obvious place to do that), and from measuring behind it I’m pretty sure it is right on the seam from the plywood subfloor. My question – he is finally coming to address the problem tomorrow – he wants to just replace those tiles. I think it will just happen again. What is your opinion, and if he needs to do more, how do I convince him?
Thanks!
Hi Gina,
If there is a seam in the denshield there and he did not tape and mud it that could definitely crack. If the seams of the denshield were placed over the seams in the subfloor it is guaranteed to crack. Per standards any underlayment sheet must be offset from subfloor seams to prevent exactly what you are experiencing. The seams need to be taped and mudded for the same reason.
How do you convince him? I honestly have no idea. You could show him this post, require him to look up the standard and show you where his method is acceptable (it isn’t) or tell him he can do it his way but you want in writing that if it happens again he will replace the entire floor properly. This is one reason communication before the project is direly important.
I have installed my hardibackeron my shower walls and I used silicone in the corners and (due to a mis-read on my part) in the plains and screw holes. I have been reading some…I know…and I am now afraid my thinset willnnot adhere to the silicone. I have been trying to remove the silicone without much luck. What to do now?
Hey Tim,
You can get a silicone remover in the paint department at any big box or hardware store. Just spray it on and wipe it off, it essentially ‘melts’ the silicone.
I read a query on “E-How” or somewhere about mudding and taping the seams in the backerboard, and the “best response” answer was “no, you don’t have to”. I laughed so hard my wife came in to see what was so funny. I think it had to do with a shower wall rather than a floor, but still…took me all of 30 minutes to do my entire tub/shower, so what’s the big deal? The Floor Elf has spoken, and he said to tape and mud them, that’s good enough for me!
Yeah, whenever I’m in a crappy mood I hit ehow or the DIY network websites and read through the tile advice – cheers me right up! (and pisses me off at the same time…)
Roger,
Any recommendations of a good tile guy in Denver? I’m doing all the prep work with the subflooring and walls but wanted to hire someone to actually lay and grout the tile. Thoughts? thanks, this site is very helpful.
Hi Casey,
Joe from Percoco marble is in Denver. You can find him here: http://www.percocomarble.com/
Jeff and Kim from Brekkus here: http://www.brekhustile.com/index.html
Larry from LP Innovation here: http://lpinnovationsllc.com/
Chris from 3 Sons Design here: http://3sonsdesign.com/index.html
Keep in mind that a lot of professional tile contractors, myself included, refuse to install over someone else’s substrate. The substrate is the most important factor in the durability and longevity of any tile installation and the good contractors won’t touch someone else’s prep work. We simply have a difficult time leaving our reputation and work on the skill, or lack of, someone else. Not that you don’t or can’t do it correctly, it’s a liability and reputation thing. They may or may not, if they don’t, please don’t take it personally.
Roger,
I just finished my first installation of backer board in my bathroom. I just installed it without any quick set or mudding. Then I read your website and am now worried about laying tile over it. Are there any recommendations you can give me to salvage my project (hopefully without removing the backer board)?
Hi EJ,
None without removing the backer. Adding an uncoupling membrane such as ditra may help, but it may not. It needs thinset beneath it to fully support it. Sorry.
Roger,
Oooo, now you are confusing the (heck) out of me. A while back I asked you a question about Kerdi and cement board vs. drywall. I said I was an old school kind of guy and somhow hanging drywall in a shower seemed wrong. You basicly convinced me that drywall is better – it works fine with Kerdi, is easier to work with (espically if, like me, you are placing niches, and it’s cheaper). OK, you did it, you got me on the drywall train. The bathroom re-model is due to start tomorrow, and lo and behold, out comes your Waterproofing with Kerdi ebook. Being the wonderful customer that I am, I buy it, and in it I see you say – to paraphrase – you can use drywall with Kerdi, but cement board is better!
Dude, my dog is on fire, my duck needs a home, and the beer supply is at an all time low. Unless you can tell me differently, for a shower (with tub) and with niches, I am going with drywall.
Thanks
-dave
Hi Dave,
Didn’t mean to confuse you. Whatever I said, whenever I said it, was not intended to make you think drywall was the better option. I tend to overemphasize the viability of drywall because people simply don’t believe it – ever. From the standpoint of strictly an acceptable substrate – backerboards and drywall are the same. From the standpoint of ease of use – drywall wins, hands down.
However! Being that I have readers from all over the world, including Canada
I put that in there for two reasons. First, anything I describe about working with backerboards applies to drywall – the opposite is not true. Secondly, drywall is not accepted by building codes in Canada for use in showers – even behind kerdi. So I needed to address that.
If you don’t live in Canada – use drywall. Really. I do most of the time. Overall, from an installation standpoint, it is a better option (easier). From a completed installation standpoint – it makes absolutely no difference at all. Provided the kerdi was installed correctly nothing behind it ever gets wet. Ever. If you could find a way to attach the kerdi you could make your walls out of paper mache and it wouldn’t be a problem.
Put out your dog, build your duck a home and, for the love of everything you hold holy – go get some more damn beer! And use the drywall.
Hello,
I’ve read your website and would like to confirm a few things. I’ve got hardie backer board around my new tub. If I understand correctly I don’t mud and tape the corners I use silicone. What name brand do I look for? I tape and mud only the in plane seams. What thin set do I use? My tiles are a mix of 3×6 porcelain with a border made up of marble and glass. I will wipe down the backer board and prime it . I’m using Redgard 2parts water to 1 part Redgard. I’ll use a brush to get the corners and inside my niche and roll out the rest. When it’s completely dry I’ll apply the Redgard only (no water)going in one direction, let it dry and repeat going in the other direction. I’ll then call the tiler. Have I missed anything?
Hi Esther,
You have everything correct. The brand of silicone doesn’t really matter – as long as it is 100% silicone. Any regular thinset will work just fine for the in-plane seams. Versabond is a good one that is readily available.
Hi Roger,
Is it an issue if there is some paint on the hardiebacker cement board prior to applying RedGuard or AquaDefense as a liquid membrane?
Some contractors painted my bathroom and painted over some of the cement board…if it is an issue, any thoughts as to how to best remove the paint without taking the hardiebacker down?
Thanks,
Chris
Hey Chris,
As long as it’s just a little and not an epoxy-based you’ll be fine. If it is epoxy-based it’s easier just to go over it with a primer like killz. The redgard can go right over that.
Hi Roger,
The paint is 100% Acrylic Latex. It seems like the contractors put some drywall mud (1/16 – 1/8″ thick) over that area of the cement board and then painted over it with the acrylic latex paint. Not sure why they did this, but I need to waterproof that area. The paint section is about 8″ wide and runs all the way up to the ceiling (about 6-7 feet high).
Am I still ok to primer over it with killz and then proceed with my AquaDefense?
Apreciate the help here.
Chris
Yes, that will be just fine.
First let me say I’m so glad a site like this exists. I just found this site and it’s just a worth of knowledge.
I’m putting in a new tile wall for my tub/shower. I’m hoping you could answer a couple questions that have me stuck.
1. I’m using a vapor barrier behind the hardi backer except for the niche I built in. Can i use redgard in that area or do I need to remove all the backer board there and redo with the vapor barrier behind?
2. When mudding the seams with thinset (ultraflex 2) do I pack with mud first, then tape, then go over again with mud? Or, do I leave the gap unpacked, tape, then cover with mud? Everything I seen so far showed doing it left unpacked, and just taped first. Not sure why I’m thinking packing the seams first would make them stronger.
3. In reference to your post on this page “Silicone in the corners so the walls can move in different directions – which they will do anyway – and tape and mud all the in-plane seams on the same walls.”
Is that saying that my corner seems don’t get taped and mudded, but silicone instead. If so you just saved me because I was about to tape and mud all seams.
Thanks again,
Hi Chris,
1. You can use redgard in the niche – it’ll be fine.
2. Pack the thinset into the gap, then tape, then skim. That is the purpose of doing it – to lock it together.
3. That’s correct, the corners do not need to be taped and mudded. The silicone is installed to prevent thinset from getting in there as you’re tiling and allows movement.
I have framed in for a shower, installed 1/2″ mold resistant drywall, installed drywall screws every 12″, primed the bare drywall, unfortunately I used an oil based primer (sherwin williams pro-block) and I have always used latex primer before. I installed Kerdi over it, but I didn’t like how the mud was/wasn’t setting up on the oil based, primed drywall. Asking the Schluter sales rep, but I don’t think I like it, and I think I’m going to tear it off, sand the walls, apply a bonding primer, then install new Kerdi. That make sense to you? Thanks much!
Hey Micah,
Yes, it does make sense to me. The oil base will likely cause problems with the thinset bond. It would be better to remove that and be safe.
Roger,
I read your article on installing tiles on a shower ceiling. You inspired me and I just started a bathroom tile project. It must be providence that I stumbled across you site. I enjoy your humor and your answers are quite informative and logical. I do have three questions:
1) What is “modified thinset”?
2) What would be the best thinset to use with porcelain tiles, especially the tiles to be placed on the shower ceiling?
3) Would the grout manufacturers “siliconized, sanded caulk” satisfy the need for silcone caulking on change of plane interfaces?
Thank-you
Clark
Hi Clark,
1. It is thinset which has powdered latex (and/or other types) polymers added. Once mixed with water these are activated and help the thinset be more sticky, give some flex ability to the cured product, all sorts of different things. The main reason is to hold moisture in the mix for a longer period of time to get a stronger cure.
2. A good modified thinset such as mapei ultraflex II or versabond will work fine.
3. It will satisfy the need, but it won’t last nearly as long as a color-matched silicone. Those tend to dry out and crack more quickly than silicone and will require removal and replacement more often. That’s normal, by the way.
Roger,
I’ve finished putting up backerboard and am ready to tape. I’m a little confused as to how to handle (tape) the inside corners or plane changes. Do I mud and tape them like regular drywall or or do I use some sort of flexible caulking ?
Hey Jesse,
Silicone in the corners so the walls can move in different directions – which they will do anyway – and tape and mud all the in-plane seams on the same walls.
I installed hardiebacker on my tub/shower walls during the construction of my house. When i came back a week later they had textured and painted it. I have removed 90% of the mud back down to the hardiebacker. I am planning on using Red Gard.Should i prime the hardirbacker with oil base primer to seal any mud that is still on the surface of just apply the red Gard?
Nope, prime it with a 2:1 mixture of water and redgard. This will fill in all the pores and lock it in so the redgard adheres well. Do not use an oil-based primer anywhere in there – it’s a bond breaker for the redgard.
Roger,
I’m about to start hanging backerboard on the walls of my first Kerdi tile shower. Do you like to use glue on the studs, along with screws?
Thanks for all the great info and help!
Hey Mike,
Nope, the backerboard screws are plenty. You can if it will help you sleep at night, though, it won’t hurt anything.
Thanks, Roger. One more. I’m having a frameless door installed (after I’m all done). The design of the shower requires the door to be hinged on the glass panel side, rather than the tile jamb. I think the hinges actually attach to the header and curb. Is a Kerdi-Curb strong enough to support the hinge, or would you use wood?
I would use wood. While the kerdi curb with tile on it is stable, swinging a door off of it would keep me up at night.
Roger
I am going to tile around my tube if I use backerboard do I still need to use a waterproofing system like
SCHLUTER KERDI-band over the backerboard like they do in the DIY channels
Hi Craig,
YES! Backerboard is not waterproof – it is water stable. It won’t fall apart when wet – but it soaks up water just like your driveway when it rains. You need either a membrane over it or a vapor barrier behind it, but you do need a barrier of some sort.
Roger,
I am going to replace a perfectly good walk in fiberglass shower with a custom slate shower. (Mamma said so.)
I will be using the Kerdi Line drain and the Schluter 55 x 55 shower tray that will drain at the wall. The shower tray will be cut to 55 wide by 36 deep.
The entire shower walls and floor will be covered with Kerdi membrane.
My question is one of dimension: Given the room is 58-1/2 to 58-3/4 wide (out of square of course.), the tray is 55 inches wide and sloping toward the back wall. What do I fill the extra 3-1/2 to 3-3/4 gap with?( say 1-3/4 to 1-7/8 on each side) Dry mud bed, Ditra and thinset, Kerdi caulking, leftover tray????? If dry mud bed, will this work with that small of groove?
Thanks again for your words of wisdom.
Fred
Hey Fred,
Yup, deck mud is the way to go. Install your tray, pack the open space level with deck mud, let it cure, then install your kerdi. The deck mud can be used to fill in any areas at all you need.
To apply thinset on an inside corner, do you use the flat edge of a trowel, or should you use a corner tool that is used for drywall compound? Thanks.
Hi Kevin,
Your trowel should have notches on the end as well. Just stick the corner of the trowel in the corner and pull it straight back. You want ridges of thinset there as well. If you use something flat then all you’ll have is a skim-coat of thinset to which nothing will adhere.