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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Roland

Roger, I have an outside corner where cement board meets green board. I’m thinking I’ll use a corner mesh and then use drywall mud to smooth it out. Please advise.

Also, should i leave a gap on inside corners?

Roland

Reply

tyler

Hey there, I have purchased Durock to go in my shower enclosure. my surrounding drywall is 5/8 in and the Durock 1/2 in. so there will be quite a difference at the butting edges. any suggestions? Shims are fine?

thx

Reply

Roger

Hi Tyler,

Yes, shims are fine, regular drywall shims work well and are cheap.

Reply

Kristen NyQuist

Dear Floor Elf:
Just want to say a big thanks for holding my hand throughout my recent DIY bathroom remodel. You might not known it, but you were there with me the whole time, on speed dial. I finished the tub surround and floor a while ago, but won’t let anyone use them because I’m so paranoid that maybe I’ve done something wrong (you know, considering the fact that I have no experience at this and everything is either perfect or you’ll rot your house away). Tonight I went back over my steps, looked at all my pictures while cross-referencing with you site, and I’m happy to say I’m gonna let the family shower in our own home tomorrow. It’s been a long 3 weeks, and I definitely couldn’t have done it without your help. Thank you!!!!

Reply

Roger

You’re very welcome Kristen! Glad I could help. :D

Reply

Scott

Sorry, correction! I am doing a tub surround.. A deep, really deep, “soaker tub” so that my wife can lounge about for hours along with several hundred pounds of water suspended above my kitchen.. The added bonus of such a deep tub, is that it will be easier for me to whack my shins getting in and out of it.. AND it’s going to be a better fire suppression system for flaming dogs than a standard tub!

Reply

Roger

Same answer, but I love the fire suppression system idea! :D

Reply

Scott

Hi Roger,

I’m doing a shower surround, 1 wall, and the entire floor in my bathroom using 1/2″ backer board on the walls and 1/4″ on the floor. I will be using Redgard (or similar liquid product) on the walls.

1 Do I still want to put a plastic vapor barrier between the backer board and wall studs if I am going to use Redgard?

2 for the bathroom floor, should I use Redgard on the backer board?

Thanks in advance for your help

Reply

Roger

Hi Scott,

1. No.

2. If you want to, but there’s really no reason to.

Reply

Kara

We are using Redgard as well…we were told by a contractor that we didn’t need to tape and bed the backer board if we do it this way-is that correct? Reading this, I see the advice about leaving the gaps, and it makes sense to still need to tape and bed those sections. We just paused our project so I could check. Thanks!

Reply

Ray

From your instruction above, if I chose to silicone the corners, that’s all that goes in the corners, right? I thought I reads in another Q&A for one of your other intructions where someone asked about why you chose one or the other options (silicone or tape/mud corners) and I think I read that you responded that you when you silicone the corners, you then tap and mud over the silicone corners, too. Did I read that right or am I just recalling incorrectly? I’m at the tape&mud step in the project and just want to get it right the first time…. thanks.

Reply

Roger

Hi Ray,

I still tape and mud over the siliconed corners. The silicone allows the board to expand (linearly) while the tape and mud prevents the two planes from pulling apart.

Reply

Ray

Got it… I’m on to the corners tonight then the Redgard membrane in another day or two. Thanks for the details.

Reply

Bill

Hello roger i am a contractor an I enjoy reading your colum with your response an they are always on the money Job well done

Reply

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