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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Wenzell

Excellent Article. Extremely informative!

Reply

Phil

Roger,
I was planning on using a !/2″ hardy backer on the walls of my shower over a 4mil vapor barrier which is being installed directly to the studs. I had my shower pan hot mopped and it looks as if the hot mop is approx. 1/4″ thick.
What is the best way to transition from the 1/2″ backer board to the 1/4″ sloped hot mop? Should I use a 1/4″ backer board for the first 18″ and float it in level to the 1/2″ backer board?
Then the 1/4″ backer board will be held in place for the top 10″ while the bottom will be held in place with the 1 1/4″ of top float on the floor.

Thank You

Reply

Larry

I have durarock for a shower surround. Is it OK to waterproof the durarock with KILZ (latex) ???

Reply

Gerald

Roger
I’m modifying the guest bathroom and I’ve installed “roxul safe & sound” in the walls because the bathroom is next to the dining area and when anybody uses it we can literally hear everything! I am about to cover it all with a moisture barrier so my questions are:
1) I was going to cover the whole area with a moisture barrier but should I just limit it to the shower area?
2)You say “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”. If I’m interpreting you comment correctly there will be a huge gap between the tile and the lip can explain this part in a little more detail.

Reply

Phill

Hey Roger,
When you are taping and mudding the walls, you say that you mostly silicone the corners. Is that all you do to the corners before applying a waterproofing liquid membrane such as redgard?
Thanks, Phill

Reply

Roger

Hi Phill,

No, I tape and mud them as well.

Reply

DB

Great article. Sorry if I missed it, but how do you keep moisture from getting around the screws that poke through the cement board and plastic?

Reply

Roger

Hi DB,

The screws seal it as you drive them in. That’s the simple explanation.

Reply

J walls

Roger,
I have a ceiling in a cook-top alcove that will be tiled. The ceiling is level in the middle of the alcove and then slopes at 10 degrees down each side. So, my question is, how should I treat that joint where level meets sloped? I’m using Hardibacker 500 as a substrate.
1) Do I consider it “in-plane” and fill that Hardi joint with thinset while taping? After all it’s only 10 degrees.
2) Or do I consider it a true “inside corner” and fill the joint with caulk prior to taping (to keep thinset out of the joint)?
3) Similar question for the tile at that joint—caulk or grout?

Reply

Roger

Hi J,

Tape and mud it as an in-plane seam, filling it. You can also grout it – 90% of the time on an angle that small that works fine. If it begins to crack at any point just dig it out and replace with silicone.

Reply

Randy

How do you joint Hardie backer board and regular drywall ? with thinnest or regular joint compound.

Reply

Roger

Hi Randy,

If it’s covered by tile use thinset, if it isn’t use regular drywall mud.

Reply

J walls

Roger,
Regarding taping of Hardibacker outside of the shower (either Hardi-to-Hardi, or Hardi-to-gyp)—–
In several posts, you mention that you can use either mesh tape and thinset, or mesh tape and “regular” drywall mud (so I was thinking you meant a drying type). But in one post, you mentioned a 45 minute cure time (so, setting type).
1) Does it matter whether its drying type or setting type joint compound? Is one more prone to cracking than the other in this application?
2) Would using thinset make a stronger joint, and thus less prone to crack?
3) As a compromise, could you use mesh tape and thinset for the first pass (strong joint); let that cure, and then use drying type joint compound for finishing the joint and texturing (easier to sand)?
4) a) Instead of “seeding” the Hardi prior to texturing, would it be acceptable to prime the Hardi (say with Kilz2) to slow down the moisture absorption into the Hardi? Might be easier, and less prone to error, than trying to keep the Hardi at just the correct wetness?
b) Same question for taping.

Reply

Roger

Hi J,

1. No, it doesn’t matter. If you can tape and mud drywall with it you can do this with it.
2. Yes.
3. Yes.
4. Yes, that will be fine.
4b, yes, taping as well.

Reply

J walls

Roger, thanks for the response.
One follow-up question, please:
—-For Hardi in a shower, using Kerdi membrane for waterproofing:
Is it OK to use the Kilz2 primer approach there also (instead of seeding)?
This would keep the Hardi from immediately sucking out all the moisture from the thinset while taping and installing the Kerdi (and I assume provide a better bond).

Reply

Roger

Hey John,

Yes, killz works fine.

Reply

Kevin Gertiser

Roger,

Unfortunately, I read your instructions “after” I had applied thinset to the cement board in my shower. I should have embedded the tape into the thinset in one step.

What I have done:
I applied thinset into the gaps between the cement boards and have allowed it to dry. I plan to now scrape the high spots of dried thinset, apply the tape and apply a second layer of thinset over the first layer, taking care to keep the wall smooth.

Is this OK, or will I have adhesion and cracking problems?

Thank you for your advice.

Kevin

Reply

Roger

Hi Kevin,

You should be just fine doing that. No worries.

Reply

Morgan

Hi Roger,
I am remodeling a bathroom for the first time and have looked at a lot of forums and yours has been the most informative…evidently the person that had my house prior to me had a leak in the shower and basically put a band aid on it to get it sold and now I am having to deal with it by installing a plastic shower kit, but no pan liner or water barrier of any kind. Some of the wall studs had some rot so I allowed everything to dry thoroughly (after i had totally gutted the bathroom) and then between the studs on the plate, I cut and screwed in treated 2X4’s and on top of those, I screwed in treated 2X4 studs to the existing studs all the way up. My house is pier and beam by the way. I don’t know what these next board are referred to but around the base between the stud’s were 2×8’s, which I guess would be a type of footer, anyway these boards had some rot and water damage and all I had on hand was 2×6 which I cut, and trimmed like the others and re-installed. Then I guess I got the cart of ahead of the mule and installed my cement board all the way down to the floor and butted them together, which after reading your article is a mistake. So my question is, evidently I need to remove the board and put tar paper or some other barrier over the studs? Replace the top half of cement board, place tar paper on the floor of the shower with metal lath and then build a mortar pre-pan, allow to dry then install a PVC shower liner and attach that to the footer boards at the bottom between the wall studs? And then install the bottom cement boards leaving about an 1/8″ gap between them, tape and thinset the gaps and then pour my shower pan?

Reply

Roger

Hi Morgan,

Almost correct. You want to do your preslope and liner first, running the liner up the wall a minimum of two inches above the finished height of the curb. Then the tar paper laps over the front of the liner so any water getting behind the tile runs into the liner at the floor. Other than that everything else sounds fine. The 2×8’s are simply backing to hold the liner up – the 2×6’s will be fine. Only attach or nail your liner at the VERY top – you don’t want any penetrations through the liner lower than two inches above the curb.

Reply

Dana

Hi Roger, I love all of the information you have here, it is extremely useful and I was wondering if you could answer a question for me.

My home has 1×12 shiplap on the exterior wall and I was wondering if I can screw my cement backer board over this(using kerdi for waterproofing). I am consider screwing it into the shiplap instead of the studs because it has so many nails I am worried I will constantly be hitting nails with the screws. The boards are pretty flat but not perfect, do you think this was cause trouble for my tile? What would you do?

Thanks again for all the help!

Reply

Roger

Hi Dana,

I would remove the shiplap. You can go over it, but it will likely cause issues in one form or another over time, not to mention the edge of the backer you’ll need to deal with because it sticks out from the wall itself.

Reply

Dana

Thanks for the response! Since it doesn’t sound like it’s impossible for me to go over it I think I will(understanding that it’s not the absolute best choice). I have found out that the shiplap extends behind the interior walls and could cause a lot of issues if I try to remove it. As long as my dog isn’t catching fire I’m happy!

So with that in mind would you say screwing it to the shiplap would be a reasonable choice or would you still aim for the studs? And what did you mean by the edge of the backer board sticking out? The shiplap is essentially at the same level as studs would be since it is all the way along the exterior walls(window sill sticks out 1/2″ past it) with the perpendicular walls built against it. This is the back wall of my tub/shower space.

Thank you!

Reply

Roger

I would aim for the studs – definitely. I did not know the shiplap was flush with the studs, so that won’t be an issue.

Reply

Patrick

Hi Roger
Very informative article. I took down a fiberglass tub surround and discovered that the whole tub was tiled which was a surprise. The tile is a bit of a mess and it looks like the previous owners opted for the surround rather than repairing the tile. The old surround was attached with construction adhesive. We just put in a new bathroom and really didn’t want to spend a lot of money to touch up this other bathroom so I guess I’m looking for lower cost options. (who isn’t) How do you feel about just gluing a new surround to the tile? Any other cost-effective suggestions? Thanks.

Reply

Roger

Hi Patrick,

Tile is cheap. :D Another surround would work, I just honestly don’t know how waterproof it would be. Most are made to go over bare walls or studs, so I don’t know how it would match up being placed over tile.

Reply

Jason

How do you attach backer board to a masonry wall? I took the old tile directly off the masonry wall but it is not very smooth. I would like to add backer board to have a flatter surface to tile over. Thanks

Reply

Roger

Hi Jason,

You don’t. You can float it out with wet mud (just deck mud with lime added to make it sticky), or, the easiest, would be to waterproof your shower with kerdi and use the kerdi installation to flatten your substrate.

Reply

Jason

Thanks Roger. I floated the wall like you said. It was a mess but it’s pretty flat now. One more question….I’m using a prefab shower base. Without using cement board on the masonry wall there is no overhang into the shower base. I was going to use redguard on the wall but how do I transition from the wall to the base? Thanks for your help.

Reply

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