# wallboard behind a tub



## Mike Milam (Mar 3, 2017)

Pat the Rat said:


> I'm working on renovating a 90 year old house. It had a cast iron/ceramic tub that never had a shower, but I want to add one with a tile surround above the tub. The room is gutted right now down to the studs. I'll be applying closed cell foam insulation to the exterior wall. I want to put the tub along the exterior wall, but I'm not sure if I should put greenboard up first or just put the tub against the bare studs. I know the greenboard or cement board should come down and overlap the rim, but I'd need a double layer of wallboard (whichever) to do that. I'd also heard not to put greenboard behind cement board. Basically I'm not sure how far away from the wall I should put the tub. Should it be a half inch away or up against the studs? I'm not going to put the tub there till I notch out the joist below the tub to fit the drain tube. It won't be much of a notch. It just sits maybe an inch below the floor. Just didn't want to make the notch any wider than I have to in order to fit.


By all means wait for a more experienced person to comment, but when I put a tub in place I set it in and make sure it is level in both directions and mark the studs at the top of the flange and the remove the tub and put horizonal 2 X 4's at the level taking into account for the distance from the top edge of the flange to the under side of the tub where it will rest on the 2 X 4's. Then set the tub tight to the studded wall. I use one layer of cement board and bring it down to the top edge of the tub flange. Wall tile "usually" is big enough to bond to the wall and stick down within an 1/8th of an inch from the top surface of the tub.


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## chandler48 (Jun 5, 2017)

What Mike said. Your biggest problem is the tub wasn't meant to have water splashed on the rim. It will tend to run out on your floors unless you incorporate a good sealing door system. No sheetrock needed behind the 1/2" concrete backer underlayment. Bring the underlayment down to just above the edge of the tub with 6 mil plastic behind it as shown in the illustration. Your tile will jump over the gap and the gap will prevent wicking of water when it hits the rim. Post a picture or two of your tub set up so we can see what you see.


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## Pat the Rat (Aug 31, 2009)

Okay I'll try to get some photos today. Thanks. I'll figure on putting the tub against the bare studs then unless someone has better info. does the vapor barrier go all the way up the wall or only about 6" or so just enough to seal around the top of the tub? Just wondering about condensation behind the plastic sheeting.


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## Mike Milam (Mar 3, 2017)

Pat the Rat said:


> Okay I'll try to get some photos today. Thanks. I'll figure on putting the tub against the bare studs then unless someone has better info. does the vapor barrier go all the way up the wall or only about 6" or so just enough to seal around the top of the tub? Just wondering about condensation behind the plastic sheeting.


I go to the ceiling or at least as far up as the studs are bare.


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## Pat the Rat (Aug 31, 2009)

Mike Milam said:


> I go to the ceiling or at least as far up as the studs are bare.


Okay, thanks, Mike.


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## XSleeper (Sep 23, 2007)

Your exterior wall is foamed, so you only need 6". I would use a window flashing tape that would stick to the tub flange.


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## SpentPenny (Dec 15, 2020)

Unless you are willing to waste your money on an amateur unsatisfactory job I would strongly advise a professional tiling contractor. As mentioned by another, that tub was not made for the installation you plan so you have a significant problem to overcome anyway, and if poorly done it will not only look bad, it will be subject to visible mold and hidden rot. I stopped using green board 40 years ago as well. Specify cement board and the correct, special fasteners.


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## jeffnc (Apr 1, 2011)

The tub goes against the studs. Drywall can be used (or cement board, not both). However this isn't an appropriate surface for a wet area. If tile is going to be applied, you need to cover with a waterproof membrane of some kind. I use Kerdi. Remember that cement board can't be damaged by water, but it can transmit water through to the studs. Cement board does not waterproof, and you need waterproofing.


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## Pat the Rat (Aug 31, 2009)

i was going to use something like Red Guard over either the greenboard or cement board. I was figuring on using cement board.


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## flyingron (Dec 15, 2020)

Cement board is not waterproof so you need a membrane as chandler's picture shows, or service coating designed for such.
I'm actually not fond of cement board. I use Denshield. Denshield goes up like regular drywall and has a ceramic facing. You "tape" and seal the seams with silicone and it becomes the waterproof barrier. No membrane behind it is required (in fact, they recommend against it). The stuff is have the weight of cementboard and easier to cut and work with.


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## Mike Milam (Mar 3, 2017)

flyingron said:


> Cement board is not waterproof so you need a membrane as chandler's picture shows, or service coating designed for such.
> I'm actually not fond of cement board. I use Denshield. Denshield goes up like regular drywall and has a ceramic facing. You "tape" and seal the seams with silicone and it becomes the waterproof barrier. No membrane behind it is required (in fact, they recommend against it). The stuff is have the weight of cementboard and easier to cut and work with.


Where can Denshield be bought?


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## flyingron (Dec 15, 2020)

Mike Milam said:


> Where can Denshield be bought?


I got it at Home Depot I think. It's a Georgia Pacific product.


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## Mike Milam (Mar 3, 2017)

flyingron said:


> I got it at Home Depot I think. It's a Georgia Pacific product.


Okay thanks. I did a little searching locally and found Menards carries it. Home depot had it listed but didn't stock it here.


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## Pat the Rat (Aug 31, 2009)

Here is a photo of the edge of the tub and where it’s going to go. I patched the floor there with 3/4” plywood but didn’t screw it down yet till i figure out where the drain will be.


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## chandler48 (Jun 5, 2017)

That tub is fine for tiled walls. Just bring your cbu down to the top of the lip, not over it.


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## Pat the Rat (Aug 31, 2009)

chandler48 said:


> That tub is fine for tiled walls. Just bring your cbu down to the top of the lip, not over it.


Okay, coo. thanks.


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## Pat the Rat (Aug 31, 2009)

the closest store that sells Denshield looks like it's nearly an hour and a half away. looks like a good choice but that's pretty far to go for a few sheets of wallboard. 

What's the best way to shim up the tub to make it level? It has small metal feet under it. I won't be able to reach under it once it's in place. Is that a matter of pushing it out again, shimming where the feet are, then trying it out by pushing it back in? and does just the shear weight hold it in place? I didn't notice any type of clips on the wall or even a board along the wall when I removed it. It pretty much sat along the left wall with the foot of the tub against the exterior wall. It didn't have a shower. Here is what the bathroom looked like before. Pretty horrible right? 😆


















I'm also going to have to build the wall out about 8" for the fixtures on the left since the room is that much wider than the tub is long. I figured I'd do that after I get the tub in its final location. Then the commode will go in the middle of the left wall and a larger vanity where the last one was. That will free up the corner (that has the second mirror on the wall) for a 30" wide closet. Will be a much nicer space. I'll need to move a heater grate that's in the floor on the other side of the tub fixtures in the second photo. It will just go to the left of where it currently is.


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## chandler48 (Jun 5, 2017)

I don't like relying on the feet for a tub to rest on. Use 3 or 4 thinset in dollops about as big as a cow patty under the main part of the tub. Once it is in place, stand in the tub and walk around to squish out the thinset to where it gives equal support.


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## Pat the Rat (Aug 31, 2009)

even for a 300 lb. cast iron tub? didn't think I'd need to do that unless it was fiberglass.


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## chandler48 (Jun 5, 2017)

How much surface bears directly on the floor (tub surface)? The thinset helps in stability, but you may not need it if you have good bearing surface. It's always a good idea to put ledgers around the three sides positioned to where the lip of the tub rests on them, especially heavy tubs.


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## Missouri Bound (Apr 9, 2011)

Pat the Rat said:


> even for a 300 lb. cast iron tub? didn't think I'd need to do that unless it was fiberglass.


You don't *need *to do that but it will ensure that the tub is solid.


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## ront02769 (Nov 28, 2008)

Didn’t read it all.....but you are fine with cement board taped and red guard. Not worth the drive for denshield.


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## Pat the Rat (Aug 31, 2009)

chandler48 said:


> How much surface bears directly on the floor (tub surface)? The thinset helps in stability, but you may not need it if you have good bearing surface. It's always a good idea to put ledgers around the three sides positioned to where the lip of the tub rests on them, especially heavy tubs.


Looks like the 4 small feet under the bottom and the front apron touch the floor. Putting 2x4 supports under the 2 ends and the back might be a good idea.


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## Old Thomas (Nov 28, 2019)

I fir the wall out a quarter inch or so and run the cementitious backer down over about half of the tub flange, not tight to the tub.


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## chandler48 (Jun 5, 2017)

Old Thomas said:


> I fir the wall out a quarter inch or so


Doing that you take a chance of the remaining walls in the bathroom being 1/4" out. That is why I like bringing the cbu tight to the studs and stop it at the tub/shower flange. Your tile will jump the gap.


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## huesmann (Aug 18, 2011)

flyingron said:


> Cement board is not waterproof so you need a membrane as chandler's picture shows, or service coating designed for such.


Which is probably why the OP says he's using RedGard...


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## Pat the Rat (Aug 31, 2009)

just saw videos of people recommending red guard, so i thought it might be worth using.


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## jeffnc (Apr 1, 2011)

Typically the back side of a tub is supported with a ledger board. Only the front side needs to be shimmed.

Considering the fact that I actually thought you had used Kerdi board to start your project, yes, I think it was pretty ugly


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## Pat the Rat (Aug 31, 2009)

Yeah that’s how the bathroom looked when my son bought the house.

looks like i’m going to have to move the tub about 4” away from the wall that’s near the window due to a joist that runs where the tub drain will go. If i move it over 4” i can straddle the joist with a 2” notch it’s an 8” joist so that should be okay. The fixtures will be on the left. I thought I’d just build a wall/shelf on the right just to the height of the window sill to fill the space.


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## Pat the Rat (Aug 31, 2009)

jeffnc said:


> Typically the back side of a tub is supported with a ledger board. Only the front side needs to be shimmed.
> 
> Considering the fact that I actually thought you had used Kerdi board to start your project, yes, I think it was pretty ugly


So if you shim up the front, won't it stick up off the floor? Is the finished flooring supposed to cover this?


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## jeffnc (Apr 1, 2011)

Pat the Rat said:


> So if you shim up the front, won't it stick up off the floor? Is the finished flooring supposed to cover this?


Well supposedly your floor is level. But we all know that doesn't always happen. If you tile, it will cover up small gaps. Personally, I trim out the front of tubs anyway, with vinyl quarter round, so it's never an issue regardless of whether they're using tile or vinyl flooring. The trim also helps with imperfect tile installation.


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## Pat the Rat (Aug 31, 2009)

floor isn't very level as I found out today. probably a 1/2" off for the width of the tub. would be pretty difficult to get it level without lifting the front apron up that much. Not sure yet, but that's about what it looks like. don't want water dripping out on the floor.


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## jeffnc (Apr 1, 2011)

You definitely want a level tub, for multiple reasons. I've seen them off that much.


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## Zukeduke (Jun 17, 2018)

Pat the Rat said:


> just saw videos of people recommending red guard, so i thought it might be worth using.


Red guard works well, dries fast. Put 2x coats, maybe three around key areas. If you use it like this, you won't need baker board, just use green drywall. The Red guard is water proof.


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## jeffnc (Apr 1, 2011)

Zukeduke said:


> Red guard works well, dries fast. Put 2x coats, maybe three around key areas. If you use it like this, you won't need baker board, just use green drywall. The Red guard is water proof.


If it's waterproof, why do we need green drywall?


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## Zukeduke (Jun 17, 2018)

jeffnc said:


> If it's waterproof, why do we need green drywall?


Lol,.... good point. I use it for peace of mind.


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## Tdmtools (Jan 6, 2021)

Pat the Rat said:


> I'm working on renovating a 90 year old house. It had a cast iron/ceramic tub that never had a shower, but I want to add one with a tile surround above the tub. The room is gutted right now down to the studs. I'll be applying closed cell foam insulation to the exterior wall. I want to put the tub along the exterior wall, but I'm not sure if I should put greenboard up first or just put the tub against the bare studs. I know the greenboard or cement board should come down and overlap the rim, but I'd need a double layer of wallboard (whichever) to do that. I'd also heard not to put greenboard behind cement board. Basically I'm not sure how far away from the wall I should put the tub. Should it be a half inch away or up against the studs? I'm not going to put the tub there till I notch out the joist below the tub to fit the drain tube. It won't be much of a notch. It just sits maybe an inch below the floor. Just didn't want to make the notch any wider than I have to in order to fit.


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## Tdmtools (Jan 6, 2021)

Pat the Rat said:


> I'm working on renovating a 90 year old house. It had a cast iron/ceramic tub that never had a shower, but I want to add one with a tile surround above the tub. The room is gutted right now down to the studs. I'll be applying closed cell foam insulation to the exterior wall. I want to put the tub along the exterior wall, but I'm not sure if I should put greenboard up first or just put the tub against the bare studs. I know the greenboard or cement board should come down and overlap the rim, but I'd need a double layer of wallboard (whichever) to do that. I'd also heard not to put greenboard behind cement board. Basically I'm not sure how far away from the wall I should put the tub. Should it be a half inch away or up against the studs? I'm not going to put the tub there till I notch out the joist below the tub to fit the drain tube. It won't be much of a notch. It just sits maybe an inch below the floor. Just didn't want to make the notch any wider than I have to in order to fit.


Research Kerdi board and membrane , would sit nicely on the tub edge and membrane is great at controlling water. I used it in two showers in my house and really like it ,


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## Pat the Rat (Aug 31, 2009)

Yes i've read a lot of good things about Kerdi board. I will definitely suggest that to my son. Turns out now that we tried to clean the old cast iron/porcelain tub that there are scratches and stains that just don't seem to want to come out and the rust around the drain won't come out either. We'll probably ditch it and buy a porcelain coated steel one instead. it's going to be fund trying to take that old cast iron one apart with a sawzall or whatever they use. I'm thinking an angle grinder would work better, but I'll probably go thru a few blades on it. I don't think the old tub is worth trying to re-enamel. Would probably cost more than a new one would.


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## Tdmtools (Jan 6, 2021)

don't some people bust them up carefully , I've busted up the old sewer stack cast pipe to remove it


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## Tdmtools (Jan 6, 2021)

Pat the Rat said:


> Yes i've read a lot of good things about Kerdi board. I will definitely suggest that to my son. Turns out now that we tried to clean the old cast iron/porcelain tub that there are scratches and stains that just don't seem to want to come out and the rust around the drain won't come out either. We'll probably ditch it and buy a porcelain coated steel one instead. it's going to be fund trying to take that old cast iron one apart with a sawzall or whatever they use. I'm thinking an angle grinder would work better, but I'll probably go thru a few blades on it. I don't think the old tub is worth trying to re-enamel. Would probably cost more than a new one would.


I used kerdi in a new steam shower I just finished , its great stuff but is not cheap


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## jeffnc (Apr 1, 2011)

Kerdi membrane is cheaper than Kerdi board, and it can be installed over drywall or cement board. You are trading labor cost for material cost. If I were doing the work at my own house trying to save as much money as possible, and time were not a factor, I'd use Kerdi membrane and install it over drywall. If I'm working for a customer, the labor cost per hour is reduced when using Kerdi board, so I can justify the extra material cost. The final cost to my customer is about the same, but Kerdi board is much easier to use and I can get their job done faster. If I didn't have other jobs to get to, I suppose I'd use Kerdi membrane so I'd have more hours to work, lol.


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## jeffnc (Apr 1, 2011)

I'd start with a sledge hammer on the cast iron tub. It will take some work. It's also dangerous. Fully protect yourself from head to toe, including a full face shield.


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## Pat the Rat (Aug 31, 2009)

Just an update. We decided to get a new tub. Got an American Standard steel with ceramic coating. Nice tub. Much, much lighter too. I put a ledger board up for the wall side. I decided to jack up the joist in the basement to help with the out-of-level floor. Now it's only 1-1/8" off from the wall to the apron. lol. So jacking it up helped quite a bit, but it's still a good inch or so out of level. I'm thinking of using floor leveler, but how do I cover the floor leveler once the tub is installed? Do I put a piece of flexible trim there along the apron? the underlayment and LVP flooring won't cover it I'm sure. I was not going to level the whole bathroom since it would be a big difference between the bathroom floor and the hallway so I will only do under the tub where it's needed most.


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