Installing Cement Backerboard for Tile in a Shower

by Roger

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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Jeff

Hi Roger…I have installed the hardibacker in my stand up shower (it is embedded 1/2 inch into deck mud..sorry…I know this no longer recommended but I used an outdated diagram from Oatley (slope is great though). However, my plumbing fixture comes out almost 1/2 inch too far, so the handle piece will not press close enough to the cover plate once it is tiled (my fault). Can I add a second layer of 1/2 inch backer board to that wall using longer backer on screws? I guess my other options would be to remove the board (which is embedded…ugh) and add furring strips to studs and redo..or last resort change the plumbing. Recommendation would be greatly appreciated! Thanks

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Jeff

I would only need a second layer of 1/4 inch backer board , not 1/2 as initially indicated above.

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laura

Hi Roger, I’m redoing a tub surround, keeping the ’80s cast iron tub. I ripped out the drywall + CBU coming down to the tub, and I’m planning to have just 1/2″ wonderboard and RedGard as substrate. The tub doesn’t actually have a flange (just a bit of a lip that curves upwards) and the wonderboard just matches that lip thickness, so I can’t bring the CBU over it (and would like to avoid furring the studs). How do I waterproof the tub/wonderboard joint? Silicone bead to stick to both? The tub lip is not completely even, so the width of the gap gets as big as 1/4″ at the edges.

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Roger

Hi Laura,

Yes, silicone the wonderboard to the lip. If the gap is too large you can do several beads, or use a backing rod (it’s just a foam rod) then silicone over that. Just make sure the silicone fully contacts both the board and the tub.

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Alfie

Hello roger, thank you for the information, when taping the joints, how long should I wait to begin tileing? Do I have to wait the recommend 24 hour for the thin set to dry on the joints before tile-ing? Or can I begin tileing right away? Thanks

Reply

Roger

Hi Alfie,

You can begin tiling right away.

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mike

roger,
great site!
i’m planning to use durarock as you describe. however, i’ve read that if you plan to install redguard (or similar) to waterproof the surface of the durarock, you SHOULDN”T use a vapor barrier against the wall as it would effectively trap any moisture that got in the middle. agree or disagree? i feel like tarpaper would be a good compromise to still breathe, but prevent direct moisture contact to the studs.

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Roger

Hi Mike,

I fully agree. You want either a vapor barrier behind the substrate or a waterproofing barrier over it – one or the other, never both.

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Andrew

Hi Roger,

Thanks for all the useful tiling info, I truly appreciate a resource such as the one you’ve created here. I’ve already commented on substrates, thinset, corner shelves and footrest, and by tomorrow all my Hardibacker will be up in the shower, including over my three-layer 2×4 curb. Bags of Sand/Topping mix and multi-purpose sand are waiting on me in the garage.

So, I’m wondering, having read your comments about half the time you caulk backerboard corners and the other half you tape and thinset them. 1) What considerations do you account for when making that call? I’ll be installing 3×6 ceramic subway to the shower walls, if that is a factor. Hydro Ban will be going over everything prior to tile. Will Hydro Ban adhere to silicone?

Also, 2) should I do anything to the backerboard shower wall to the pitched deck mud shower floor transition? Would that be considered a corner that needs tape and thinset or caulk too? If so, do I wait to thinset and tape everything once the floor is formed? And speaking of the shower floor, Laticrete provides installation instructions for a bonded (wire lathe and cleavage membrane attached to substrate) and unbonded (cleavage membrane with 2×2 wire mesh reinforcing the deck mud, but not secured to subfloor). The unbonded method seems to allow for more deck mud thickness at the drain, which I’ll need 2″ at the drain, while the bonded method specifies 1 1/4″ thickness at the drain. Will my dog burst into flames if I use the bonded method and have 2″ of deck mud at the drain? Laticrete of course assumes one is using their 3701 product. Not sure of the ratio in that mix, but I’m making my own.

One of the shower walls is recessed 2-3″ from the main bathroom wall, thus creating a small outside corner inline with the inner face of the curb; 3) tape and thinset this corner too, or is there a special corner bead for this application?

Thanks again,
Andrew

Reply

Roger

Hi Andrew,

1. I always do both. The silicone is in that gap to keep thinset out of it in the corners, then tape and mud are applied over the top of that to tie the boards together on the different planes. If you have thinset in those gaps the ‘swelling’ of the board may crack it and lead to problems. If you fill them with silicone first that eliminates that issue.

2. You can use the bonded method with 2″ thickness at the drain.

3. Tape and mud it, or you can use a plastic corner bead (NOT metal).

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Andrew

Thanks, Roger.

What about where the deck mud meets the backerboard? Does anything need to happen there before the Hydro Ban goes on?

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Roger

Nope. But you can embed mesh into the hydroban as you install it, that never hurts.

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Kristen

Hi Roger –

We are installing durock above a cast iron apron tub. We plan on tiling the surround, as well as the ceiling. I have taped and mudded all of the in-plane joints and corners, except for where the ceiling meets the walls. Should I also tape and mud those joints or caulk them with silicone? I don’t expect that area to be wet at all, but don’t want to screw myself with the tile installation.

Also, should I caulk the joint at the flange prior to applying redguard? I have read conflicting info about this since redguard won’t stick to silicone caulk.

As always,

Thanks!

Reply

Roger

Hi Kristen,

Yes, you should tape and mud that corner as well. You can silicone it first, or silicone it after. I would install the redgard first, then silicone. (Silicone will bond to redgard).

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Gene

Hello Roger…

I installed 1/2″ Durock for a full shower and taped the seams with the appropriate cement seam tape. Also the boards are screwed with the correcet screws and at the right distances between each fastener. I filled the seam gaps with thinset, placed the tape overtop the wet thinset to embed, then smothed over the top CLOSE to the tape to make a nice flat surface but could see the texture of the tape (very slightly) through the top coat of thinset. I then had to let the shower set for about 3 weeks prior to application of AquaDefense because my tile contractor was 3 weeks out before doing the tile appication.

When I went back to apply the AquaDefense 1 day prior to my tile guy showing up i noticed 2 of the in-plane seams across the whole width of 2 of the shower walls had cracked (almost a full 1/8″ gap) basically rendering the seam taping “open” again.

How could of this happened and should i just apply a second mesh taping with thinset right over top of the new tape and then waterproof? Waiting for thinset taping reappication will cause me to have to wait another 24 hours for drying and my tile guy will then have to reschedule. Will new taping over these seams with the CLOTH seaming tape (not the mesh type) embedded in the AquaDefense application Fix the seams just the same?

What do I do?

Reply

Roger

Hi Gene,

It’s likely too much water in the thinset you used over the top. The cracks are due to the shrinking of the thinset, not movement. Simply going over it again with correctly mixed thinset will solve it.

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Darrin

Hi Roger,

I’m getting ready to install Durock and tile above a new alcove steel tub. Most of the information I find, including your post says to NOT overlap the tub lip with the Durock. I’m having a hard time convincing myself to follow this advice since it seems more logical to overlap the lip which would ensure water will flow down the wall and into the tub. I plan to apply 2 coats of Redguard over the Durock. With the Durock overlapping the lip it would seem nearly impossible for water to work it’s way over the lip and into my wall. If the only reason to NOT overlap is that the Durock would bow the wall out I think I can resolve that. The distance from the studs surrounding the tub to the tub lip varies (this is a remodel). I plan to attach thin 2×4 strips on the face of the studs to address this. The thickness of the 2×4 strips can bring the studs flush with the inside of the tub lip. Then my Durock can slide right over the lip without any bowing out. Does this seem like a good approach or are there other reasons for NOT overlapping the tub lip?
Thanks for the great, well written information you provide. It truly helps
Cheers!

Reply

Roger

Hi Darrin,

If you shim out the studs so the durock will be a flat plane all the way down over the lip of the tub you’ll be just fine. Yes, the bulge is the only reason not to overlap it. I actually prefer it shimmed out and overlapped. Place a bead of silicone between the bottom inside edge of your durock and the tub flange before you redgard it to seal that contact.

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Wes

Hey Roger – I’m doing a traditional shower with a pre-slope and liner. This will be a walk-in shower but more of a step-down version instead of a curb. I have 2 questions:

1. How do I waterproof the curb if it’s only step-down and not two-sided?
2. Do I install the backerboards (Hardiebacker) BEFORE the final layer mud bed or after? If I do it before – what’s the waterproofing method used for the walls? I’m assuming I would either do Redgard on the walls (not the bed) or use the vapor barrier behind the CBUs stapled to the studs? If I do Redgard – is that done before the board gets installed?

That was actually more than 2 questions, but I know you can handle it :)
Thanks for the help.

Reply

Roger

Hi Wes,

1. With a liquid or sheet membrane like redgard or kerdi. You need to run the liner up to wherever the step down is and mud over it, then use the membrane over that.

2. You can use either, redgard would be better since you have to use it on the curb anyway. :) Install the backer before the final bed, then paint the redgard on the walls and about two inches out onto the floor around the perimeter.

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