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Old 03-26-2012, 09:03 PM   #1
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Basement Bathroom Help needed

Ok, hello all. I am in the middle of finishing my basement and I'm at the point where Im ready to hang cement board-durarock, hardie backer board etc. I have a plastic shower pan in place, plumbed and secured to the floor and studs.

Here is my question:
I've been reading all over online and some suggest a vapor barrier behind the cement board, I dont know why I wouldnt right? better safe than sorry? But my big question is the flange or lip on the top of the fiberglass shower pan. Do I run the board all the way down, leave it a 1/8-1/4 inch from the base, but over the flange? is it preferance or is it a science? I know if I leave it short, it will allow me to use tile adhesive later at the base. Just wanted someone elses opinion, Id rather do it right the first time!

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Old 03-27-2012, 08:57 AM   #2
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Cement board is nice because it is completely mold resistant. It doesn't contain any paper material that can get wet and be a mold spore's dream.

However, that being said, cement board is not waterproof.

A common misconception is that tile & grout is waterproof. This is not true. Certain tiles absorb less water than others, but in the end, water will leech through your tiles and hit your substrate.

If your substrate is cement board, you don't have to worry about mold, BUT, the cement board will also leech water which can and will make it to the wood studs behind. This is a recipe for disaster.

This is where the vapour barrier comes into play.

You can either have a vapour barrier installed behind the cement board (i.e. sandwiched between the studs and the cement board). What this does is prevent any water from reaching the studs. In this scenario, the cement board is still allowed to get wet, but it can't penetrate any further.

Another option is to use some sort of surface membrane. One popular option being a Kerdi membrane. This option brings the water proof to the front most layer (i.e. right behind the tile). In this scenario, the cement board will never see water as the Kerdi membrane will stop it first.

There is even option for Kerdi board, which is a foam board pre-wrapped in Kerdi which is lightweight and super easy to install. If you go with the Kerdi board option, you can forgo the cement board.

Me personally? I'd do something like Kerdi board. It's completely mold resistant and waterproof. You also don't have to start messing around with vapour barriers.

Now, as for the actual install, whether using cement board/vapour barrier, cement board/Kerdi or Kerdi board, the substrate (i.e. cement board or Kerdi board) does not sit over the flange. It comes down to meet the top of the flange but does not go over it. When you tile, the tile will cover the flange.

Hope this helps. Please feel free to ask more questions!

Here is a picture to describe the flange install:
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