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.

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Leave a Comment

  • Linnae Neyman

    What kind of caulk should I use in corners when I am taping cement board seams? Mold resistant paintable caulk since I will put seam tape in it and mortar just up to the corner caulk when I do the in-plane seams? What kind of caulk would you recommend? The redgard needs to stick to it. I like your instructive website. I’m trying not to set the cats on fire.

    Reply
    • Roger

      Hi Linnae,

      100% silicone. The redgard will not stick to it, but it doesn’t need to. You still need to tape and mud the seams once you silicone them.

      Reply
      • Linnae Neyman

        Thank you Roger! I’ll mud them when as I’m installing the tile. Today is a Silicone/seam tape and RedGard day!

        Reply
  • Brenda

    My tile guy left an inch and a half space between the lip of the cast iron tub and the cement board. Then he started to install the 12 by 24 tiles. Heonlyput 1 row the next day he didn’t want to use the same patern of tile. The tile was already set to the wall. He useda hamer to smash the tile to remove it. When he did he smashed all the cement board. I asked if he had to replace the cement board, he said No that the tape that holds thecementboard together was till there so he could fill it with mud and put the tiles back up. I feel uncomfortable with his procedure. My insurance company wants it redone. The contractor will not remove all The tile he installed to complete the shower suroundandis saying whether it is fine. I just can’t believie have a goid solid back behind my tile, especially being the first row of tile. What should I do? Except the job or get a new tile guy to take down all tiles and cement board and redo the entire shower surround?

    Reply
    • Roger

      Hi Brenda,

      Get a new tile guy! That’s BS and it doesn’t sound like he even waterproofed the shower. There should be NO gap there either.

      Reply
      • Brenda

        The largest gap at the lip of the tub surround should be 1/8 – 1/16 , then mud tape and mud, right? So with smashed cement board around the bottom I don’t have a sturdy waterproof shower surround, especially with the cement board that was smashed is on the first row of tile on the 2 short walls. He is insistent that the job is done correctly and will go to court to stand up for his job. I’m sure it will last a bit but I wasn’t ever planning to retile. What do I tell him? How do I prove he is wrong?

        Reply
        • Roger

          That is correct about the gap. The tile substrate has been compromised, period. If the substrate is compromised it compromises everything installed over it. Don’t let him BS you, he would lose his ass in court. Ask him to show you on the manufacturers website, or in the TCA manual, where his shitty fix is an approved method.

          Reply
          • Brenda

            Thank you for your help. This was an insurance job. I’m trying to find out how much money is left on the claim to get someone else to redo the floor and shower surround. It has been a nightmare. We were without a bathroom for 2 1/2 months and still having leaking from shower plumbing, besides the tile job. I can’t thank you enough for your expertise! Have a great day!

            Reply
          • Brenda

            Thank you so much for you advise. This is an insurance job. I have to see how much money is left for me to hire a new tile guy and redo the job. Have a great day!

            Reply
  • Dan

    I have gotte so much knowledge from your website, thank you for sharing your time and experience.

    I’m using NobleSeal (similar to Kerdi) and it says it has crack isolation. Does that negate the need to mud and tape the seams? I know backer seams don’t go flush when you mud and tape them, but if you tile while you tape it’s all good. In this application, the mud and tape will have to dry prior to installing the membrane and I’d hate to have lumps on my wall.

    Reply
    • Roger

      Hi Dan,

      If you’re using a sheet membrane like nobleseal or kerdi you do not need to tape and mud your seams.

      Reply
  • chris

    Can you address a shower niche? I framed out a niche with 2×4, used flashing and planning to cut cement board and tape and thinset. Should I also use finerglass tape, redguard or silicone?

    Reply
    • Roger

      Hi Chris,

      If you go over it with redgard that would be ideal.

      Reply
      • chris

        Ok so cement board, thinset the corner and Redguard. Should I use the fiberglass seam tape or is the taped and thinset joins good enough?

        Additionally I did vapor barrier behind the cement board about 7 feet. I left 1 foot at the top for ventalation. There will be tile almost to the ceiling. can I Reguard the entire Cement board area. I keep hearing about this mold sandwich, not sure how if the area is water tight. Thanks for your advice.

        Reply
        • Roger

          If the backer is trapped between two vapor barriers moisture can still form between them. Think about the outside of a glass of ice water after it sits in the heat for a bit. Condensation, no matter how small, can build up over time and begin to feed mold. That said, you have an open area at the top for moisture dissipation, so you’ll be fine.

          Reply
  • Ryan

    I don’t understand how you finish the space between the horizontal part of the shower base and the bottom of the cement board when you leave the cement board 1/8″ from the top of the shower base flange. I get that you don’t want to push the cement board out by overlapping the flange and make a bulge at the bottom. Do you just leave the flange showing? My shower base also says to put screws with washers right above the flange to attach it to the stud wall. So I am also figuring I need to account for these. I plan to notch my studs but am wondering if I can avoid that and somehow make the tile to base transition look good.

    Thanks for creating and maintaining this website.

    Reply
    • Roger

      Hi Ryan,

      You don’t do anything there. Leave the flange showing and the tile just hangs over it down to withing about 1/16″ of the base.

      Reply
      • Ryan

        Ah. Guess I should mention I am doing a pebble wall. Does that change your answer? Don’t think I can hang those down very well. Gonna have to notch and recess the base I think.

        Reply
        • Roger

          Oh. :D In that case you need to either notch and recess the base or shim out your wall stud and run the backer over the flange lip down to within 1/8″ or so of the base.

          Reply
  • Jim

    You are awesome. Bought your manuals on liquid topical waterproofing and tile tips. Worth every penny. In it it states you put your Hardibacker up first and then form your mud pan. Do you just embed the walls in the pan? If so, does it matter if the bottom edge is butt against the floor? Thanks in advance for your help!

    Reply
    • Roger

      Hi Jim,

      Yes, I embed the walls in the pan. With topical membranes it doesn’t make any difference. You do need to leave them up off the floor about 1/4″.

      Reply
  • Colleen Clifford

    Hi,
    I’m ripping out old vinyl or linoleum shower walls that the previous owner put in our back unit. The sheeting had been glued onto plywood. The seams had been caulked but eventually failed and the vinyl sheeting has started rotting.
    I’m ready to drape plastic sheeting and then install hardibacker, then tile these walls, but wondering if the plywood will cause a problem, or if I need to rip that off to get to the studs. I’d much rather not! The ply has been nailed in, not screwed, so it would be quite the process to remove it.
    Thanks,

    Reply
    • Roger

      Hi Colleen,

      Sorry, the plywood really has to go. Plywood moves WAY too much to be behind a tile installation. It’s likely at least part of the reason the seams in the previous surround failed.

      Reply