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Unread 09-07-2015, 08:21 PM   #1
Bamer101
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Flood Test with CBU Installed

Hey all,

Super noob here trying to redo my master 5x3 shower. I already installed the hardie backer and fiberglass taped/thinset the joints. After removing the cardboard that I used to protect the hot mop I discovered an indention in the hot mop the size of a golf ball, and it looks like some of the tar has been removed. It doesn't appear that the tar has been completely punctured, but I want to flood test again before I have someone float the pan.

1) Can I just go ahead and flood test below the curb even if my hardie backer will be well below this point?
2) Will the hardie backer withstand 24 hours of being submerged in water during the flood test?
3) What if I just go ahead and use RedGard over the entire surface of the hardie backer and hot mop. Will that be sufficient and remove the need to flood test the hot mop again?

Thank you all so much in advance!! Please let me know if you need more pictures or information.
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Unread 09-07-2015, 10:46 PM   #2
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Welcome, Byron.

1. You can do that.

2. The Hardibacker will survive the test quite handily. The problem you have is that the manufacturer of the Hardibacker does not permit it to be embedded into the final mud bed of a traditional shower pan construction, which is what you'll be doing. The tile industry, as represented by the TCNA also does not recommend that application.

Up to you whether you continue with what you've got.

3. Probably not, unless you also remove the top flange from your drain and install the RedGard and the required reinforcing material in there. But then you have the problem of having installed RedGard over a material not recommended by the manufacturer. I wouldn't do that.

My opinion; worth price charged.
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Unread 09-08-2015, 09:13 AM   #3
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Thanks so much for the quick response CX! May I ask why it is not recommended to have the hardie backer embedded in the mud pan? I was under the impression that this would reinforce the hardie backer installment since I didn't drill below 8" on the hot mop.
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Unread 09-08-2015, 09:36 AM   #4
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You'll hafta axe James Hardie. It's his product and he apparently thinks there is a problem with that installation method. Having tested a number of backer boards for water absorption and penetration, I don't see the problem, but it ain't my backer board, eh?

And the installation method published in the TCNA Handbook for showers is different for CBUs and for Fiber/Cement boards. Hardibacker is a Fiber/Cement board.
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Unread 09-08-2015, 12:13 PM   #5
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Gotcha, I guess the theory is that if water gets to the pan then it will go up the hardie backer rather than travel down to the hot mop. Path of least resistance?

1) So I guess I should take the first level of hardie backer off, then re-flood test, have someone float the pan, then reapply the hardie backer and caulk the joint that connects the hardie backer to the pan, and then redgard the entire shower?

2) Alternatively, could I just have the pan floated with the hardie backer embedded and then just redgard everything?

3) Assuming I restart and remove the first level of hardie backer - do I leave 1/4" between the board and the pan, and then caulk? Is that all I have to do?

I really appreciate all of the help! This has been a long and arduous process. A good learning experience.
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Unread 09-08-2015, 12:49 PM   #6
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Nope, not it at all, Byron. Some true CBUs wick water much more than does Hardibacker. It's expected if there is water to be wicked. If your waterproof membrane is properly sloped, there will be no standing water at the perimeter of your shower to be wicked by any wallboard. There will still be a little wicking and there will even be some moisture gravity feeding down the wallboard itself, but it's not expected to do any damage. And I've personally witnessed no such damage to Hardibacker, but, again, you'll do well to read and follow the manufacturer's instructions and the industry recommendations.

Trying to seal the top mud bed of a traditional pan to the membrane on the wallboard will not help the situation if a wicking situation exists. Trying to cover the entire shower floor of a traditionally constructed shower receptor with that waterproofing membrane will cause more problems than the one you think you'll be correcting.

Choose one waterproofing method for your shower, install it correctly, and don't worry about it functioning.

My opinion; worth price charged.
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Unread 09-08-2015, 01:03 PM   #7
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Okay thanks, CX! I will go ahead and remove the first layer of hardie backer and then have someone float the top mud bed. The mot mop was pre-sloped.

I'm beginning to second guess everything that I've done. Would you be so kind to answer a couple more questions?

1) I drilled holes to affix the hardie backer 8" from the hot mop floor. I had the hot mop done 12" up. Is this okay? I read that anything above the curb is okay.

2) Do I need to fill the holes in the hot mop where I drilled with some sort of sealant?

3) When I install the hardie backer to the top mud bed, I am supposed to leave a 1/4" gap and then silicone caulk the joint? Is there anything else I need to do?

4) I will only apply redgard on the walls and shampoo niche.
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Unread 09-08-2015, 01:26 PM   #8
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1. The requirement for you shower pan liner is that it rise up the walls a minimum of six inches or three inches above the top of your curb, whichever is higher. You must not have any penetrations in that membrane except in the top inch to hold it up.

2. I would.

3. You are supposed to leave a quarter-inch gap. I would not caulk the gap as that would seal the only place the wallboard has to drain.

That installation method also does not provide a good way to secure the bottom of the wallboard. I don't particularly like it, but that's the installation method prescribed for the material.

My opinion; worth price charged.
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Unread 09-08-2015, 02:04 PM   #9
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Thanks CX for all of the insight, I really appreciate it.

Is there a product you'd recommend to fill in any drill holes in the hot mop?

When you say, "That installation method also does not provide a good way to secure the bottom of the wallboard. I don't particularly like it, but that's the installation method prescribed for the material", are you implying that embedding the wall board into the top mud bed would be more secure? Is there another product that you would recommend that I can replace that section of hardie backer with that would allow me to embed it into the top mud bed? I understand if you don't want to answer that question.
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