drywall compound over backerboard

kevin1727June 11, 2008

Should I be concerned about this? The shower hardibacker meets the drywall at seven feet high where the tile ends. The drywall guys have spread drywall compound at the joint and also went over the seam in the hardibacker at the six foot level. I believe there is thinset on the joint underneath. (The pipe sticking out will be the showerhead)

I am thinking the tile is not going to stick to this very well.

Is this okay, and if not, how to fix it?

Bill, I'm trying to be calm and not PANIC again.

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It's not a good thing, but being as high as it is, and especially on the showerhead wall, I don't think it'll be all that big an issue.

    Bookmark   June 11, 2008 at 8:31PM
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Are you sure it's not a setting compound? If it is, then no worries.

If not, then it's nothing that a little water and a scrub brush won't cure.


    Bookmark   June 11, 2008 at 11:49PM
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That could be bad news, I'd get rid of it - all of it. Any floating and feathering that needs to be done in a shower should be done with thinset.

You may be able to cover it with a liquid waterproofing but even that is arguable.

    Bookmark   June 12, 2008 at 12:19AM
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Mongo, I easily scratched a spot with my fingernail. I've done enough drywall patching that I am sure it is drywall compound. I forgot to say, the tile is 13"x13".

In the supplies pile there are bags of:
"Sheetrock lightweight setting-type joiunt compound."
"Mapaei Ultraflex 1 polymer-modified mortar."
"Mapaei Ultraflex 2 tile mortar with polymer."
"Jamo Multibond Polymer-modified thinset."

In the trash is:
"Sakrete Type S Mortar/Stucco Mix." They used this under the tub and for the shower pan.

    Bookmark   June 12, 2008 at 7:04AM
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I talked to the contractor this morning and he said the thinset being used is rated over all surfaces, but he will make sure the drywall guy doesn't use any more drywall compound on the hardibacker. He said he will make sure the tile has a good base. I'll see tonight if that means he is taking the compound off.

    Bookmark   June 12, 2008 at 8:37AM
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Kevin, I'll be a little more specific, and since you've done a bit of drywall work you probably already know this, but I've already gotten a couple of emails from readers asking what "setting compound" is.

So...there are essentially two families of drywall compound.
Premixed mud in the infamous drywall bucket that hardens by drying, and "setting compound" that hardens via a chemical reaction.

Premixed mud is water soluble. If that's what you have on your cement board, I'd take a sponge to wet and soften it and a bristle brush or taping knife and get rid of it.

If what you have is not water soluble (it doesn't soften when wet) then it's a setting compound. Setting compounds come as dry powder, you add water to them. Add water, mix, and the chemical hydration reaction starts. Some are designated by a certain amount of working time, like "20-minute mud". You have about 20 minutes to get it on the wall before it gets to tough to work with. It's also referred to as "hot mud", I always figured that's because the chemical reaction gives off heat after it's mixed with water.

So...if it's water soluble, get it off your cement board. If you tile over water soluble compound, the moisture in your thinset can be absorbed by the compound, which can soften the compound and destroy the bond between the tile/thinset and the wall.

If it's a rock-solid, non-water soluble setting compound, I'd say it's okay to tile over.


    Bookmark   June 12, 2008 at 6:44PM
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I figure what's on the wall is the "Sheetrock lightweight setting-type joint compound". So that should be no problem from what you are saying. It is not rock-hard but there are several hardened piles in the dumpster that indicate his mix hardened before he could use it all.
I have used dry mix compound but don't remember it being called "setting-type". It was just something that could be kept longer than pre-mix without drying out between patch jobs. I didn't pay attention to the details at that time.
I donÂt see that any of the stuff he is using is hardening by chemical reaction. I know cement "hardens" and does not "dry". When I grind up the hardened drywall stuff from the dumpster and mix it with water, it makes a sticky paste. When I grind up hard stucco mix and mix it with water it just makes sandy water and settles out.
DonÂt say it I need to get a life and stop playing in the dumpster. :)

    Bookmark   June 12, 2008 at 7:34PM
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Yup, cement "hardens" via a chemical reaction, just like hot mud.

If those look familiar, then yup, you have a setting compound. Easy Sand is somewhat soft, which makes it...easy to sand! You get some other setting compounds and they are impossible to sand. Good for a first coat as they give a strong drywall joint, but you'd want to use a lightweight compound on your second and third coats for easier sanding/finishing.

That softness is what allows you to scratch it with your fingernail.


    Bookmark   June 13, 2008 at 12:25AM
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Yes, there is the exact bag. Easy Sand 20.
Even if it's not my post, I learn something almost every day on this web site thanks to you, Bill, Bud and others. Thanks for taking the time to explain the details.

The drywall guy sanded much of the compound off the hardibacker areas anyway. My panic level is down to just my normal rapid heartbeat.

    Bookmark   June 13, 2008 at 5:40AM
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Do we need to mud and tape the seam at the top of a tub/shower surround where the cement board and sheetrock meet? If so do we use the fiberglass mesh that we used on the cement board or the joint compound that we used on the sheetrock? We plan to tile to the ceiling and we have 1/4 to 1/2 inch gap between the cement board and sheetrock.

    Bookmark   January 6, 2009 at 7:13PM
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I'm going to assume that the tile will cover that seam (atleast it SHOULD, anyway), in which case, tape it off with self sticking fiberglass mesh tape, and then coat it with thinset when you spread to set your tile. That way, you won't end up with a "speed bump". Same with all the other joints under the tile.

    Bookmark   January 6, 2009 at 9:50PM
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I have similar issue--not sure I understand advice. Contractor put up cement backer board for me and said he'd tape up seams and I'm ready for tile (I'm DIY)--but I realize 2 problems. 1--no one put up a vapor barrier behind it and 2--it looks like the guy put drywall compound in the seams, not thinset----I feel pretty confident b/c the guy in there patching and taping did the drywall in room and around shower at same time. So not only is there compound onto a lot of the cement board along top and outside edges of tub, but through the horizonal center seam and on edges of the niche. I figured my remedy would be Hydroban membrane over whole thing--which I already bought, but now I read of someone using similar product and it didn't adhere to the compound. Advice? Sand and remove it all and replace with thinset before hydroban? Or is it okay b/c it's the "setting" kind of drywall compound? If I just coat those areas with latex primer before hydroban will that solve my issues? ThanksS!!!

    Bookmark   March 11, 2014 at 1:22PM
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