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Old 06-29-2020, 10:01 PM   #1
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Backer board in bathroom


Hi, novice here, I’m wondering what folks use behind backer board to ensure it sits flush once screwed in given that it needs to sit over the tiling flange of the bathtub, which is about 1/4” e
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Old 06-30-2020, 12:26 AM   #2
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Re: Backer board in bathroom


Backer board typically sits right on top of the flange so that shims are not necessary. If for some reason you want to drop it over the flange, you have to shim the entire wall out by the thickness of the flange, whatever that may be. 1/4" plywood... or what have you. If you just shim the bottom your wall is no longer plumb which really pisses off the tile layer.
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Old 06-30-2020, 04:48 AM   #3
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Re: Backer board in bathroom


Agree. Sit the cbu on top of the flange.. Your tile will jump the gap with no problem and it will prevent wicking.
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Old 07-01-2020, 06:13 AM   #4
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Re: Backer board in bathroom


Larry, on that illustration how far up does the vapor barrier go? I've seen where the VB is skipped and Redguard or similar painted onto the cement board. The gap to the tub is covered with mesh tape and cement before applying the paintable membrane all the way down to the tub.

Getting ready to install new tub and tile in coming weeks.
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Old 07-01-2020, 09:53 AM   #5
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Re: Backer board in bathroom


I think it depends what you're doing at the top. The tile is supposed to go up to at least 6' above the bottom of the tub/shower, so I would think the VB would go up to there. But if you're tiling all the way to the ceiling, I think it's easier to just go all the way to the top plate and staple there.

As far as Redgard or similar, I would think the same would apply up top, except I don't think it's necessarily beneficial to "paint" above 6' (but if you've got the extra Redgard you might as well).
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Old 07-01-2020, 10:00 AM   #6
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Re: Backer board in bathroom


I'm tiling to the ceiling, and plan on painting the Redguard on the entire surface just as extra precaution.

Does the double vapor barrier of plastic and Redgard pose a problem? If water somehow gets trapped between them it has nowhere to go or to dry to.
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Old 07-01-2020, 10:30 AM   #7
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Re: Backer board in bathroom


If you plan to use Redguard, then no, you do not want a membrane behind the backer board. The whole point of liquid membranes is to keep the moisture in front and to let the back of the backer board dry out if it should ever get wet.

So if you plan to use Redguard and you hang your board like Larry shows in his illustration, that poly that flashes the flange would just be a thin strip... maybe 4-6" wide. Some guys flash their tub flanges with window flashing tape since it sticks.

If you shim out the studs and let the backer board extend over the flange, that is fine but it should NEVER sit on the tub. Water would then soak into the bottom of the backer board. That's why most people just sit it on top... to keep it up out of the wet zone.

Or you could do some combination. Tape the flange, then shim the wall, then let the backer board extend over the flange slightly. But in all cases you want the cement board to be plumb... not kicked in which would cause the tile to taper at the corners.
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Old 07-01-2020, 11:03 AM   #8
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Re: Backer board in bathroom


No, I'm not going to shim out the wall - I'm aware of the issues cause by allowing the backer to overlap the flange.

I like the thin strip of flashing/plastic idea - I was thinking of that. Thanks!
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Old 07-01-2020, 11:47 AM   #9
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Re: Backer board in bathroom


Quote:
Originally Posted by chandler48 View Post
Agree. Sit the cbu on top of the flange.. Your tile will jump the gap with no problem and it will prevent wicking.
I'm just a DYI'r, but this is how I have done the past remodels on my homes.
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Old Yesterday, 05:42 PM   #10
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Re: Backer board in bathroom


Been reading that redgard is a no-no if you're tiling.
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Old Yesterday, 06:05 PM   #11
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Re: Backer board in bathroom


@flummoxed , where did you read that? Redgard is a fine membrane.....just expensive at $65 a gallon somedays.
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Old Yesterday, 06:58 PM   #12
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Re: Backer board in bathroom


My mistake- read wrong, looks like it just states not to use pre-mixed mortar on redgard, that's all.
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Old Yesterday, 08:01 PM   #13
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Re: Backer board in bathroom


Right. You shouldn't use premixed anything in a wet area. Roll your own.
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