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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L (LANELL)

Roger
Easy for me is Durock. Extra work and expense is buying more ‘stuff’. The wall is not my handiwork and is lumpy. :bonk:

Thanks

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L (LANELL)

Hi Roger

About not attaching Durock to block. I drilled and screwed other things to it with what was told were proper screws. Lots of screws just in case. I have worn out and have many more masonry drill bits. But if no screws will keep it up , as they have for me elsewhere, what easy store bought mix to screed over block could I get at box stores or where I have bought laticrete things. Still waiting on glass estimates from FOUR places, BTW.
Thanks mucho

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Roger

You can screw it (to a wall, it will last), there is just no reason to when you can skim coat it with regular thinset or just go right over it with your waterproofing. You’re creating extra work for yourself.

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Ross

Roger,

Regarding the corner edges where the walls meet I’m not sure if I should silicon and be a rebel or keep to industry standard and tape/mud. I’m using 6×6 porcelain with a horizontal mosaic row if that helps determine. 1/8 grout line.

When you use the silicon method what it the reason for half the time you doing it that way, why not all the time?
What is throwing me off is why james hardie doesn’t mention this (like you state in your article), however, your silicon method makes perfect sense.

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Roger

Hey Ross,

The size of the gap determines whether or not I put silicone in there. You can fill it with silicone then tape and mud. The two do not do the same thing. The tape and mud ties it together to compensate for movement, the silicone prevents water wicking. The tape and mud with silicone in the gap will not wick enough water to do any damage before it dissipates or drains.

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L (LANELL)

I will have to use something to level out block wall since it would never work to TILE on. And thinset only levels up to 1/4″ as I recall? So no Durock directly drilled and screwed to block wall. Ok.

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L (LANELL)

Next problem /question. I have one wall that was thin cedar strips over concrete block wall (condo wall, end unit). I wondered whether to leave the metal 1″ thick metal square strips I screwed to block as studs.Or would be better to take them down and Durock over the block directly. Just bare block, only painted on outside of Condo. Would be easier. The corners of abutting walls I have put very sturdy corners in. So, Durock over block? This will be topical drain and Hydroban troweled.

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Roger

You can install durock to the studs you’ve created or you can remove them and hydroban and tile directly to the cinder blocks. Do not attempt to install durock to a concrete wall.

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John Walls

1) “Do not install Durock to concrete”—
Why? Because it’s hard to do or because of some other issue–compatibility, expansion, or something else?? Does this apply only to Durock or would the same apply to Hardi or any other cement board?
2) Why is thinset limited to 1/4″ for leveling?

I’m asking because I need to install granite or tile on a fireplace front. It’s an Isokern modular masonry fireplace (blocks of ground up volcanic rock that have been molded together with a binder of some sort). Unfortunately, the fireplace face (firebox and smoke dome) is out of plane with the surrounding wood framing by about 1/2″. So, I need to taper the fireplace face from 0″ to 1/2″ (with thinset, I assume) to flatten the surface to match the framing, then install Hardi over the whole assembly (framing and fireplace), and then install the stone or tile on top of that. Note that much of the load taken by the Hardi would be from attachment to framing, and also note that Isokern allows nails or screws up to 1″ long to be used for drywall attachment to the face. Also, note that changing the framing to match the fireplace is not a reasonable option at this point. I would appreciate your advice on this.

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Roger

Hi John,

1. Because there is no way to reliably fasten cement board to concrete that will last long-term with continued use. And it is redundant. Cement backer (yes, it applies to all of them) is a product used in lieu of cement. It gives you a cement-based substrate for tile where cement is not available.

2. Because a bed of thinset over 1/4″ will shrink considerably over time, throwing your tile installation out of whack and cracking your grout.

If you need to put a wall in-plane with a concrete-based product as your substrate use wet mud. It is 1 part cement, 4 parts sand and 1/2 – 1 part powdered masonry lime. That is what you float your wall out with, not thinset. Once cured it is stable, solid and will not shrink. It is still the absolute best wall substrate available.

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Amanda

Hi Roger,

We are renovating the Master bath in the house we recently bought (it was terrible!). You’re site has been a great resource! We did a mud pan with the PVC liner (pre-sloped!). We’ve got backerboard up and are planning to Redgard after we mud & tape the seams of the backerboard.

We read that we could install the backerboard before the second mud bed (embedding it in the floor) or install it afterwards with a 1/4″ gap above the floor. We opted for the later, as that is what the backerboard manufacturer indicates in their instructions. Our gap above the floor ended up being closer to 1/2″-3/4″ on one side – we put the cut side down and it unfortunately broke a bit jagged, so the gap varies a little – and 1/4″ on the other wall.

Now our question is, do we need to fill this gap with something, so we can Redgard all the way to the floor? I’ve searched around, but it seems that most people just say to embed the backerboard in the mud (too late!). If we do need to fill the gap, what should be used – thinset & tape, caulk? Since there is no vapor barrier behind the backerboard b/c we’re doing the Redgard, I’m worried about water wicking up behind the backerboard through the gap. Any advice would be much appreciated!

Thanks!
Amanda

Reply

Roger

Hi Amanda,

You don’t need to fill it with anything, tile can just hang over it. Water is not going to wick up the board if there is not a constant source of moisture into which it is embedded (like your top mud deck :D ).

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John Walls

Roger, I’m new at this, so please accept these questions in the spirit in which they are intended—educate me.
1)Regarding the requirement that every edge of backerboard be supported:
a) I have a case where the studs range from 2 to 6 inches from the corner. Should I still put in more framing or is this close enough? What about using metal corner clips here (instead of more lumber), like the kind used on regular gyp board for two-stud corners?—Like the Prest-On Corner Back. Do you think the cement board would be too weak or fragile for this? Or maybe the clips would be too weak for this particular service?
b) What about the horizontal edge just above the tub—do I need horizontal blockers across there to support the cement board bottom edge? Same question at the top if I don’t go all the way to the framing top plate.
c) Would the answers to the above be different for granite slabs (or engineered stone) instead of tile? Maybe the slabs would be more forgiving since they are in one piece and not grouted?
2) Regarding leaving expansion gaps between cement board corners:
If these gaps are then filled with thinset and then embedded in tape so as to lock everything together, then what good is the gap from an expansion perspective? It seems that the expansion function of the gap is eliminated. You mention that you use silicone in the corners sometimes (instead of tape embedment). Why not all the time? Why do you think the manufacturers recommend tape embedment instead of caulk? Help me understand this.

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Roger

Hi John,

a. 2-6 inches is fine provided you tape and mud the corners. DO NOT use a metal corner clip or bead – they rust.
b. No, you do not need horizontal backers at the top or at the tub.
c. No, same answers.
2. It isn’t any good. That’s why I fill the gap with silicone first, than tape and mud. The silicone allows for expansion (in a lateral swelling sense) and the tape and mud tie the two backers together so they move as one. Manufacturers recommend taping and mudding, because most of their specs are dealing with a simple wall construction without taking wet areas into the equation.

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John Walls

Roger, thanks for the reply. I have a few followup questions:
a) Would you still have an issue with corner clips if they were galvanized or zinc plated? Also, they should be behind the waterproofing (for a topical installation), right? Also, do you think there would be a problem structurally? Just asking for future reference.
2) If you use Kerdi for waterproofing, do you still need to tape embed the corners? Will Kerdi be strong enough for this on its own, or do you need the extra strength provided by the tape in the corners? You mention this in your eBook, but I think the Kerdi thing may apply only to in-plane seams and not corners?

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Roger

1. I would have no problem at all with any material provided it was installed behind a topical waterproofing. no structural issues I can see.
2. No, the kerdi does the same thing as the tape and mud.

Reply

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