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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Newt

We built a partial wall for a new walk-in shower. By partial, I mean it doesn’t go all the way to the ceiling and it’s open-ended. It wobbles. We have put about ten thousand screws into it but it still wobbles. Will it stop doing that after sheetrock & cement board go up? I’m guessing (seriously-I’m a rookie) this is not suitable to tile if this condition exists?

Reply

Roger

Hi Newt,

Yes, it will solidify once you get the substrate on both sides of it. It will still move a bit, but once you get tile installed it won’t move.

Reply

lee

Just bought a house with a stand alone shower that has never been finished. Has plumbing, studs, then greenboard that is taped floated, textured and painted. No shower pan yet we have to pour one. My question is should I leave the greenboard, redguard over it, then install backer board on top? Or should I demo down to the studs, put in red guard, then backer board?

Reply

Roger

Hi Lee,

You should remove the greenboard. It doesn’t belong anywhere near a shower. You can not use redgard over greenboard. If you want to use redgard you would install the backerboard then install the redgard over the face of it, not behind it. The tile is installed directly to the redgard.

Reply

keri

I have a question about where tile meets shower base. We installed our cement board with gap at bottom so it doesnt touch tile shower floor. After we tile, can we just fill the gap with grout or do we need to use silicone? Thanks

Reply

Roger

Hi Keri,

You need silicone where the wall tile meets the floor tile.

Reply

Brendon

Perfect. Thanks a ton for everything you’ve done here.

Reply

Ray

You mention plastic up between the studs and backer board? My shower is in the basement, I have rigid insulation against the poured concrete wall and a plastic sheeting against that. I have placed a 2×4 wall against that. I am undecided if I need to place more insulation between studs, or leave it open to allow breathing. My concern is if I place more plastic sheeting up, won’t I potential have a sealed barrier, that could build up mold and mewdew?
Or leave the insulation out, and only have the studs between plastic?

Thank you.

Reply

Roger

Hi Ray,

Either should be fine. It’s normally just sealing the backer or substrate between the two layers that causes problems. A larger area like 3 1/2″ between studs allows more vapor dissipation than a sealed 1/2″.

Reply

Jeff in SK

Hey Roger!

The mother-in-law’s bathroom is coming along nicely, but I have a question regarding finishing the bottom edge where the tile/wall meets the shower base:

So, when the cement board was installed, we left a small gap between the CBU and the shower base flange as you recommend, and filled it with silicone to seal that space. The plan is now to paint the Redgard overtop the silicone and onto the shower base (it will be masked off so that no Redgard will show once the tile is on).

My question is this: After we install the tile to within 1/8″ of the top of the shower base and grout everything (including weep holes!), is it necessary to silicone the bottom edge of the tile against the shower base being that there is already Redgard overtop of the silicone to prevent any moisture getting into that gap?

Thanks again in advance!!
Jeff

Reply

Roger

Hi Jeff,

No, you don’t need to silicone it at all. Silicone at the bottom in a properly built shower is only for aesthetic purposes, it doesn’t serve any purpose at all as far as waterproofing. All waterproofing is behind your tile, as it should be, as you’ve stated.

Reply

Brendon

Regarding silicone below the bottom row of tiles, how doesn’t it prevent water that gets behind the tiles and runs down the plastic from getting back into the pan? Does it go back through the tile, above the bottom line of silicone? Is silicone not actually waterproof?

Reply

Roger

Hi Brendon,

Silicone is waterproof. And it does prevent it unless you install weep holes.

Reply

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