Installing cement backerboard is one of the more popular choices for a shower wall substrate. Cement backerboards include Hardiebacker, Durock, Fiberboard, wonderboard, and similar products. These materials bridge the gap between expense and effectiveness. When installed properly they will give you many, many years of durable shower construction.

The advantage of cement backerboards is that, while not waterproof, they are dimensionally stable when wet. That just means that when they get wet they do not swell up. Any swelling behind tile is a bad thing. It will lead to cracking grout, tile, and all sorts of bad things.

Waterproofing your studs

To install the backerboard you must have a vapor barrier between it and the wooden wall studs. While the backerboard will not swell when wet, your wall studs will. You must prevent any moisture from reaching them. The preferred material for a vapor barrier would be 4 mil or thicker plastic sheeting which can be purchased at places like Home Depot or any hardware store. You can also use tar paper or roofing paper, the thick black paper used under shingles. Although I personally do not use that, it is an acceptable barrier.

Starting with your bare wall studs on your shower walls simply take your plastic sheeting and staple it to the wall studs completely covering the entire surface which will be inside your shower. You can also use silicone instead of staples to adhere it to the studs. Make sure you overlap all the edges. Just hang it all up there like you’re hanging wallpaper. You want it covering the framing enough that if you were to spray the walls with a hose the wall studs and framing would not get wet.

At the bottom of the barrier you will want it to overlap on the shower side of the tub or shower base. That is you want it so that any water that runs down the plastic sheeting will roll off into the tub rather than behind the tub. Overlap the lip of the tub or base and silicone the back of the barrier to keep it in place.

Installing the backerboard

Now for the backerboard. Lay out your backerboards for the best fit on the walls. They can go up vertically or horizontally, it makes no difference. With a regular tub surround with a five foot back wall it is usually easier to use two horizontal sheets along the back wall and one vertical on each of the sides. (This assumes 3 X 5 foot backerboard sheets.)

All backerboards are cut by scoring and snapping. You do not need a saw for them. While there are special scoring tools specifically for this you can easily do it with a regular utility knife. While all these backerboards are essentially identical in their effectiveness as a substrate, some are more easily cut. Durock, in my opinion, is the most difficult. I personally prefer hardiebacker or fiberboard. Make sure you check the website for whichever you choose for specific instructions.

To fasten the backerboard to the framing you have a couple of choices. A lot of professionals simply use galvanized roofing nails. While this is perfectly acceptable, I prefer screws over nails when possible. Hardi makes specific screws for their backerboard which can also be used for all backerboards. These are manufactured with ribs beneath the head of the screw which help it cut into the backerboard and countersink so the head is flush. If your local big box or hardware store carries them, they will be in the tile section. You can also use just about any type of corrosion resistant screw. Anything that can be used for an outside deck can be used for your backerboard.

Fasten your backerboard to your shower framing with a screw or nail about every 8 – 12 inches. I would also suggest using a straight-edge along your wall while doing this so that you can shim out any areas where the wall studs may not be straight. The flatter your backerboard is installed, the easier your tile installation will be. Take your time, the beer isn’t going anywhere.

Allow for movement!

You do not want to butt the backerboards against one another. You need to leave a small gap at every change of plane. That includes corners, walls to ceilings, and walls to tubs or floors. There needs to be room for expansion and contraction.

Wood moves – always. It’s just a fact of life. The secret to dealing with the movement is to ensure the movement will not interfere with the tile. Leaving this small gap will allow for movement of the sheets enough so that they do not force against one another and push out. While the backerboard itself is very stable, you are still attaching it to wood.

If you have a tub or shower base you will also want to stop the backerboard about 1/8 inch above the lip. You do not want to run the board over the edge of the lip because it will cause the backerboard to bow out and your wall will not be flat. It will also allow the tub or shower base to move a bit – it’s attached to the wooden studs as well. Tubs also move when they are filled with water. You need to allow for that movement.

I usually leave about a 1/16 to 1/8 inch gap between the sheets of backerboard. This allows for thinset to lock into the entire thickness of your backerboard when you tape and mud your seams. We’ll cover that part in a minute.

Don’t allow for movement! (Confused yet?)

If your shower framing is such that you cannot place the edges of all the backerboards directly over a stud you will need to add more studs. You may do this with regular 2 X 4’s screwed to the present framing vertically or horizontally as needed. You must make sure that every edge of the backerboard is supported so if the wall is pushed or leaned on in that spot it does not move. You want solid walls.

Final step

The last thing you must do is mud and tape your seams. Similar to regular drywall all of your in-plane  joints must be taped. To do this you just use regular thinset and alkali-resistant fiberglass mesh tape. You can find the tape in the tile section – it’s similar to regular fiberglass drywall tape, but it specifically manufactured to be alkali resistant. Make sure it is alkali-resistant because your thinset contains alkali which will gradually erode regular tape thus defeating the purpose.

There are two ways to address the corners. The industry standard, and the way you should do it, is to tape and mud the corner joint as well. Most backerboard manufacturers recommend this, as do the handbook standards. I only do that about half the time – I’m a rebel like that. 8)

*The other half of the time I only tape and mud the in-plane joints – the gaps in the same wall, not the corners. With the corners I fill the gap with silicone. I do this to allow the different planes of the walls to move in different directions, which they will do whether you like it or not. Allowing this movement in the substrate compensates for excess stress in certain applications. This is something that I do, it is not industry standard and you will likely not find anyone else recommending doing this. So when you get the conflicting information about this – that’s why. :D

Fill all the gaps in your seams with thinset (you left gaps there, right?) then embed the tape into it. Then go over the tape with more thinset to smooth everything out. This will lock everything together and give you a continuous, solid substrate for your tile. That’s what you’re looking for.

When properly installed cement backerboards will create a rock solid, extremely durable substrate for your tile installation. Taking time and care to solidify what is behind or beneath your tile is the only way to guarantee a lasting installation. Your tile is only as durable as what it is installed upon.

As always if you have any questions at all please feel free to leave a comment.

Hardiebacker Website

Durock Website

Need More Information?

I now have manuals describing the complete process for you from bare wall studs all the way up to a completely waterproof shower substrate for your tile. If you are tiling your walls and floor you can find that one here: Waterproof shower floor and wall manual.

If you have a tub or pre-formed shower base and are only tiling the walls you can find that one here: Waterproof shower walls manual.

{ 1498 comments… add one }

Leave a Comment

  • Ben

    Couple of questions:
    1) I’m putting in a fiberglass shower wall insert over a bathtub, and the edges of the tub don’t line up with my studs. Should I (A) take the cement board to the nearest stud past the tub or (B) stop the cement board at the edge of the tub/shower insert? If (A) what’s the best way to match the thickness of the backer board to the thickness of the regular drywall? If (B) should I put in another stud at that edge so I don’t have the edges of my drywall and backer board just hanging over empty space?
    2) As I was reading through other questions and responses to see if my question was already addressed, I saw something about not trapping moisture in the outside wall. The long wall of the shower is an external wall, and has an old part with insulation foam sheets covered by painted wood siding, and a new part under the window where I have put a sheet of plywood and will cover with wood siding for a “closest match” to the existing. I still put a plastic sheet on the inside between the backer board and the studs/insulation, right?

    • Roger

      Hi Ben,

      1. Either one will work. A) Shims. B) Yes.

      2. Yes, with acrylic surrounds you do.

  • Craig

    Hi Roger,

    I am putting Hardibacker board down on the floor. Once piece got caught on a pipe when putting it into place and cracked. Does it matter I have a crack through the board or do I need to put a new board to replace this.

    Can I just tape and mud the crack. One is not very viable – the other has gone through the entire board.

    I appreciate any assistance.

    Cheers,

    Craig

    • Roger

      Hi Craig,

      Yes, you can tape and mud the crack, it’ll be fine.

  • doroteo arango

    I’ m building a walk-in shower in my basement. The shower concrete base is 4’ x 4’, surrounded on 3 sides by a 16” high concrete wall [part of the original house configuration which I took advantage of] and the entry side to the shower has a 5” concrete lip. Below the concrete base I installed a vapor barrier which extends below the concrete slabs to the entire bathroom.
    My question: should I waterproof the concrete base and the concrete surrounding the shower or can I tile the shower and base as is?
    thank you very much,
    Regards,
    Doroteo A.

    • Roger

      Hi Doroteo,

      You need to waterproof your shower floor.

  • kevin

    hey , im doing a shower stall with a traditional floor and durock walls with hydroban .My question is ,is it ok to do the top mud deck before doing the hydro? i was going to coat 4-6 inches of the floor along the walls as well ,is this ok or should i hold off and hydro before top mud deck goes in (i added 1 foot of poly on my walls overlapping my pan liner because i was worried about wicking)hope thats ok…rock wool insulation underneath..thank you for your time

    • Roger

      Hey,

      Yes, you can do it before doing the hydroban.

      • kevin

        thank you

  • Westbrook

    As I go through my master bath renovation I’m questioning the use of liquid waterproofing for a tub surround.

    I understand the need for it in a soaking wet shower stall. What benefit does it serve when applied to cbu?

    If moisture is wicked by the cbu to studs, then where does that moisture go if you have waterproofed the cbu? Has to go somewhere, right? :bonk:

    • Roger

      Hi Westbrook,

      I’m not too sure what you’re asking me. The benefit of waterproofing your cbu is to have waterproof cbu, it is not otherwise. If you have it waterproofed then no moisture gets into the cbu to be wicked to the studs.

  • Russ

    After stripping my shower to the studs, I have replaced rotted 2×4’s, put up the Hardibacker, taped, mudded and waterproofed with Redgard (one primer coat plus two regular coats). I opted to keep the fiberglass shower pan. This pan has a 1 1/4″ lip. My Hardibacker goes down to the lip, and I left a gap between the backer and the fiberglass that I filled with silicone. The backer, silicone and fiberglass make a smooth, level transition. Finally, I will be installing 6×6 porcelain tile using FlexBond Crack Prevention Morter. My question is this: I want the bottom row of tile to extend down the backer and over the fiberglass, where i will leave a gap at the bottom to be filled with silicone. Do I just use the thinset on the entire tile (i don’t think the thinset adhears to fiberglass) or is their a different component I should use on that 1 1/2″ fiberglass lip?

    • Roger

      Hi Russ,

      You don’t need to put anything behind the tile over the fiberglass. Just bond the tiles to the backer, the portion over the fiberglass will just set over it without being bonded.

      • Russ

        Roger,

        Thanks very much for your quick answer. This is my first tile project and I have gained much respect for the artisans who do the job right.

  • john

    When placing backerboard in shower floor how much space can you leave around hole for drain? Or, how should you place backerboard around the shower drain?

    • Roger

      Hi John,

      I’m not sure what you mean since backerboard should not be placed anywhere in your shower floor in any application. It needs to be deck mud.

  • Pablo

    Hi Roger,
    I am using 6 mil poly behind the baker board, what type of silicone should I use to attached to bathtub lip? The kitchen and bath type? Doesn’t it get moldy there after a while?
    What type of thinset should I use? Modified or unmodified? Does the modified thinset leach chemicals?
    Great advice in your books!
    Thank you,
    Pablo

    • Roger

      Hi Pablo,

      100% silicone. They are all essentially the same. It doesn’t really matter much. If you are talking about thinset to bond tile to the backer use modified. Modified thinset may leach latex, but that happens in very moist, VERY hot environments such as steam showers that run all day (like in a health club) or similar environments. That is not a problem in regular showers.

      • Pablo

        Hi Roger,

        What about the seams between the Backerboard sheets? Should I use the same thinset but unmodified to help it drain better?

        Thank you,

        Pablo

        • Roger

          Either one will work fine.

  • Pat

    I am in (hopefully) the last weeks of a grueling bath and plumbing remodel – we were down to the studs on all walls. When the tile person installed the ceramic tile in the tub/shower combo he used a 1/2 inch Wonderboard Lite base and used Red Guard everywhere. He left about a foot of Wonderboard on all edges and told us that the sheetrock person could join directly to the Wonderboard and then tape and texture everything leaving the ceramic tile bullnose trim sitting above the surface. When the sheetrock person came in he told us that he could not put texture on the surface of the Wonderboard. He then put up 1/2 inch sheetrock first and then 1/4 inch sheetrock on top of that which is now basically flush with the top of the bullnose trim. He plans to put a silicone caulk in the small space between the now textured wall and the tile. We find this to be a very odd look. Most all tile I have ever seen sits above the surface of the sheetrock. With all that said, can you tie sheetrock to Wonderboard Lite and then texture on top of that?

    • Roger

      Hi Pat,

      Your sheetrock person is incorrect. Wonderboard (or any backer) can be taped, mudded, textured and painted just like regular drywall. Primer may need to be used to paint it, but it can definitely be textured.

  • Bobbi

    First time tilers here.
    Just so we get this right… we took out a old fiberglass tub/surround combo and installed a tub and want to tile the walls. We have already put cement board up on the walls (does it matter what side is facing which way?) with no vapor barrier be hide it. We plan on using redgard or aqua defense on top of the cement board.
    So I want to get the next steps right:
    1. So places say you can use the Fiberglass tape and mortar to do the seams then water proof and others say use the fiberglass tape with one of the water proofing membrane like redgard then do a coat over the whole thing. Is one better than the other? Does it matter?
    2: Our new tub has a lip- should we put a small (6 in or so) plastic liner of some sort going up the wall at little be hide the backboard and over the lip of the tub or do we silicone the crack between the tub and backer board? The tile will go over the lip of the tub onto the tub.
    I just want to get this right because there will probably be at least 1 more tub redo in the future.
    Thank you

    • Roger

      HI Bobbi,

      No, it doesn’t matter which side is out.

      1. With in-plane seams do tape and mud, then waterproof. With change-of-plane seams fill with silicone first, then either tape and mud or just tape and membrane. Tape and mud, then membrane, is a better choice.

      2. Silicone.

  • Cory

    Hey Roger,
    I have my durarock up and taped etc…
    I am using Laticrete hydro ban for my waterproofing on the walls (nothing behind the durarock). Should I waterproof the walls before I do my final mud floor (over my pvc liner), or put the 2nd layer of the mud floor down first.

    • Roger

      Hi Cory,

      I would waterproof the walls first.

  • Tom

    Hi Roger,
    I am installing a shower and am currently down to the studs and ready to install the moisture barrier (4mil plastic sheeting) then cement board. I understand you can use silicone over the staples attaching the plastic sheeting to ensure no water reaches the studs. My question is, what can be done to prevent water from reaching the studs where the screws attaching the cement board to the stud pierce the plastic? I don’t want the studs to swell at every screw point.

    • Roger

      Hey Tom,

      The twisting of the barrier as you drive the screw in creates a pucker in the membrane which water will run around. But if it makes you feel better you can always place a dot of silicone on the backer and drive your fasteners through the middle of it. It will seal the hole as you drive the screw in.

  • Randy

    Roger,
    I am using Redguard on the inside of my shower wall backerboard. Do I also use the plastic behind the backerboard? The shower has one wall that’s on an exterior wall while the other two are interior walls.
    Thank you, Randy

    • Roger

      Hi Randy,

      One or the other, never both.

  • Cindy

    We had a leak behind our shower wall and had it cleaned and dried by a local company. Then I had the backerboard put on the wall with what appears like scraps put together like a puzzle. There is a hump/bump in the middle between the showerhead and tub spout. I would like a 12 inch square title on my wall. I am not sure if our wall is prep for titles. Is there a certain title or better title to use and a better way to install backerboard?

    • Roger

      Hi Cindy,

      There are all sorts of ways to prepare a shower for tile, just slapping backer up in a puzzle form is not one of them. :D You can use whatever tile you want, the key part is the preparation of the substrate. Have you downloaded my shower waterproofing guide? It explains the basics and different methods of shower preparation.

  • Corey

    Hi,
    I am installing a new shower in an existing bathroom.
    I installed plastic sheeting on the studs as an vapor barrier, and then
    installed Durock on top of the plastic. My question is can I now install
    Red Guard on the Durock to waterproof ? I understand that you shouldn’t
    have a vapor barrier on both sides of the Durock but is the Red Guard considered a vapor barrier?
    Or is it just a waterproofing and crack repellant?
    Thank you

    • Roger

      Hi Corey,

      You need to have one or the other, not both. And yes, redgard is a vapor barrier as well.

  • Karen Sharpe

    I have a Mustee shower floor and my shower alcove is down to studs. It appears I should install the shower floor first and then the Hardie backerboard, as it seems to fit just above the floor flange edging, with a space for the tile in a sort of grove of the floor flange edge just in front of the backerboard, but lower.

    • Roger

      Hi Karen,

      If you are speaking about an acrylic floor with the channel around the perimeter then yes, it goes right above it. If you’re talking about anything else I have no idea what that would be.

  • Darin

    You mention that you sometimes fill the corner seams of cement backerboard with silicone instead of taping and mudding. If I am using a liquid such as redgard for waterproofing, can it be used to fill the corners instead? I have never used redgard, but I would be afraid that the redgard wouldn’t adhere to the silicone if I were to go that route. I’ll be re-tiling my tub/shower walls in the next few weeks, and I have found your site really helpful. I’m just trying to get all my duckies in a row before I start the job.

    • Roger

      Hi Darin,

      I silicone before taping and mudding. The silicone allows for expansion of the board itself (swelling) and the tape and mud tie it together so the boards don’t move independently. Silicone, tape and mud, then redgard. The redgard will shrink and will not span any considerable gap in your corners.

      • renee parks

        I am putting silicone at the corner seams of my cbu in my shower. How long do I have to wait after applying the silicone to put the tape and thinset over it? Also, when I silicone between the tub and cbu, do I mud and tape over that?

        • Roger

          Hi Renee,

          You can normally do it within six hours or so, after the silicone firms up. No tape and mud at the tub.

  • Bob

    I have read that some topical waterproofing membranes don’t adhere well to Hardiebacker. Does mapei’s Aquadefense work well with that board? I’m doing a shower remodel.
    Thanks,
    Bob

    • Roger

      Hi Bob,

      All of them bond well to hardi. The problem arises when the hardi is not properly prepped for it. It needs to be wetted down and a primer of 1 part membrane and 2-3 parts water should be installed to it initially to prime the board.

  • Aaron

    Betty,

    There is no way not having a water barrier on the walls would leak that much water. You either have a shower drain or shower pan issue.

    • Roger

      Hi Aaron,

      If you would like to reply to Betty’s comment please hit the reply link in her comment so people (including myself) know what you are referring to. There is absolutely a way that a shower would leak that much water without any type of waterproofing barrier (although she did not mention a certain amount of water). Your shower sees roughly 1.5 times as much water yearly as a rain forest.

  • Jerry

    Okay so I saw the comments about either using poly behind the backer board or the Kerdi material on top of the backer board, so does the same apply if I use a roll on water proofer, meaning if I go that route I don’t need the poly behind? Also I know you must insulate on exterior walls, but can I do it on interior walls as well without an issue, I ask this cuz the interior wall is a bedroom on the other side and want to keep it warm?

    • Roger

      Hi Jerry,

      That is correct, no poly behind if you are using redgard. Yes, you can insulate the interior walls.

      • kevin

        if you insulate interior walls u must use insulation without a vapor barrier (no paper covering on one side) or use Roxul (rock wool) fire/water proof and sound deading

  • robert

    Hey Roger – your site is the best resource out there for bathroom renos. thanks so much for developing this site and maintaining it!

    I have two questions for you.

    1) i have my cement board up and ready to go in my shower enclosure. Some of the gaps where you suggested 1/8th or 1/16th of an inch are actually are in some cases 1/2″. is it okay for me to just build up the thinset in those larger gaps to bridge between the cement board pieces? (then tape, and add more thinset).

    2) I’ve been to Home Depot a couple times, and they have dozens of thinset mortars, and all of them seem to have slightly different descriptions about the product. Is this product below okay to use?

    http://www.homedepot.ca/product/premiumplusltsupgtreglt-supgt-standard-thin-set-mortar-150-white-2268-kg/968110

    Once this process is done i will be using Redgard as the liquid membrane.

    • Roger

      Hi Robert,

      1. Yes.

      2. Yes, that’s fine.

  • JP fitting

    Thnx for all the great info! Wish you were in Burbank! I tiled the floor in my pool house bath room (1st tile job)& there is some grout in places fairly proud..I tried to wipe as I went but didn’t get it all.. My question is if it’s almost as high as tile surface I have to scrape /cut away right so grout can have a Home correct? Thnx ..p.s,trying to get built in shore waterproofing Manuel but I seem to be clicking in circles? Cheers & this! JP

    • Roger

      Hi JP,

      If you need to remove just a little bit of grout in a grout line you can sand it down with a drywall sanding sponge. Seriously. :D

      Did you get the manual? The elves were drunk last night…

  • Michelle

    Thank you for your instructions about installing cement board. We purchased Durock cement board and will use your instructions to guide our installation. My question is about using waterproofing membrane ie: Kerdi membrane on top of the cement board. Should this be done? Can we use Kerdi on top of the cement board? You talk about using thinset to seal seams of cement board. Would we use modified or un modified thin set to seal seams with the mesh tape you mentioned? The porcelain tile should be applied on top of Kerdi using what type of thinset?

    • Roger

      Hi Michelle,

      Yes, you can use kerdi on top of cement board. If you do DO NOT install the barrier behind the cement board. You want your waterproofing behind the backer (plastic or felt as stated above) or a topical membrane (kerdi) over the front of it – never both.

      When you tape and mud, when not using kerdi (seal the seams) you want to use modified thinset, although either will work fine. However, when you use the kerdi membrane you don’t need to tape and mud the seams, installing kerdi does the same thing. It essentially seals and locks all the individual boards together. Taping and mudding is not required if you are using kerdi.

      Schluter requires unmodified thinset be used between porcelain tile and the kerdi membrane. They also require that unmodified thinset be used to install kerdi to cement backerboards.

      • Michelle

        Thank you! One of the walls in the shower is an outside wall so it has insulation(Roxul) and 6mil poly vapour barrier. Are you saying I need to remove this poly in order to use cement board and Kerdi membrane? Or not put the poly on the two other interior walls that will be in the shower area? But leave the poly on the exterior wall? I understood that that exterior wall requires the poly vapour barrier

        • Roger

          You can either remove the poly or cut slits in it between the studs. The problem is trapped moisture between the kerdi and barrier has no place to dissipate, it will fester and feed mold. It would be best to remove it. The kerdi on the face of the backer also acts as a barrier for the exterior wall, it just has an extra layer of substrate in it.

  • Betty

    I have a newly completed porcelain ceramic tile shower with epoxy grout that is leaking thru to the first floor. We thought it was the drain, but have pulled out the ceiling and found that it is coming from the side of the monolithic shower base. First thought was that it was the bottom grout joint at the base, so I caulked that. No difference.

    After reading your website, I’m having a strong suspicion that the guy who did the installation put up cement board with nothing behind or on top of it for waterproofing. I think he thought the epoxy grout would be waterproofing enough. If he told me that it was, I would have believed him.

    So, my question is, is there anything I can topically apply, that will waterproof the shower walls? The tiles are 6×24, and the grout joints are 3/32″.

    • Roger

      Hi Betty,

      No, there is nothing that can be applied over tile to make it waterproof. If it’s leaking now it will always leak. Waterproofing needs to be incorporated into the tile substrate, it should be completely waterproof before a box of tile is opened. Your guess is my guess – he figured the epoxy would waterproof it. Unfortunately it happens more than you might think.

  • jamey

    This is what I was wondering about ….

    *The other half of the time I only tape and mud the in-plane joints – the gaps in the same wall, not the corners. With the corners I fill the gap with silicone. I do this to allow the different planes of the walls to move in different directions, which they will do whether you like it or not. Allowing this movement in the substrate compensates for excess stress in certain applications. This is something that I do, it is not industry standard and you will likely not find anyone else recommending doing this. So when you get the conflicting information about this – that’s why.

    I was wondering what kind of silicone you were using? thank you very much.
    -Jamey

    • Roger

      Hi Jamey,

      All 100% silicone is the same. I normally use GE silicone II. Just FYI, I no longer do that. I still put the silicone in there but I also tape and mud them. The silicone allows for ‘swelling’ movement, the wall pressing against the adjacent wall, but I tape and mud over that.