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.
Hi Roger,
I decided to do some remodeling in my bathroom which previously had carpet for flooring ( yuck!). Predictably the sub floor underneath the carpet is rotted in several areas so I need to replace it. I was wondering when replacing the sub floor do I need two layers of plywood then cement backer board then tile, or can I use just one layer of plywood then cement backer board then tile. I’ve seen websites that say both ways and I just want to make sure I do this right. Thank you so much for all of your help.
Hi Dessa,
Per standard a double layer of plywood is required under your tile installation. You can use 1/4″ backerboard rather than 1/2″ if you need to save some height, but you do need two layers of plywood at a minimum 1 1/8″ above the joists. The second layer is only screwed into the first layer – not into the joists.
Hi Roger,
I’m getting ready to install 12″ x 12″ ceramic tile in my Kitchen,dining room, bath and laundry room.(all Interconnected) I had the house built about 10 years ago and the floor is sheet vinyl over Luan over 3/4″ ply. I want to use 1/4″ Hardibacker. Do I need to tear up the Luan? If so, wouldn’t this be glued down? I know standard plywood is very strong, and with the Luan attached on top wouldn’t this be a pretty good base for the Hardibacker as long as I thinset and screw it down? This area I’m doing covers about 300 square feet. Don’t want to see tile failure down the road, but I don’t won’t to tear any more up than necessary.
Thanks,Steve
Hey Steve,
The luan really needs to be removed. The problem is that luan has a lot of hollow areas. Even though it is only 1/8″ – 1/4″ thick, there are several voids in the product itself. Even those tiny voids can move beneath your tile which will lead to problems. It is likely not glued down, but I’m sure they stapled the crap out of it. I know it sucks, but I also know redoing a large tile installation sucks too, I learned this from firsthand experience – not the good kind.
Roger,
This is a big project for our home and I want it to last my lifetime. I’m also going to use the load calculator to check the existing subfloor. I may wind up adding some additional bracing also.
Thank, Steve
Hi Roger,
I’m renovating my kitchen and approaching the floor/cabinet time frame. Would you recommend installing the cabinets first, then putting down the floor (and lose a bit of height being the cabinets won’t be on that floor) or put down the floor and set the cabinets on those?
Also, what’s the preferred method for cutting the boards? And do you predrill your screw hole or do the screws go in easily enough?
Thank you for all your help!!!
Adam
Hey Adam,
You can do it either way. It’s normally easier and quicker to put the floor down first – less cutting, but either way works fine. I just use a razor knife and scoring tool to cut the boards. I also have a roto-zip for specialty cuts. As long as you use the correct backerboard screws they will go right into the board without problems.
Roger,
One more thing. Looking at your pictures, I am curious if you mortar the entire room, then go back and screw it down, or if you screw one row at a time like you do the mortar.
I really want to thank you again for what you do here on your site. You freely share a lot of information and show a tremendous amount of patience with people, like myself, with our questions. I think it tells a lot about your character.
Thanks,
Jeff
I normally do two or three sheets at a time. Put down the thinset, lay the backer in it, then screw them down and move on to the next three.
Hi Roger,
I’m finally getting to the “big project”. I’ve already got the plywood down and will start laying out and cutting the backer tonight. I’m using the 1/4″ Green E Board stuff for my backer. Since I will be working on this project in my free time, I will not be placing tile for at least a few more days after the backer is down. I’ve read your responses where I can walk on stuff, which is great, cause I’ll need to. However, do I need to fill the gaps and tape the backer right away since I’m waiting a while to finish the tile part, or can I still tape and fill while I’m tiling like you do?
Thanks, Jeff
Hey Jeff,
You can still tape and mud the seams as you set the tile. No need to do it right away.
Hi Roger,
Thanks in advance for your expertise and help. I am doing a small power-room bathroom and this is my first time tiling. I ripped out the old tile and found that it was cemented down. The concrete broke very unevenly as I hammered out the old floor. I knocked the concrete ‘lumps’ down as flush as I could then poured that self-leveling stuff down. The floor is now pretty even but I am almost 1″ below the toilet flange. To fill the gap, I was going to use backerboard on top of the ‘concrete’ but I think from reading your answers, that’s not advisable. Plus I would still be a bit below the flange even after tile installation (including thinset and mudbed). I didn’t really want to have to use concrete to make up the space now that I have already put down the self-leveling stuff (hindsight….), but I didn’t know if I could do something like put down 1/2″ plywood with thinset, then apply the tile directly to that using mud. I read that you aren’t supposed to use wood less that 1/2″ under tile, is that true? If I was able to use thinner wood, I could put down maybe 1/4″ plywood first, then backerboard (with thinset between both), then the tile onto the backerboard and the height would be good. Ugh. So sorry for the long explanation. When you have time, please help me out. Many thanks!
Joe
Hey Joe,
The 1/2″ rule is true for the most part – as is the inability to install wood or backerboard over concrete or SLC. Let me ask the obvious: if you are doing all this simply because of the toilet flange why don’t you just lower the toilet flange? Much quicker, easier and cheaper. If not your best option now is SLC to the height you need it.
Hi Roger,
Thanks again for the input. I have an answer to your question. I didn’t know that was an option. But if it is, that’s great, and I will look into how to do that. If I can easily lower the flange, can I then just apply the tile directly on top of the self-level ‘concrete’ stuff with the normal mudbed under?
Thanks,
Joe
Yes, you can tile directly to the SLC.
well i would like to know if i can put hardie backer over vinyl. thin-setting it and screwing it down.
Well, yes.
If there is luan or particle board beneath your vinyl – no. Or – yes, if you screw your backer down every four square inches.
Hi Roger,
Great article. I was wondering if this backer board can be used when you have a concrete base with no ply?
I have ripped up the old ceramic tile in my kitchen and at the moment there is tile adhesive stuck onto the concrete base of the floor. I am pretty certain the concrete base of the floor underneath the adhesive is neither level nor flat so am questioning whether I even need to spend countless hours trying to remove the old adhesive.
What do you suggest?
I am questioning if I can use thinset and then the backer board to give me the proper substrate.
I guess I would need to pre-drill all the backer board screws if it is going into concrete?
Thanks you in advance for your advice.
Regards
Gavin Ogden
Hey Gavin,
Nope, no backerboard over concrete. Your four options, in order from best first would be:
Scarify or otherwise remove the adhesive from the concrete and begin with bare concrete, use a medium-bed mortar to compensate for any unlevel or uneven spots.
Prime the floor (with the adhesive still on it – PROVIDED THE ADHESIVE IS ADHERED WELL!) with a proper primer and use a self-leveling cement to level out the floor – install directly to that.
If the adhesive is adhered well simply install a membrane such as ditra with thinset and use a straight-edge to make it flat. Install tile directly to that.
Float out the floor (again with the adhesive if it is adhered well) with thinset, let it cure, then install your tile.
Backerboard over concrete will not last long-term and will cause more problems than it solves. The fasteners must be drilled and tapped, and even then are very, very difficult to get flush with the board. Given that you’ll likely end up with a floor more uneven than it currently is. Any of the above four will give you a proper substrate.
Thanks for helping me and giving me some options Roger. Better get started.
Getting ready to put porcelain tile down in our kitchen (approx 70 sq.ft. 18″x18″ tiles.) I knocked a wall out and put an island in, I positioned the island out into the dining area about 5″ to make the kitchen bigger. The old kitchen was carpeted and the dining area has hardwood floors. The hardwood floor is about an 1/8″ higher than the floor I’m tiling. Is it ok to use the thinset to fill in the difference? I’m using 1/4″ hardy backer, I have 1/2″ plywood and 3/4″ OSB substrate. This is only an issue at the two entrances to the kitchen. I hope I explained the situation clearly. Another way to ask it, should I sand the hardwood floor down 1/8″ to be flush with the floor I’m tiling before I lay the backer board on it, or is that unnecessary?
Hey Matt,
No reason to sand down the wood floors – that’s gonna make it all look like crap.
You can build up 1/8″ with thinset if needed – just use a larger trowel, 1/2″ x 1/2″ would work. A better option would be the 1/2″ trowel with a medium-bed mortar. It contains more sand and won’t shrink as much.
Hi Roger…..Great site.
Sooner or later I’ll get to tiling my entire place. I’ve gotten the carpet torn out and old linoleum scraped off the kitchen floor. What’s left there is the linoleum backing and under that, the GLUE. I’m trying to get it all off, but it’s very messy and time consuming and expensive using strippers, and I don’t know how else to do it. However, am I wasting my time trying? The backer for the linoleum is quarter inch particle board on top of OSB. All the cabinets, appliances and everything sits on top of that particle board. Can I get away with placing tile on top of that after I get it all stripped?
Thanks so much for taking the time and effort to help others. Keep up the smartassery!
Best, Steve
Hi Steve,
Absolutely not! Particle board has absolutely no business anywhere near a tile installation at all. Don’t bother attempting to get the glue off of it, just tear it all out and the glue will go with it. You can buy or rent a toe-kick saw to cut right up against the cabinets to remove it. If you put tile over particle board your dog will become engulfed in flames – now that’s not very nice.
ROGER I AM LAYING TILE OVER BACKERBOARD THAT IS ON TOP OF BLACKTOP!! OR AMESITE, I HAVE LEVELED THE DIPS WITH CEMENT BUT DID NOT COVER THE TOTAL SURFACE, AM I RUNNING INTO TROUBLE? THE BLACKTOP FLOOR HELD AN ANTIQUE POOL TABLE FOR 30 YEARS WITHOUT ANY INDENTATIONS, SO I FELT IT WAS FAIRLY SOLID!! THANKS GEORGE
Hey George,
Unless that backerboard is fully supported with thinset beneath it and firmly fastened into the blacktop then yes, you’re running into trouble. While the initial floor may have been solid unless that backer is supported you essentially have another full floating floor above it which may have hollow spots beneath it. When stepped on it may dip down to the solid part – the original floor – but that movement may cause your tile installation to fail.
I appreciate your answer, I have walked/crawled/and tapped my way across the floor (22X16 ) and heard few if any variances in pitch, I did cover the low spots with thinset, before laying backerboard and have doubled any areas with extra screws without any lowering of the backerboard, all seams are flush with people walking along them while I lay and watch, Boy I hope I’m ok!!! thanks for your help!! george
If you don’t notice any abnormal movement you may be fine. As long as it’s supported well it should be all right. I can’t guarantee failure, I can only give advice with methods that guarantee success.
Good luck.
Hey Roger,
I was talking about the backer. Would you recommend just skipping the backer and adding another layer of plywood to reach my desired floor height? I thought the cement board would add a little extra stability, but it is pretty much just for the height. If I do use the backer board do I still need to thinset it since I’m using the ditra on the top layer?
Thanks,
Grant
Yes, the backer still needs thinset beneath it. You can add another layer of plywood – backerboard lends absolutely no structural support whatsoever.
Hey Roger,
Thanks for the tip on the flange. It confirms my suspicions of the plumber who wanted to get work from me. He suggest the washer idea as the “best” way to install the flange.
Anyway, a quick follow up. You mentioned not screwing into the joist, because of the different rates of movement in the wood. Does this also mean that since I have two layers of sub floor one solid wood slat boards and then plywood, that I only want to penetrate the top layer of sub floor? If so then I need shorter screws.
Also, does the notch size matter on the trowel?
Thanks,
Grant
Nope, just don’t screw the backer into the joists. Notch size depends on the tile and substrate.
You talking about notch for backer or tile? For the backer I normally use 1/4 x 1/4″, but you can use a bigger one, no big deal. For tile: Bigger tile, more unlevel substrate = larger trowel. A 3/16 x3/16″ is what I normally use for most tile installations.
Roger,
I need help. Had a contractor put down backer board in my kitchen. He didn’t leave a gap for expansion, he lined up all the joints, didn’t use thin set between the plywood subfloor and the backer board ,and used drywall screws to hold it down.The kitchen was completely tiled and cabinets are in place with a huge island in the center of the kitchen. The floor sucks. It’s making popping noises the tile and grout are cracking. Do I have to rip up the hole tile job including the backerboard and start over? What about the existing cabinets and the island and all the granite on them. To make matters worse The kitchen has radiant heat that runs between the floor joists so I have to be very careful when putting down screws. I need HELP. Thinking of putting down that orange polyethylene mat down. I have it in the bathroom and no problems. The kitchen is brand new 3 yrs old. NEED HELP PLEASE!!!!!!!!
Hey Danny,
Yes, the entire floor needs to be replaced – sorry. You can rent or buy a toe-kick saw for tile with which you can cut the tile right up against the cabinets to remove it. When you redo it ditra would probably be your best option, no screws and much easier to work with, and a better tile substrate.
THANKS ROGER, I WAS AFRAID YOU WOULD SAY THAT. GONNA TRY THE DITRA. ANY ADVISE ON THE INSTALLATION? DO I HAVE TO REMOVE THE BACKER BOARD?I’LL LET YOU KNOW HOW IT TURNS OUT. THANKS DANNY
If the backer doesn’t have thinset beneath it then yes, it should be removed as well. Be sure to cut the tile around the cabinets first, then just go to town.
Hi Roger,
I have my bath fully gutted and installed 3/4″ plywood on top of the slat boards. I bought 1/4″ Hardiebacker for the floor and 1/2″ cement board for the tub/shower walls. (Question #1) Would you put the the Hardiebacker under were the tub will go? I also thought of using the 1/2″ under the tub to raise it up a little and make the top of the tile more flush with the bottom of the tub. Is that a concern or do you just cover that bottom half inch of the tub with tile?
Also, I wanted to get the floor down so I can finish the plumbing, install the tub and the toliet flange. (#2) Do you have a preference of how the flange sits when it comes to the tile install? Would you sit the flange right on top of the Hardiebacker or use washers to raise it up a little and be able to set tile underneath? Just trying to think ahead.
I also have some other steps before tile. Install the window, drywall etc. (#3) Do I lay my Hardiebacker as a step in laying the sub floor or do I wait right before tile? I don’t want to damage the Hardiebacker while I’m doing other work, but I thought of it as a step in laying the subfloor and then laying the Ditra membrane right before tile. Am I mistaken?
Sorry for all the little questions. Again I think you have a great site and I really appreciate all the time and effort you put into helping us common folk out. Like the tip in screwing from the center out.
Thanks,
Grant
Hey Grant,
1. You can do it either way. I prefer having the backer only up to the tub, not under it. It really makes no difference, though.
2. Technically the proper way is to have the flange installed after the tile is in. That way the flange is flush on the tile surface and solid. Do not use washers to shim it up, if you are 1/16″ off it can create problems. Either wait until the tile is in or cut the tile around it.
3. You can install the hardi whenever you want – you aren’t going to hurt it. It’s very, very durable. Do not install the ditra until you are ready, or a day before you’re ready, to install tile.
Roger,
My question concerns tiling over a transition in the substrate from hardibacker on 3/4″ plywood (on 2″ x 10″ joists) to solid concrete. The exterior door to an apartment unit sits in the center of a solid 8″ wide concrete sill. The top of the sill is the same height as the top of the hardibacker, but there is a 1/4″ gap between the two. I’d like to tile up to the door threshold which would put about 2 -1/2″ of the 16″ x 16″ tile over the concrete. I know if I set the tile without some kind of membrane installed, I’ll have “issues” in the future. Is there a product you suggest keeping in mind I only need a piece for this one small area.
Thanks,
Scott
Hey Scott,
You can use any product with crack-suppression properties to compensate for the differential movement in the concrete and plywood. Ideally the backerboard could have just had the end butted up to the threshold. As long as there is thinset beneath it and it is only 2 1/2″ you could have places a full piece there so you had over 2’9″ over the wood with just the small sliver over the concrete. However, attempting to do that after the fact will not last long-term since the differential would be on two separate pieces.
You can use ditra or ditra-XL over the concrete part (you can stack ditra for height) or you could actually use a really, really good modified thinset. It would have to be one with excellent flexibility like Ultraflex III, but it would work.
Roger,
Getting ready to install tile floor in my bathroom. Do I need the full 24 hour cure time for my Laticrete thinset under my concrete board before begining to tile. I understand the need for the full 24 hours before grouting, after tiling, because of the grout inhibiting airflow to allow the thinset to fully cure, but don’t see that as a large issue for concrete board when grout joints will still be open. Was looking to lay concrete board and tile on Friday the 27th and grout on Sunday the 29th.
Thanks,
TJ
Hey TJ,
No need to wait at all. You can begin tiling right after you install the last screw into the concrete board.
Hi Roger,
total beginner installing tile, and getting ready to tile my entryway with 18″ porcelain tiles. I’ve heard I need to remove the OSB that’s above my plywood floor before putting backboard down, true? Also, because it’s the entryway and we have snow here, do I need to waterproof the backboard somehow before installing the tile?
thanks
Hi Joe,
Nope, not true at all. Make sure you put thinset beneath your backerboard (to fill voids and support it – not to adhere it to anything) and use the appropriate screws. No need to waterproof it unless you want to, it won’t get enough moisture beneath the tile to hurt anything.
Hi Roger,
I am so sorry to be asking so many questions. And here goes again.. and sorry as well if I posted this in another section in the past.
Having had one shower failure and pretty devastating health efects from that, I am a bit paranoid.
I am wondering about using a redguard over the cement board in a bathroom at least where the shower entrance flooring is and around toilet?
And what about a membrane (plastic?) between the plywood or most likely OSB subfloor and cement board?
And not to always do with tile, but can… what do you think about a barrier under flooring in any entrance where there is a potential build up of moisture, like in snowy climates?
I’m not clear on when a barrier with a floor can create more damage than it can help.
We are contracting with a builder this week and want to get these details settled.
Thank you so much!
Robin
No problem at all Robin,
The only type of membrane (waterproof) you want under your floor tile would be directly under the tile – never beneath the tile substrate (backer). These can be redgard, ditra with kerdi-band, hydroban, etc. But the membrane needs to go directly under the tile. If you have a membrane beneath your substrate the moisture or water will soak the substrate and be trapped long enough (because it’s in a solid material) that it may begin to grow mold or mildew. With the membrane right under the tile it can dissipate fairly quickly the same way it go in – through the grout lines.
You can paint redgard essentially anywhere directly under where you’ll be tiling, including floors, without a problem.
Roger, you are my hero of the day (and beyond!). Thank you so much!
Robin
No problem at all Robin,
The only type of membrane (waterproof) you want under your floor tile would be directly under the tile – never beneath the tile substrate (backer). These can be redgard, ditra with kerdi-band, hydroban, etc. But the membrane needs to go directly under the tile. If you have a membrane beneath your substrate the moisture or water will soak the substrate and be trapped long enough (because it’s in a solid material) that it may begin to grow mold or mildew. With the membrane right under the tile it can dissipate fairly quickly the same way it go in – through the grout lines.
You can paint redgard essentially anywhere directly under where you’ll be tiling, including floors, without a problem.
had a builder screw up a cament slab that was supposed to be the same height as my wood living room floor. The cement slab which i wanted to tile is 1 3/4″ lower. is there any way to build it up to match the existing wood floor? someone said cement board. others said no
Hey Bill,
No viable way to attach cement board to concrete for a long-term installation, especially just to build it up. About the only proper method would be a mud bed. You can mix deck mud and create a very flat, very stable, perfect tile substrate over that concrete for your tile installation. I would absolutely love every floor I do to be 1 3/4″ too low so I could set all my tile over a mud bed. Seriously, I’m not being a smart ass. Setting tile over a flat mud deck is a joy.
Hi Roger,
1.25″ minimum under tiles? Yikes.
1.25″ + 1/4″ backer + 1/4″ tile = 1.75″ ignoring mortar! Abutting floor is 1/2″ subfloor + 3/4″ hardwood = 1.25″ to top. That puts the tile more than 1/2″ higher.
Do you consider it too risky to drop it down to 1/2″ subfloor + 1/2″ ply + 1/8″ backer? My guess is that you’ll say too risky since you already said 1.25″ minimum!
In general, do you prefer a backer board or Ditra?
Thanks!
Hey Rob,
Yup, I don’t write ’em, I just report and follow ’em.
There are now several products available which state you can use them as substrate beneath tile over a single layer 3/4″ ply. DitraXL is one and Tec has a liquid substrate which they say can do the same (can’t remember the name of it just now). With DitraXL you are looking at the 3/4″ layer plus 1/2″ for ditra and mortar plus your tile height, if 1/4″, your total height from joists would be 1 1/2″, so you would meet flush with the wood.
TCA states that manufacturers recommendations and methods trump standards, given that one procedure covering, say floor tile substrates, cannot possibly incorporate every available method and product. We are still attempting to get a standard testing method to determine levels or amounts of ‘uncoupling’. Since the manufacturers are the ones that will warranty the installation PROVIDED THEIR INSTALLATION INSTRUCTIONS ARE FOLLOWED EXACTLY, their methods are fine. Keep in mind you must follow their method to the letter, this means unmodified with the ditra, taping and/or silicone in subfloor seams, etc. If you do that you can follow any of their methods over the TCA standards.
I always prefer ditra. Easier to work with and more benefits than a cement backer.
I just took a look at some of the Ditra documents. For 16″ oc plywood or OSB with, they require a minimum subfloor of only 5/8″. I have 1/2″. They also recommend a minimum of 3/8″ plywood / OSB if you have to build up. So I guess I’ll either remove the subfloor and replace with 5/8″ or screw/glue 3/8″. Adding 3/8″ sounds easier to me.
So I’ll have 3/8″ underlayment + 1/8″ Ditra + 1/4″ tile = 3/4″, the same as the hardwood. But I’ll still have mortar above and below the Ditra. Looks like I’ll need to use a marble threshold.
Rob
Ok, so I didn’t go back and re-read your instructions…I had a very nice and very flat subfloor of plywood. I then mixed up thinset and troweled it on with a 1/4×1/4 notch trowel, laid the board down, screwed it into place and did the taping of the seams at the same time. I see that you do the seam when you lay the tile. Right now that looks like a really good idea to me, cuz now I have a lump in my floor at the seams and another at right angles down the middle of the 4×4 backerboard. Wud I do?
and of course, wud I do now? 
I’m doing a running bond diagonal, 4×9 bathroom, slate 12″ tile, started the whole project a month ago and trying to pee down the middle of the flange is getting pretty old. Thanks for a great forum!
If you use a 3/16 x 3/16″ trowel you can adjust for that lump as you tile. You can adjust the height of the tile over as you set everything. As for the whole middle of the flange thing – that’s a good aim! The taller you are the better the aim required.
Thanks Roger, Im relieved to know that I can save my project, and my life! So here may be a dumb question; I understand that the depth and width of the trowel notches determines the amount of thinset under the tile, but (not good at math here) if the depth and width of the notches are equal, what changes when you go with a 1/4 vs a 3/16? and last I checked 3/16 is less than 1/4? Somewhere I heard that with an uneven floor you want to go bigger?!? I wanna make sure I understand what I am doing here . Screwed up once already this year and it is only January. thanks
Sorry, I meant 5/16″, musta been a bit tipsy.
A 5/16″ will give you a little more thinset beneath the tile and that should be enough to make up for those little speed bumps.
Roger,
. so backerboard the arch or just waterproof it? On the other side of the niche is the living room so backerboard the inside in the shower for strength? LAST ONE- doing tile baseboard, the floor is going to be diamond shape. should i continue that pattern up onto the baseboard or just rectangles for the base?
After I ripped out my newly installed unprotected green board in my tub/showed (thanks) I put one piece of plastic up (6 mil) and covered the whole shower area. I dabbed silicone over all the staples and I understand how to silicone the plastic to the tub.
1) What do I do with the ceiling transition from plastic to drywall? 2) What do I do with the transition from wall to plastic in shower? 3) I live in a 1 story house with a concrete foundation. all the tile in my house is directly on the concrete and looks fine after it was built in 86. Can I do the same for my bathroom or do i have to put down 1/4″ backerboard? 4) Also doing an arched niche that probably is gonna look like an afterthought after reading your Niche page but its kinda late and I have a great dane that’s on fire
Thanks again, The wall that I built connected to the front of the tub looks legit!
Hey Jastin,
1. Nothing at all. Water doesn’t run up. You need to silicone between the tile and ceiling is all.
2. Nothing there either, the plastic waterproofs it, the backer is just there to adhere the tile to.
3. You can install tile directly to concrete, although I prefer a membrane such as ditra.
4. Backerboard and waterproof your niche. You can just use thinset to adhere the backerboard to the back of the living room drywall.
5. Nope, just rectangular base. It really finishes off the look.
6. oh shit, no number six. Sorry.
What about applying backerboard to a concrete surface. Before you laugh too hard, let me explain. I am finishing a basement bathroom and using Dricore in the room outside the bathroom (if you’re not familiar with Dricore, it basically just raises the floor 1″ and allows you to have some drainage/dead air space between your feet and the concrete… the end result, however, is that my “normal” floor is 1″ higher than my bathroom floor). As a result, I need to raise the floor of my basement bathroom (and kitchenette and entry) between 5/8″ and 3/4″. This seems to be exactly what a 1/2″ backerboard and thinset would be. You say that thinset doesn’t stick the backerboard to the subsurface, thats what the screws do… obviously I’m not going to be putting screws into the basement concrete, so there’s nothing to adhere the backerboard to the base concrete (which I am not worried about moving too much… although it could crack… but SLC won’t help with that either). I like backerboard because its flat… I’m not great with concrete, and I’m worried SLC won’t end up perfectly flat (did I mention I was laying down 20″ x 20″ tiles?).
So… if I needed to raise my bathroom floor 5/8″-3/4″, would you suggest SLC or backerboard adhered to the concrete only with thinset (and stuck between walls).
Thanks so much for all you help… also… I can keep a fire extinguisher handy for my dogs if need be.
TomB
Hey Tom,
Never said thinset wouldn’t stick backer to the substrate, only that it’s not why it needs to be there. It will stick it to concrete – not a good idea, though. No movement allowance at all in that particular method and rather than simply cracking your grout it would debond the entire sheet.
The way to do that is with deck mud. You can install a mud deck over any areas you need to raise, if correctly installed (screeded) it is the absolute flattest substrate you can install tile over, and it will compensate for any movement at all in your slab. Oh – and you can raise it from 3/4″ up to about four inches if needed – really. I think a mud deck would be your best bet.
Thank you… but one more question… you say a mud bud “will compensate for any movement at all in your slab”. So, does that mean you’d make it an “unbonded” mud bed? Lay it over felt?? Or bond it to the concrete?.
Again… thank you!!!
Nope, I would still use a bonded mud bed installed over thinset to the slab. By movement compensation I mean that if the slab beneath the mudbed moves or cracks the stress from that movement will be dissipated throughout the depth of the mudbed and completely gone way before it reaches the underside of your tile. The sand in the bed allows movement to be dissipated throughout the mudbed to any stresses disappear before reaching tile.
Hi, Roger.
I’m getting ready to tile a 65sq ft bathroom. You’ve convinced me to put down thinset beneath the 1/4″ Hardie board. However, I’m not sure how much thinset to use? In the pictures I don’t see any ridges. Do you just use the flat edge of a trowel?
Thanks for all of the work you put into helping others!
Dave
Hey Dave,
Nope, I normally use a 1/4″ x 1/4″ square-notch trowel. I may go larger if the floor is a bit more wavy.