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. 
*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. 
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.
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.
I am fixing to mud the floor on the firberglassed pan but the weep holes were fiberglassed over, should i drill them out or let them be.
thanks, jack
Hey Jack,
Yes, drill out the weep holes – they need to be open.
I have two questions. First is i am building a tile shower and i had the floor pan fiberglassed in. should i put the mortar bed directly on it and then the thinset after it sets up. Second i used durock for the walls should i use just regular thinset on the walls. I haven’t decided whether to tile the ceiling or not but if i do what would i use if i do.
Thanks, Jack
Hi Jack,
Assuming you mean you had the fiberglass installed as a waterproofing membrane and not that you had a fiberglass pan meant as the final floor similar to an acrylic pan then yes, put the mortar bed directly onto the fiberglass. It is a correct waterproofing membrane technique – just one that isn’t used around here which is why I rarely mention it on my site.
And yes, just regular thinset to install the tiles to the wall. Regular thinset as well to tile the ceiling if you choose to do so. Keep an eye out in the next day or so – my next post is actually about tiling a shower ceiling. See, sometimes you get lucky with timing. Sorry it was with my blog and not a lottery ticket.
Hi Roger,
In your previous note, you mentioned you were going to be having a “tiling a shower ceiling post” coming soon. I haven’t been able to find that on your website. Am I missing it?
I’m still at “tear out stage” so I’m trying to learn as much as I can before I get into the “jackassery” of tiling anything!
Your website is fantastic! You rock!
Thanks so much!
Hi Michael,
You haven’t missed anything – I haven’t gotten off my lazy ass and written it yet.
But since you need it I will write it up tonight and post it. Check back in the morning for requested jackassery!
YOU ROCK!
I would like to tile around my bathtub and I know I have to use cement board but I have one question. In the article you said not to put the cement board over the lip because the board will bow. I understand that idea. My problem is that the lip around the bathtub is a about an inch high. How can I cover that lip around the tub so that it is not sticking out as much?
Hi Jim,
You have a couple of options:
1) Use drywall shims or something similar behind your cement board to nudge the board out far enough to cover the flange without sticking out.
2) Stop the cement board above the top of the flange and install your tile all the way down to the top of the tub. Do not attempt to stick the bottom inch or so of the tile to the flange – it won’t work. But it will be just fine. You may still need to bump the cement board out enough so that the face is at least flush or out further than the flange.
I usually prefer number one but in certain circumstances, and with certain tub flanges, number two is sometimes necessary. Just use whichever would work best for your particular tub.
I was wondering if there would be a way to install ceramic corner shelves securely to a corner that is about a 1/4 inch out of square as I tile the walls?Thanks
Hey Dave,
That really depends on a couple of things.
When you say ‘out of square’, that could mean two different things. Is the angle more than 90 degrees or less?
If less you can, if the shelf is the type that can be cut, simply cut it to fit. I’m thinkin’ it isn’t that type, though, or you wouldn’t be asking, right?
In that case, if less than 90, you can have each outside corner of the shelf touching the wall and the inside corner will be 1/4″ off of the wall. There should still be enough to hold the shelf on the field tile and silicone that extra space.
If more than 90 it creates a more difficult problem. Your tile should still be thicker than 1/8″ though so you can simply place the back corner of the shelf against the wall substrate and split the 1/4″ on each outside corner of the shelf. In that case the outside corners would each be 1/8″ away from the wall. You should still have plenty of support for your shelf.
All of the above is assuming that you are tiling up to the row of tile where you want the shelf placed, setting (thinsetting) the shelf in place against the wall on top of that row (so the tile supports the shelf) then cutting the remaining field tile around the shelf.
In either case, so long as your tile is thicker than 1/8″ you should be able to split the difference between the two sides of the shelf and have it reasonably supported and any spaces between the shelf and wall covered with the field tile. For additional support you can always use epoxy to install the shelf rather than just thinset.
Just don’t go dancing on it.
Okay. We are installing tile (again!) on the walls of a tub/shower combo which has quickly become the remodeling DIY project from you know where! The original tile was installed on top of hardibacker which set on top of sheetrock thus requiring the tiles have a “capped” edge tile (which we hated). We are installing the new hardibacker flush with the rest of the walls so we can just use the flat tiles (which has caused other issues that I wont’ get into!) :-) My question is that when installing the hardibacker over the tub lip do we tile down “to” the lip; slightly covering the lip”; right “above” the lip; slightly above the surface of the tub or all the way down to the surface of the tub?
Stressed out in Texas!
Hi there Ellen,
You know, when I walk into someone’s house and see one of those oh-so-special ‘capped edge’ showers I try really, really hard not to laugh my ass off – I’m rarely successful.
You want to get your tile as close to the tub surface as you can. I usually run the tile all the way down to 1/16″ above the tub surface and caulk that space. The hardibacker, after installed, should be sticking out beyond the lip a bit to allow the tile to go all the way down to the tub surface. The lip of the tub should never stick out further than your tile substrate.
Don’t stress – it’s just tile.
I guess I should have also asked “how far down do we run the hardibacker to?”
Thanks!
Ellen
And I guess I should have told you anyway.
You want to run the hardi down as far as it will go without needing to bump out at the bottom. In other words, if your tub sticks out from the studs – usually about 1/8″ or so, then just run the hardi down to about 1/16″ above the tub lip. If the lip is flush with the face of the studs then run it down to 1/16″ above the tub surface just like your tile.
You want it down as far as you can while maintaining a flat surface that does not bow out toward the tub.
Thanks! I really appreciate the input! We are usually quite handi at DIY projects but we are both about to pull our hair out over this one! Seems like one little thing after the other has gone wrong! Started out as a simple re-tile and has grown into an almost total gut job! Craziness!
I had a tile base and surround shower installed about a year 1/2 ago. The backerboard was attached directly to a stud on the left and around the corner to a stud on the right for a corner shower. Due to the size, there was approx a 3 inch strip on the left side that did not get tile put on it. The installer told me to treat it like the green board we currently had up, tape/float/sand and paint which we did. It looked great. Now we’re seeing bubbles form where the green board and backer board meet. What is causing this?
Hi Lisalue,
The tape/float/sand/and paint is fine so long as the 3″ ‘strip’ in question is not actually inside the shower.
If this strip is outside of the shower and sealed away from moisture then the only thing I can think of that would cause this would be expansion in the wall studs. While all the wall studs will expand at fairly similar rates the two different substrates installed over them may not. This uneven expansion and contraction would eventually separate the two substrates and may cause the paint to bubble up.
If this strip of wall that is not tiled is inside the shower – and exposed to moisture – then moisture has gotten in behind the paint, likely dissolved the drywall mud (I assume that’s what you floated it with?), and caused the paint to bubble. While not really recommended, if you are going to leave an untiled ‘strip’ inside your shower it needs to be taped with alkali-resistant mesh tape, floated with thinset (which is not affected by moisture or water), sanded then painted. Even that, though, will not last a very long time as the stability of it is entirely dependent upon the water resistance of the paint. If this strip is inside your shower it should be tiled. Did your installer know that tiles can be cut to size?
If I am going to apply tile in a shower in a basement (with concrete walls) do I need to put up backerboard where the cement is or can I put the tile directly on the cement wall?
Hi Tammy,
You can but it does absolutely nothing to waterproof that wall at all. You need to install some type of topical waterproofing on that wall to prevent water from sneaking to places you don’t want it. These are products such as Redgard or kerdi. As long as the wall is in good shape (no cracks, bowing, etc.) and you install waterproofing on it you will be fine installing directly to it.
Another option is to install stud to that wall then build your shower as you would a shower with normal studded walls using whatever waterproofing method you prefer.
Hey Roger,
Considering I leave 1/4 ” space between cement board and fiberglass shower pan. The board is thicker than the pan lip, then after my tile is installed it will be even further away from the lip, my question is , Will there always be a void here or can I seal it with silicone caulk or is there another method?
Hey Rick,
The space behind your tile over the lip of the tub can be filled with silicone if you wish. I usually fill it with thinset as I set the tile. The thinset won’t adhere to the fiberglass at all but it will fill the void and give you a solid backing for that portion of the tile. I also only usually leave 1/16″ to 1/8″ rather than 1/4″.
I read that an alternative to filling the gaps between bb sheets is to use silicone rather than tape and thinset – your thoughts?
Hi Ryan,
You can do that but it would defeat the purpose.
The information you found may have been noted as acceptable for backerboard but someone is confused. The silicone between the gaps is used for substrates such as denshield which is a gypsum (normally) product with a waterproof layer on the front of it. The silicone basically ensures that the waterproofing is continued from sheet to sheet without water having the ability to seep through the space between them – silicone stops that.
The purpose for taping and thinsetting the seams in cement backerboard is to ‘lock’ them together and create one large monolithic structure. The studs behind the wall will always move – they’re wood. This may cause the individual sheets to move independently and in different directions. Every time I see a crack in a shower or floor which is in a single, straight line I can usually guarantee that they are nearly exactly three feet apart – the width of backerboards. Tile will crack right over that seam if they start moving differently – taping and thinsetting the seams prevents that.
Tape and thinset is for structural purposes – silicone is for waterproofing purposes. Hope that helps.
I currently have a plastic shower enclosure which will be removed. Behind it is Greenboard. The top 8-10 inches is painted. I have seen very few posts anywhere that clarify if backerboard can be used directly on top of greenboard. Assuming the bottom sections of the greenboard is in good shape (no mold,mildew or otherwise deteriorated), can I or should I redguard the greenboard (3 coats), install backerboard, then tile? I have read posts about redguarding the backerboard. Which is the best method? I am certain there is not a vapor barrier behind the greenboard. Any concerns about waterproofing around plumbing fixtures?
Hey Ron,
You can, but why would you want to? If you don’t plan on tiling all the way to the ceiling you will need to deal with the top edge of the backerboard. If you do tile to the ceiling you will still need to deal with the outside edge of the backerboard. In either case the backerboard will stick out further than your greenboard.
If you do choose to do that I would Redgard the face of the backerboard rather than behind it. If you are spending the money and time to Redgard anyway you may as well have the waterproofing directly behind your tile, eh? Then you don’t have to worry about moisture reaching anything at all.
No concerns about the fixtures at all. I would just run a bead of silicone behind your escutcheons (that’s just a big fancy-ass french word for the metal trim around your fixtures) before installing them. This is especially effective if you have a topical waterproofing membrane (Redgard). I would just install the backerboard either over the greenboard or, ideally, in place of your greenboard, and use Redgard on that.
I am installing hardibacker in a tub surround, and plan on using Redgard over the hardibacker. One of the walls is an outside wall that will need to be insulated. Does the vapor barrier need to be removed from the insulation in this wall?
Also, I was thinking of putting insulation behind the hardi on the inside walls to help muffle the sound of the shower in the bedroom on the other side of the wall. What do you think?
Hi Mike,
I would not remove the vapor barrier from that outside wall. The barrier that is presently there extends the entire length of the wall from floor to ceiling and does not stop at the lip of the tub or shower pan. The problem would be if it did stop at the lip then redgard were applied to the backerboard effectively “sealing up” the backerboard between two waterproof layers. With the current plastic I don’t believe that would create any problems for you. I do not remove it when I run into the same situation.
Placing insulation behind your backerboards in any other wall would be fine and fairly effective sound dampening. You do not need to worry about placing plastic over it behind an interior wall.
Roger,
Just to follow up my last question. I am guessing you are going to recommend a ledger board. If I use a ledger board I need to screw it into the hardi. I am going to be using Redgard over the hardi. The screws will make holes in the redgard and the hardi. So after the tile is set above the ledger, remove the ledger and recoat the holes with thinset. Then wait for that to dry…….recoat with redgard, wait for that to dry……then finish tiling the rest below the ledger? Seems like a awful lot of waiting. Please tell me there is a better way???
Hey Ian,
Yes, you should tape and mud (with thinset and fiberglass tape) all the corners as well as all the edges or corners of your shower niche.
The Custom’s M & G is fine. It’s a good mortar for heavier tiles.
I usually use a ledger board with larger tiles but will usually start at the second row of the shower and support it from beneath rather than screwing or nailing it through my waterproofing.
You can redgard over those holes or you can also use silicone to fill them if you do nail or screw it to the wall.
Thanks Roger. That is what I was thinking, but wanted to get your input. I backed out all of the “Rock On” screws that were not flush and installed “Hardi On” screws. All are now flush.
Do I need to thinset and tape the back corners were the walls met as well as all other joints? I also made a shampoo and soap “box” out of hardi inset in the walls. Do all the edges, inside and outside of the box, need to be thinset and taped as well?
I am using 3×6 honed limestone tiles and Custom building product’s Marble & Granite Fortified Premium Mortar to set the tiles. Does this sound like the right setting material for these tiles? When setting the tile would you set 1 or 2 rows of tile then let those set over night and tile the rest, use a ledger board or just go ahead and tile the whole thing. I am concerned about the tiles being heavy and just sliding down the wall as I begin to lay them. Any thoughts?
Roger,
Fishished putting up my hardibacker yesterday, but I am a little concerned about the screws I uses. They are not totally flush with backer. Some are flush, but many are sticking up maybe a 1/16″. I used Rock On 1 1/4 screws. I tried to drive them down as far as I could. After I did some research, I found out about “backer on” screws. So much for listening to the guys at HD saying the screws I bought would work. Should I be worried about this for the tile?
Thanks,
Ian
Hey Ian,
The heads of the screws should really be flush with the backerboard – I know that isn’t what you wanted to hear. If you have one that sticks up even 1/16″ it may prevent the piece of tile that is installed over it from laying flush with the wall and, in turn, even with the tiles around it.
You can try to back the screws out about half way and screw them in again without stopping, sort of a “running start” if you will, or just pull them out and use the hardi screws that are made for it.
If you don’t want to do either of those there is always the option of using galvanized roofing nails. Although I do not use nails they are an acceptable method of attaching your backerboard.
How long does one have to wait before taking a shower after applying the tile and grout sealer?
Hi Steve,
There should be specifications on your sealer container. With most I’ve used 24 hours is usually sufficient.
I installed hardiebacker on shower walls to drywall ceiling. Should i tape and mud or caulk that joint? I also have outside corner with hardiebacker and drywall joint and flush joint with hardiebacker and drywall? thanks , Tom.
Hey Tom,
I would use just caulk or silicone at the ceiling joint. As for the others the general rule of thumb is to use tape and mud for everything that will be covered by tile and regular patching compound for any others. Do not use regular drywall mud anywhere inside the shower – it’s water soluble.
Roger –
Thanks for the info. Here is why I wanted to mount the brackets for the shower doors first:; I am replacing the shower walls and not the shower base, the base is exactly where it was before. The old wall was a polystyrene shower kit which was 3/32 added to the 1/2″ greenboard. I tore down the greenboard and put in wonderboard, same dimensions. Now if I add 1/4 to 1/2″ (thinset and tile) before I put the brackets back I will alter the placement of the wall/door/wall unit.
I could trim the wall/door/wall unit 3/8″ and it will be back in the same location AND I could put the bracket on after the thinset and tile. I thought I had the first method available, but if that will be non-standard I dont want to do that.
Any tips on how to cut the polystyrene bracket, and the end of the wall section that slides into the bracket?
And one more issue; I thought I used thinset to seal the wonderboard sections, and then mix up mortar to hold the tiles to the wall. (then I saw tile adhesive at the hardware store, which confused me more) Am I on the wrong track? whats the best way to attach the tiles to the wonderboard?
Thanks for your help
Hey Bud,
I learned that the hard way.
Cutting around the door brackets is not really standard, it simply makes available the option of removing and replacing them if needed or wanted. You can leave them and tile around them if you choose to do so – it’s up to you.
You should be able to cut the bracket with a hack saw. If you have the plastic glass(ish) looking panel it can be scored and snapped. Just use a utility knife and a straight-edge to score BOTH SIDES then you should be able to snap it cleanly. You can clean up any edges with sandpaper. Do not use power tools – that stuff melts.
Mortar and thinset are the same thing (well, mostly). You use it to both seal the seams and adhere the tile to the wall. The “tile adhesive” you saw was likely mastic – don’t get it anywhere near your shower. Read my post here for more info about setting materials.
Hope that helps.
Great tips here!
I am doing a shower enclosure with tile. I have done the vapor barrier and wonderboard already. Now I am wondering how it goes from here.
1. Should I do the tape and thinset on all seams, and the outside edge where it meets regular drywall?
2. Next I will install the enclosure’s wall brackets, 1 1/2 in wide from the shower pan up 66″. How can I seal the plastic bracket to the newly thinsetted wonderboard? Silicon?
I will have tile inside the enclosure and 6″ outside of the enclosure, what do I do to the joint/margin where tile/wonderboard meets drywall?
Hey Bud,
1. I usually do this as I am setting tile. It’s easier because you already have the thinset mixed and everything is in process. Just run your tape and fill the seams as you tile.
2. I assume you are speaking of the brackets for the shower doors? If so they usually are installed on top of the tile after is is set and grouted. They are (the plastic ones) usually only installed with silicone.
As long as it is not inside the shower doors you can just tape and mud it like regular drywall. Then just paint it up to where your tile will be.
Hope that helps.
Thanks. I cant seem to counter sink the screws well, and nailing is easier. But what do you do with the head of the roofing nail?
I am visually sure I didn’t puncture the liner. What could I do to test that? I’d hate to have to wait a month or so to see if my dining room ceiling is getting soft.
The screws are difficult to countersink. The specially made screws have “teeth” on the underside of the head to cut into the backerboard to countersink them. You can just place the nail head flush and the mortar for the tile will make certain they don’t effect your tile.
You can do a “flood test” on your shower floor to make sure it doesn’t leak. Inflate a small balloon and place it in the drain to block it. Make sure you push it down far enough to plug the weep holes as well. Fill the pan and let it set overnight. There should be no significant water loss in the next 24 hours – just evaporation.
Here I go again. I have two issues this time.
1. Should the hardibacker be screwed or nailed. If you can nail, what kind of nails should you use?
2. When stripping the tiles off the shower floor, I also tried to get the cement up as well. I did great until I reached the final corner where I managed to crack a 6×6 triangle off. I took it all the way down to the rubber. Can I patch this, or has too much been taken out? In my earlier discussions with you above, you mentioned using a sand and topping mix or concrete to fill the voids I was creating at the borders. Since I’ve made such a big hole here, should I use sand and concrete or can I still use a topping mix? What kind of concrete? Whatever the the tile section has?
Hey Steve,
You can use regular galvanized roofing nails to put up your backerboard if you want. You can also use specially made screws for the backerboard or any type of coated deck screw as well.
Do not use regular concrete. If you want you can use the sand and topping mix or, better yet, deck mud. Either one should work fine for that size area. You need to make absolutely certain you did not puncture your liner before you fill that area in.
hi, got a question. never having tiled a shower wall before, you can imagine the questions and redo’s i’ve encountered… i have more than half of my hardibacker up, i wanted to make sure i’m on the right track-i tend to be like a bull in a china closet and get ahead of myself. so i did some more research and realized i forgot to put up a vapor barrior between studs and backerboard, duh? . is there any other way to ensure a “vapor barrier” at this point without having to take down the backerboard? could a membrane be installed over the backerboard? suggestions ???? want to do this right the first time. thanks
Hi Joanie,
There are membranes that can be installed over the backerboard. The difference in what you decide to do will probably be dictated by how much you’re willing to spend for it.
Redgard is a brush-on or roll-on membrane similar to thick paint. I have a post about it, just click on redgard in the tag cloud.
The other option would be ditra which is a complete tile membrane for your wall, and it’s expensive. Just google ditra and you can find it pretty easily.
The Redgard would, depending on your shower size, run about $45.00 or so. The ditra is about 1.50 / square foot. The cheapest and easiest, of course, would be to take down what you already have up and use 4 – 6 mil plastic as a vapor barrier. I know it sucks – sorry.
On a floor, is it absolutely necessary to thinset hardiebacker to plywood? Why or why not? (I read this somewhere, but wonder if it is an urban legend.) for vapor barrier purposes, wouldn’t it be better to put down felt between the plywood and the hardibacker?
I am going to install electric radiant heat under my tiles and therefor am adding this extra thickness as it is. It’s going to be 25 feet thick before I finish if I listen to all the gurus!!!
The floor and the wall are two whole different animals. You do not need a vapor barrier under your floor substrate. (Unless the entire bathroom will be a shower.) You do need to use thinset beneath the hardi. The purpose of this is not to adhere the hardi to the plywood – it is to make absolutely certain there are no voids. Voids = movement. Movement is bad. If you fill all the voids you will have no movement. You still need to screw the hardi to the plywood.
If you are concerned with height issues, especially with underfloor heating, you may want to go with a decoupling membrane such as Ditra. With Ditra you can install it directly to your plywood with the heating mat beneath it or on top of it. The tile is then installed. The total height of ditra when installed is about 1/4″ with thinset and all, a bit less, actually. This takes the place of thinset and backerboard and saves you about a 1/2″ if not more.
It is a bit more expensive than just backerboard but the upsides vastly outweigh cost in more than just height issues.
That makes a lot of sense. Thanks. Now to the next part. The floor drain: I had 4×4 ceramic tile (1/4). Now I am putting mosaic squares (1/2 x 1/2 by 5/16). The difference of height between the ceramic cement and the thinset will lift everything up a tad. (Yes???) Is it possible to chip around the drain screw to loosen and raise it, so that I not only have the correct height but also change the pattern of the drain cover from round to square?
First try to unscrew the drain by inserting two screwdrivers into the holes in the drain cover on opposite sides. You are doing this only for leverage purposes so a phillips or flat-head will work, whichever fits. This will give you the leverage to twist the drain out.
If it still won’t move (and it probably won’t) it is because the bottom of the screw-in drain flange has nubs on it. When the floor is fabricated it is done in a manner that prevents movement. Sometimes it will move, sometimes it won’t. You do not need to chip around the threads – they won’t stick to the mud bed. You need to chip down underneath the drain from the edge to the center until the nubs are free. Do this until you can unscrew the drain.
The tiles you want to use will only raise everything 1/16″ which you could probably manage with just the amount of thinset. You do need to remove it if you want to change to a square cover, though.
You can fill this back in with sand and topping mix or regular concrete when you install the new screw-in part of your drain.
Thanks for the quick response. Everything you have told me makes sense. Now to continue with the adventure. I have a mortar bed shower pan. I chipped off 4×4 ceramic tiles and cement to get to the original surface. Since I have pulled the walls off I have an empty 1 inch by 3 inch channel around the perimeter. Could I fill this gap with thinset or do I need to use a mud mix? Also would I put the backer all the way down into the channel or should I now keep it above the level of the mud floor? Should I seal any of the joints between floor and backer?
You can fill that channel with thinset but you need to make certain it has enough time to thoroughly cure so it does not shrink after you get tile on top of it. It would be better just to pick up a bag of sand and topping mix or regular concrete mix and use it. Since it is such a small area you really don’t need to mix additional sand in it. You can also use the same to fill any gaps or dips in the shower pan to flatten it out.
It would be better to keep the backerboard about 1/8″ off the floor but with cementious backerboard it is not really necessary. As long as you have the vapor barrier running between the pan liner (which should be at least 8 inches up the walls) and the backerboard there is no need to seal it. Just seal your wall tile to the floor tile when your done.
Added to the above: You can keep the backerboard above the mud bed as long as you can keep the bottom of the backerboards solid without putting screws into it. You do not want to put screws through your pan liner. If they stick out or are loose enough to move you need to insert them into the channel and keep them in place with concrete packed into the channel. (That’s why the channel was there to begin with.)
I hope this is where I can ask a question. Thanks for the hint about the vapor barrier. There wasn’t one behind our walls. It appears that the shower walls were 1/2″ green board and 1/2″ duroc. I don’t know if it is mold or mildew that was behind the walls. What do I do about making sure this is all dead before I recover the wall. We plan to put up 12/12x 3/8 marble tiles. How thick does the substrate really have to be? I was thinking about just doubling up the Hardieboard to get the one inch thickness, but if I don’t have to, I’ll go for a half inch instead.
Hey Steve,
To ensure that any non-welcome things (mold or mildew) is left just mix up some 50/50 bleach and water and spray the heck out of it. Then let it dry. The most important factor is dry. As long as there is no moisture left in these areas none of those things can live nor multiply. (That’s why you want the moisture barrier.)
The “thickness” of your substrate does not matter so much as the solidity of your framing. As long as you have solid studs in good shape and all your backerboard edges are fully supported (i.e. you can’t push them in and out) you’re fine.
If you use 1/2 hardi that would be fine. Strength-wise for a wall you are more concerned with “sheer strength” rather than “load strength”. That just means it will fully support your installation. Since gravity is pulling along the board rather than against it, it’s not a concern.
Hope that helps. If you have any more questions at all please don’t hesitate to ask.