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Old 01-21-2019, 12:52 PM   #1
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Mortar recommendation for RedGard coated tile backer


Hi Guys,

I'm renovating my bath and installed Greenboard around the tub. Now about to mud/tape/seal joints and apply RedGard water membrane. Now my question is .. I'm getting different opinions what mortar I should use. Some say use unmodified because of Kerdi niche, some say modified.

I understand the reasoning behind both but since RedGard is the last layer over everything (actually 3 layers as I need it to be really waterproof) ... should I just follow RedGard recommendations?

What would you use to tile? Any product recommendations?
BTW I have porcelain 4x12 tiles.

Thanks for Your advice.
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Old 01-21-2019, 02:37 PM   #2
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Re: Mortar recommendation for RedGard coated tile backer


Is there a good reason why you chose to use greenboard in a wet area?
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Old 01-21-2019, 03:00 PM   #3
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Re: Mortar recommendation for RedGard coated tile backer


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Originally Posted by chandler48 View Post
Is there a good reason why you chose to use greenboard in a wet area?
I was looking for something manageable as far as cutting, weight, install etc. Normally CBU would be my first choice but I needed something easy/quick. Going full Schulter crossed my mind but Greenboard with RedGard is faster / easier / less expensive.

The old (removed) setup was tile over drywall and lasted previous owners for 40 years without even one leak ... If this with a proper barrier will last 20 I will be glad. By then my tiles will be ugly anyway :D
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Old 01-21-2019, 03:53 PM   #4
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Re: Mortar recommendation for RedGard coated tile backer


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Is there a good reason why you chose to use greenboard in a wet area?
It's a trick question - the answer is NO !!!
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Old 01-21-2019, 03:55 PM   #5
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Re: Mortar recommendation for RedGard coated tile backer


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Originally Posted by misuszatek View Post
I was looking for something manageable as far as cutting, weight, install etc. Normally CBU would be my first choice but I needed something easy/quick. Going full Schulter crossed my mind but Greenboard with RedGard is faster / easier / less expensive.

The old (removed) setup was tile over drywall and lasted previous owners for 40 years without even one leak ... If this with a proper barrier will last 20 I will be glad. By then my tiles will be ugly anyway :D
Tear it all out and do it right with CBU and loose the kerdi too, you don't need it
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Old 01-21-2019, 04:34 PM   #6
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Re: Mortar recommendation for RedGard coated tile backer


Let's see greenboard runs about $15 a sheet and you would need 3, so $45. Redgard will cost about $60 per gallon, and you may get it covered with a gallon, not sure, so now it's up to $105 your way.

CBU runs $14 per sheet and you would need 4, so that's $56. You don't need to Redgard cbu.

Nothing difficult about cutting cbu.

Miss one pin hole with your Redgard and it will tell on you in short order using greenboard.
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Old 01-21-2019, 06:37 PM   #7
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Re: Mortar recommendation for RedGard coated tile backer


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Originally Posted by chandler48 View Post
Let's see greenboard runs about $15 a sheet and you would need 3, so $45. Redgard will cost about $60 per gallon, and you may get it covered with a gallon, not sure, so now it's up to $105 your way.

CBU runs $14 per sheet and you would need 4, so that's $56. You don't need to Redgard cbu.

Nothing difficult about cutting cbu.

Miss one pin hole with your Redgard and it will tell on you in short order using greenboard.
Interesting, you guys don't use RedGard on CBU ? We do on all jobs
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Old 01-21-2019, 06:43 PM   #8
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Re: Mortar recommendation for RedGard coated tile backer


I use it on cubbies, and in corners, but let the thinset and tape do the job on the flat surfaces. I've only done one entire walk in shower from ceiling to floor, including the floor in RedGard, but the customer specifically requested it. I just don't think Redgard on this greenboard will have long lasting effects, especially if he misses a spot and water can eke through.
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Old 01-21-2019, 06:53 PM   #9
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Re: Mortar recommendation for RedGard coated tile backer


Definitely the green board is a bad idea - I don't think he'll take the advice and will stay with it and RedGard over it, big mistake but live and learn I guess right, it's his project.

We only use CBU with 2 coats of RedGard or we don't do the job.
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Old 01-21-2019, 07:53 PM   #10
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Re: Mortar recommendation for RedGard coated tile backer


Esp gypsum board that close, or touching the tub. That joint, and the corner, almost always cracks. I grout then cover with silicon caulk. Caulk keeps the cracked grout from falling out.
If you can, cut about 3/4 of the board away from the tub surface. Fill this with tile glue or thinset as you tile.


Modified means it has acrylic extra in the thinset. Not more special than that. Acrylic makes it more sticktuitive and bit more water resistant. I think modified is recommended for marble or such that could warp from straight water mixture as well as more sticky. Ask google, "why use modified thinset?" Depends on the tile you're using but you can make the similar thinset by using admix (name?) and water to mix your thinset. But this kind of thinset or grout is harder to clean away if allowed to dry too much. You can also use glue, if your tile is not that heavy. I used whichever over redguard. I don't think redguard is incompatible with any of the building adhesives.
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Old 01-22-2019, 06:27 AM   #11
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Re: Mortar recommendation for RedGard coated tile backer


Not a pro - just have done a 4 showers. (3 of them the same one over a 20 year period - so I have had to correct my mistakes and see the results. Green board is useless in a wet area.



I don't think Redgard is approved (by them) to adhere to Green board, because of the makeup of green board. I would check with them before proceeding. It they say its OK you may have a shot with 3 careful coats.



CB with Red guard (or similar) over it is the way to go for DIY.



If i was the OP - I would cut out the bottom and replace with CB. Even 3 feet would help a lot.
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Old 01-22-2019, 07:56 AM   #12
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Re: Mortar recommendation for RedGard coated tile backer


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I don't think he'll take the advice and will stay with it and RedGard over it...
You "don't think" based on what exactly? I'm here for advice so it would be counter-logical to work against it while seeking it at the same time

I picked a greenboard because I had plenty already delivered and came across this video when starting work on the bath. Seems easy.



When I will go with CBU I will definitely cover it with RedGard ... it puzzles me why some would use CBU without it if you go all the way with CBU already.

Anyway..BACK TO ORIGINAL QUESTION:

Assuming CBU is on the wall with coats of RedGard.... whats next? / which product you would recommend for tile install? (I have porcelain 4x12" with 2mm groutline / have leveling system)

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Old 01-22-2019, 03:43 PM   #13
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Re: Mortar recommendation for RedGard coated tile backer


I'm pretty sure I used Versabond modified thin-set from Home Depot. (The kind you mix) I was sure I saw that Redgard specified modified, but can't seem to find it in their specs.
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Old 01-22-2019, 04:12 PM   #14
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Re: Mortar recommendation for RedGard coated tile backer


The green board would be fine with just kerdi over it. Why not just do that since you already have the kerdi nitch? No need for redguard then. Trying to mix systems is not a good idea. Go with one or the other. kerdi is very easy to use, like wallpapering.
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Old 01-22-2019, 06:25 PM   #15
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Re: Mortar recommendation for RedGard coated tile backer


"You don't think I will based on what exactly?"

Based on the fact that you put up green board in the first place

If you're seeking advice why didn't you seek advice on using green board vs. CBU before you put it up ?
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