Liquid membrane over cement board

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Ukrop

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Hi there!

I'm in preliminary stage of my bathroom reno - ripping old stuff out. And planning new stuff :)

It's my first bathroom project - I read ALOT online that's why have questions :)

I'm planning on putting cement boards on bathtub's alcove walls. From what I read I need to put membrane behind - it's Ok, I'll use 6mil plastic.

But what about sealing cement boards themselves? I read people use RedGard... Do I need it for tub walls?

thanks!
 

Jadnashua

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There are numerous ways to do this. Cement board (cbu) is not waterproof, but is not damaged by being wet. Because it can let some moisture wick through it, you have two choices: put a moisture barrier behind it to protect the structure, or, waterproof the front of it. On a vertical surface, very little moisture gets through it, and the moisture barrier, typically a sheet of plastic, works fine. You lap that moisture barrier over the tiling flange of the tub, then after the board is up, cut off the excess. This directs any liquid into the tub, rather than behind the wall.

either way typically works fine.

My preference is to use a totally different product. Something like KerdiBoard from www.schluter.com. It's a structural, tileable, waterproof foam-core panel. You screw it in place, seal the seams and penetrations, seal it to the tub lip, and tile it. Totally waterproof, light, easy to carry, install, and makes the entire tub surround totally waterproof rather than being just water resistant.

The liquid applied waterproofing can be used. Personally, I think it's a bit harder to install properly, and I don't like working with or carrying around cbu. You'd think painting or rolling it on would be easy, but to get it the proper thickness (not too thin, not too thick), is IMHO, harder to do that most people think. Mapei, Custom Building Products, and Laticrete all make liquid applied waterproofing materials, and each can work, if done right. FOllow their instructions if you go that way.

There are other ways to do this that work as well. Suggest you check out www.johnbridge.com .
 

Ukrop

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Thanks for reply!

Yes, I don't feel comfortable with liquid membrane...

My co-worker put Kerdi membrane in his bathroom and liked it. There is roll in Lowe's - it's 3'3" wide and 16'5" long. It means I need to put to horizontal strips? How hard is it to put such membrane? Do I need to use special glue or just regular mortar?

Thanks for the link.
 

Jadnashua

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Kerdi is installed sort of like wallpaper, but the glue is thinset. You can either butt the seams and use their thinner tape (Kerdiband), or overlap the sheet membrane. It does not matter how you put it up. As long as there's at least a 2" overlap by either the tape or the membrane itself, it's waterproof. Their preferred backer is plain drywall, which is cheaper, easier to install, and because it's behind a waterproof membrane, works quite well. CBU is one of several approved backing boards. If you go with KerdiBoard, you do not need or want any panels up on the wall...you use the board as the tileable surface...it just happens to also be waterproof. For someone new at it, it's easier to use smaller sections than larger ones and say wrapping it around corners. Schluter has loads of videos you can watch to get an idea of how to install the stuff.

Hydroban Sheet membrane is similar (they have a Hydroban liquid waterproofing too, it's not the same thing), and some feel that sheet is easier to use than Kerdi, but both work. They're quite similar. Those are the only two that have been tested and approved for use over plain drywall, but again, you can use other boards if you really want to. When used in a tub surround situation, you would want to buy a tube of KerdiFix, which is used to seal the membrane to the tub surface and make a waterproof connection between the walls and the tub.

Each of those two choices requires you cover the area at least up to the height of the showerhead (you can go to the ceiling, and even cover the ceiling, but it's not required on a ceiling unless you're doing a steam shower).
 
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