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.
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.
*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.
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.
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.
Do you have any experience using GoBoard foam tile backer board? I was planning on using this for a tub surround. I was also considering using CBU and a topical membrane because foam boards with sealant at edges and corners seems a little “weak”.
Any advice or recommendations?
is it ok to put 1/2” plywood on top of studs for cement board to be screwed on to? I’m kinda getting help from my bother in law since I don’t have much knowledge on this and I’m re doing my bathroom and that’s what he recommended but I’m kinda questioning it. So he wants to do 1/2” plywood on studs with roofing paper on plywood and 1/4 cement board after. Thanks in advance.
I did Lear heard good information for shower instalation thanks.
I’m building a custom shower with a mud pan. Therefore, I’m confused about the best method of backer board installation. I’ve read that you should install backer board about 1/4-1/2 inch above the finished mud pan to prevent wicking, I’ve also read that you should install the backer board before the pan is formed and the pan mud is leveled above the bottom of the board, encasing the bottom of the board within the pan. What is the best method of installing the backer board to insure success? Thanks
My general rule is that if a topical membrane is being used (redgard or kerdi, etc.) then it’s fine to embed the backer. If I am going with a traditional system with the membrane and two layer base I leave about 1/2″ above the top deck.
Cement board will wick water. With a topical membrane it isn’t an issue as it will never see water. With a traditional system it may wick water from the top mud deck if embedded.
I’m installing a new shower in an existing stall. The studs are 48 1/2″ apart and my shower pan is 47 7/8″ wide. Whats the best course of action for the extra 5/8 inches?
This is all new plumbing on elevated floor in my basement. I just don’t want to reframe existing walls.
You NEED to add at least one additional stud between those. Two would be better, which would give you the standard 16″.
If you mean joists (under the elevated floor) you need to add at least one additional joist between those two.
You don’t have to reframe anything, but you do need to add to the existing.
NO! Sorry… I’m not a professional. The stall is 48-1/2″ wide, the shower base is 47-7/8″. Should I throw a double layer of hardy-board on one side to fill the gap? or is there a way to fudge this to make it work? It’d be a hassle to reframe one end with a soffit on the other side of the wall.
What type of shower pan are you using? Is it one that will be tiled, or is it a prefab acrylic type pan?
Prefab acrylic. Delta laurel 48×34.
You can center the base in the opening, you’ll have about 1/4″ on each side where it doesn’t touch. The tile will more than cover that, you just need to make sure that whatever waterproofing method you use is wrapped across that gap and bonded to the face of lip. You want to make sure that any water running down the wall behind the tile ends up in the base and into the drain rather than between the shower base and wall.
So, where the backer board meets the tub base lip, I don’t wanna take the board over the lip bc it’ll create a slope. So do I fill that gap with thin set down over the lip? Or how do I create a smooth surface for my tile to meet the tub?
You don’t fill that gap with anything. It should be less than an inch and the tile doesn’t need to have support behind it for an area that small which won’t see any foot traffic. The tile just hangs over that. If you fill it with thinset or anything it will end up cracking out and creating more issues.
Why is a gap/seam required for the in plane joints, or any of the joints for that matter when they are being filled with thinset and tape? Thinset has very little give and doesn’t flex either – wouldn’t making this whole structure ‘monolithic’ defeat the purpose of leaving a gap in the first place?
Leaving a gap in the in-plane joints allows thinset to get in between them in order to create a monolithic structure. Leaving the gap at changes of plane allows for expansion due to swelling (rather than movement due to the framing on which it is attached).
That probably doesn’t help you unless you’re a complete overinformation junkie like me.
Information junkie I am, the devil is in the details!
Usually for installs I’ve seen corners tightly butted and then taped. In the instances where there is space, it seems that the thinset usually fills it up regardless. Wouldn’t both of these approaches effectively eliminate the movement zone?
Also, how come you say it’s more for swelling rather than the movement of the framing? All of these materials have varying expansion coefficients and would surely have some differential movement between each other? Is it minimal as to not cause concern?
On another note, how would you recommend doing backerboard walls to drywall ceiling transitions? Leave them floating or tape them?
Welcome to the club! Overinformation junkies anonymous meets on tuesdays at…
Framing is a macro-movement, swelling in the SUBSTRATE ITSELF (not from the framing), is a micro-movement. IF you fill this with thinset, the swelling will lead to cracking, which leads to larger movement.
I leave my walls floating at the ceiling and fill with silicone (after tile). When I prep I isolate separate planes as much as I can unless they are tiled to one-another, then I tie them together as much as I can.
I am remodeling my walk in shower. I have removed all the old tile from the shower floor. The shower pan is poured concrete, and old. Should I waterproof the shower pan before installing new floor tile?
You can, but if there were no issues with the waterproofing of the original pan then it’s really not necessary.
I am installing a shower pan in a repair .I would like to know how to you fasten the Hardie board to the wall over the vinal pan yo keep it square if you can’t put hardie board into the cement base.
I usually leave the tiling to Tile man after pan installiation , but this is my house.
You screw it onto the studs down to three inches above the finished height of the curb (NO penetrations below that). You can either shim out the studs above the liner to give you a flat plane, or notch the studs below so the liner sits flush with the face of the top portion of the stud.
I have a six month old concrete slab, and new stud walls on a double floorplate. All new. My contractor guy screwed in Hardibacker into the studs from the ceiling to the floor. What concerns me is the Hardie is either touching the slab, or maybe 1/32″ above the slab – as in maybe I could slide a taping knife under it in some areas. Do I need 1/4 inch of vertical clearance there? If yes, do I need to unscrew the hardie, trim off 1/4″, and reinstall? Not pleasant. I was thinking instead to put fibre mesh where the hardie meets the slab, thinset the mesh into place. Then 3 coats of redgard over that seam and up the wall 6 inches. Then build then preslope, and then redgard the surface of the preslope and the walls with two coats of redgard. Two points of concern. One is aforementioned – hardie is touching slab. Other concern is some folks say hardie can never be embedded in the preslope mud as the fiber in hardie will grab moisture from the pan … but if the hardie has redgard on its face and the wall to floor seam is fibre taped, thinset, and redgard – not sure how that could actually occur.
Hi Rdur (if that IS your real name),
If you are using redgard as your waterproofing on your shower floor it’s not an issue. Once you get the redgard on everything is should be a continuous layer from the wall, over the floor into the drain – all the water stays above the redgard, there is no water to lift into the hardi. I would probably put silicone in that tiny gap just to prevent it from absorbing moisture from the concrete itself, but that doesn’t have anything to do with your shower waterproofing.
Well it sort of is my name … abbreviation of my wife’s name. Thank you for the tip, yes, I’m redgarding everything, twice. So … my shower odyssey continues, as in the next shower enclosure, my brand new hardi wall was 3/4 inch out of plumb if measuring from the ceiling to the floor, and there was a bow in the middle of the 32×96 inch wall. The contractor did not bother to shim the studs (all brand new), over torqued some screws (causing distortion in the hardie as it sucks against the studs), and messed up in thinking the 1/2 inch hardi will seam well into 1/2 inch drywall. Things were so inconsistent I screwed some 1/4 hardi on top of the 1/2 hardi in spots at the worst places, then I floated the wall over several days, with hot mud. I am now “mostly flat”, and mostly plumb, except for the width between the two opposing walls of my 31 inch shower starts at roughly 31.5 and then tapers to 31. So I’m definitely out of square at the pan. By the way, this is an exterior structure, few people will see the insides. but it still bothers me. Thus where to go from here … I have about 100 pounds of mud on the hardi – I know … insane, but it’s better than what I had. My options going forward: demo the whole wall, and my rough shower curb, and my shower pan. Materials not super expensive, but, demoralizing and my dry packed shower pan and curb are redgarded and water tight. If I demo, rip out everything down to the cement slab, install new hardi after shimming, or use drywall. Then regdgard. Or option 2, leave everything as is… skim coat everything with thinset (to help with structural integrity of the built-up wall?), regard twice, and then tile. When I floated did I use stucco mesh and cement? No, I figured the hot mud would hold itself up and at my age realistically, mixing and floating with cement is too much for me. I know this all sounds stupid, but my contractor bailed on me and thus the issue of a callback and fix was not on the table, and no, I did not have a written contract as he was a “friend” who most likely had some personal problems.
If you are using larger tile (not subway or mosaics), I would just use a medium-bed mortar and make sure everything is flat as I set it. If you insist on having it plumb for the aesthetics of the inside corner you can do that with the same method, just more mud under the portions of the walls which lean away from plumb. The issue, however, is going to be your termination at the outside of the leaning walls, you should only have about a 1/4″ difference there, though, so it shouldn’t be too terrible.