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-planeĀ  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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Brown

Hey Roger,

DO I have to use thinset before and after I apply the tape to the backer board for bathtub walls.

Reply

Roger

Hi Brown,

I’m not quite understanding your question. You comb thinset over your seams, then lay the tape into it, then smooth it out with more thinset if necessary (it normally isn’t, there’s usually enough thinset to flatten and feather it out).

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Mary

Hi Roger –
We are redoing our main bathroom. The tub and shower area is on an outside wall with a window in the center. We are tiling the entire enclosure on hardi backer. Should we also put green board against the studs first? Or is hardi backer alone (with insulation behind it) ok? (We live in Michigan). When we demoed we found a layer of green board behind the hardi backer which is why we are asking. Is this normal for a shower on an outside wall?
Thanks for your help!

Reply

Roger

Hi Mary,

No, it’s not normal at all. I have no idea why that would have been done. Just the backer. However, in your shower you need either a moisture barrier behind your backer over the studs or a membrane over the front of the backer. Cement board IS NOT waterproof – you need to have a waterproof barrier in there somewhere.

Reply

Mary

Hi Roger –
Thanks for your reply. To confirm – we should either 1) put a 4ml vapor between the studs and hardi backer or 2) put up the hardibacker and then paint it with redgard -correct?

Reply

Roger

Absolutely correct.

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judy mcgill

After my backer board was installed, the installer found that the shower stall is not square. He started the tile installation on the back wall which is the most prominent visual point. He started laying the 6 inch tile from the left corner and moved to the right. This left an odd width for the last tile on the right corner to be only 1/2 inch wide or so. It looks really bad. What was the proper way to install the tile- start from the center and move outward left and right? Can you suggest a way to make the corner look better?

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greg

Your installer should have started with a 3 1/4″ on the left, and ended with the same width on the right. Now, to cover the problem, you could buy a trim tile for both ends and attach with caulk. It will still look funky, but hopefully, not as bad.

Reply

Roger

Hi Judy,

Yes, it should have been started from the center with either a grout line down the center or the halfway mark of a full tile down the center. Unfortunately, short of removing it and beginning again, there really is no way to make it look better.

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Chris

1. How do you handle where cement board meets drywall on an outside corner? I plan to tile flush to edge on cement board side but paint drywall. Do i use a vinyl corner bead or l bead or j channel?

2. I taped and used thinset on the horizontal seams for the cement board all at once, but i didnt get to finish and do the corners. Will it create too much of a hump to do the corners now with thinset? Should i caulk the inside corners instead?

Thanks!

Reply

Roger

Hi Chris,

1. I use a vinyl corner bead.

2. It shouldn’t create too much of a hump. You should caulk the gap in the corners, let it cure, then tape and mud over it.

Reply

Mike

Roger,

I’m a little confused about something you say here as it conflicts with the tile prep video that Lowe’s has on their website. Their video follows what you say pretty closely about using plastic sheeting over the studs, followed by backer board, and taping and mudding the seams, which I have done so far. I used 4 mil plastic and 1/2″ Hardiebacker. But their next step is to also use a membrane material (like Red Gard or some other product) over the backer board before you start to tile. You mention in an answer to another post above that you need one or the other, but not both. (I think I also saw a similar answer from you on another post or article you wrote). So, my question is, is Lowe’s recommendation wrong; i.e., will it hurt anything to do both? Or should I just start tiling at this point and skip the Red Gard? Thanks — your website has been a big help!

Reply

Roger

Hi Mike,

Yes, if you have the barrier just start tiling. If you have both barriers you have two waterproof layers with backer trapped between. Even if it’s completely dry between them when you install them the temperature differential of the shower and inside the wall can cause vapor and moisture to form with no way to dissipate. This can lead to mold being trapped between those layers – lots of it.

Reply

Mike

Thanks!

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Grant

Hi Roger. I’m planning on using hydroban to seal my shower walls and floor. Any reason I wouldn’t be able to use a pre-mix thinset prior to this to mud the backer board seams? I realize that it is basically a mastic with sand, but I have some left over from a backsplash project I’d like to use. I appreciate your help.

Reply

Roger

Hi Grant,

It needs to be thinset. Thinset will grow cement crystals INTO the board to bond and lock them together, mastic is simply a glue which will remain on the surface and can break free with movement.

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Josh

do you also mud and tape the gap from backerboard to mud showerpan?

Reply

Roger

No. Just the corners and seams on the wall.

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Joshua

what do you put in the gap between the backerboard and the mud shower pan? and what to put in the gap from the backerboard to ceiling? I also have a 1/4″ spacing from the backer board to the shower pan on the side corners but in the middle it dips a little its about 1/2″ can I thinset on the shower pan to get that 1/4″ along that side so the floor tile cant slide under the backerboard? Thanks for the help, your a lifesaver..

Reply

Roger

Hi Joshua,

You don’t put anything between the backer and the mud bed, it just hangs there. Yes, you can float out the pan to get rid of that dip with regular thinset.

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Ed

Hi Roger. I’m gutting most of my bathroom. To come out even with the doors’ framing (yes, there’s 2) I need to build up to a complete inch. In other words, the studs are one inch deeper than the door frames. Assuming that the cement board is 1/2 in. thick. What’s the best way to fill he remaining 1/2 in.? Could I install wall board against the studs and put cement board over it? Or could I (should I?) build up the studs with 1/2 in plywood strips and attach cement board to that?

Also, do I need to cement board the entire 3 walls (i.e. between the tile and the ceiling) or just that which is behind the tile? This is a typical bathtub and shower arrangement with 3 tiled walls up to 5′ or so. The remainder would be painted.

Finally, if I put cement board up above the tile and paint it, will the surface look the same as the painted plaster portions of the bathroom?

Reply

Roger

Hi Ed,

The easiest way is to cut 1/2″ ply strips for the front of the studs. You only need backer behind the tile. If you go to the ceiling you can prime, texture and paint it just like regular drywall.

Reply

Bob

Hi Roger,
I am building a mortar shower base with Kerdi and a Schluter drain. For the walls, I want to use Hardiebacker with poly behind it for a vapour barrier.

1. How do I overlap the wall vapour barrier to the kerdi on the floor so moisture will run into the drain?

2. I am going to use 3 stacked 2×4’s for the curb. Can I just run the kerdi over the wooden curb and hold it down with thinset?

Thanks for the great site!

Reply

Roger

Hi Bob,

1. You can not. If you use a topical waterproofing membrane on the floor you need to use one on the walls as well.

2. No, you need to cover the 2×4’s with backerboard. The wood expands and contracts too much to be directly beneath the kerdi, you need that buffer there.

Reply

Bob

Thanks for the response. A couple of followup questions:

1. So if I use the Kerdi on the walls, is there any reason to use cement board instead of regular drywall?

2. Can I mix Hardiebacker and regular drywall? Say, use the cement board on the bottom half and drywall on the top?

3. How do I handle the gaps in the transition between sections? My shower will run half way down a long wall. Beneath the tiled shower section the gaps will be covered with tape and thinset. But the gaps in the untiled section of drywall need tape and drywall compound. Should the drywall tape be put up first and then overlap the Kerdi over it with thinset?

Reply

Roger

1. Not really. Either will work just fine.
2. Yes.
3. In the sections with the kerdi you don’t need tape and thinset, the kerdi does the same thing. It’s not necessary there.

Reply

Massoud Taheri

Hi,
You stated that we should leave some gaps between the backer wall for expansion and if I understood correctly we are to fill the gaps with Thin set, will that not defeat the purpose of the gap or will the thin set allow the expansion?
Thank you

Reply

Roger

Hi Massoud,

You do not fill the gaps between the backer in the corners. I fill them with silicone, then I tape and mud the corners. It keeps the thinset out of those expansion gaps. The in-plane gaps between the boards on the same wall get filled with thinset to create a monolithic wall that moves as one, but you still need expansion capabilities in the corner.

Reply

Matt

I have installed all the backer board to tile shower walls in my bathroom. However, being the first time I have attempted a project like this, I did not pay attention and now I have uneven joints at the backer board drywall joint. Some places are over 1/8″ difference. What is the best way to remedy this problem. All joints have been mudded.

Thanks

Reply

Roger

Hi Matt,

You can float it flat with thinset, let it cure, then install your tile.

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Stuart

I have a cast Iron tub in an alcove which is finished with metal lathe and plaster. Very, very hard plaster. The plaster is about 1″ thick and solid as hell. The end wall that contains the plumbing had to be removed to replace the shower/tub faucet and plumbing. I’ve replace that end with hardibacker. Here’s the question, how do I go about finishing the new backer board and tie it into the plaster? I’m thinking thinset/tape in the corners and seams where the backer meets the plaster, then skim-coat the new backer board. I’m thinking I’d then redgard the whole shebang and then tile. Is this a bad idea? The main reason I didn’t tear all of the plaster out is; 1, it’s a real bitch to get out, and 2, it has a arched ceiling in the alcove which matched the lavatory alcove and there’s no good place to stop tearing it out. (it’s just not feasible to gut the whole bathroom since the plaster is nearly perfect)

Any pointers you might have would be appreciated. I loved your ebook by the way.

Thanks, Stuart

Reply

Roger

Hi Stuart,

Yes, tape and mud where the backer meets the plaster and redgard everything. It will work just fine.

Reply

Stuart

Sweet! I love it when I make correct assumptions. I’m now wondering how the thinset will bond with the plaster where it meets the backer? It’s really smooth. Any tips for that? I’ve got to patch the plaster in the corner and where the end wall meets the curved ceiling so I want to get the most solid substrate repair and the best bond between the differing materails.

Thanks again!

Reply

Roger

Thinset will bond tenaciously to plaster. You won’t have a problem.

Reply

Steve

Roger,

For Hardiebacker on shower ceiling, what thickness do I need?
Also, do I need to add 2x4s in between the ceiling joists (rafters) so they are 16″ apart (instead of 24″)?
And what is the best Hardiebacker thickness for the shower walls?

Thanks, Steve

Reply

Roger

Hi Steve,

1/2″ for ceiling and walls. Yes, ideally you should add 2×4’s to have more joists to screw into.

Reply

Steve Ray

Thanks Roger

Also, I will be installing Swanstone shower base, and my wife wants a shelf about 12″ off the floor to put her foot on for shaving. Can I build out a small corner shelf from the stud framing (just a shelf, open underneath), and then do the normal cement board and tile, or is there a better or easier way.

Thanks, Steve

Reply

Roger

You can just lock it into the wall tile like I do with shower corner shelves.

Reply

Hockey16

Hey Roger,

In my bathtub remodel research I’ve been told by the locals at the local hardware store that vapour barrier is not necessary behind cement board on interior walls, as they need to “breathe”.. Agree/disagree? Should I use both vapour barrier and Redguard? (neither of the bathrooms I’ve ripped apart so far have had either a vapour barrier or sealant).

Reply

Roger

Absolutely disagree. Tell them to purchase a TCNA handbook, which describes that one or the other is absolutely necessary. It’s a common misconception that backerboard is waterproof – it is not. You need either a barrier or redgard, one or the other, not both.

Reply

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