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I'm going to be using the Kerdi shower pan and membrane to seal a steam shower. I understand the process of installing the membrane, deck and preparing the wall and ceiling substrate.

This question is specifically about the subfloor.

Do I need to apply a waterproofer such as Redgard to the subfloor ply before I apply the thinset to set the Kerdi shower pan?

The same answer, I presume, is applicable to a dry pack pan as well?

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up vote 3 down vote accepted

No I would not apply water proofer to the subfloor under the shower pan (unless we are going to be waterproofing the entire floor it might be easier to just continue it under the shower pan).

I think The Evil Greebo does bring up a good point, because this is a steam shower I would be even more concerned about mold developing in this humid environment. IMHO, if you will be tiling the floor I would put a water proof membrane under the entire floor and not worry about using pressure treated wood for the subfloor. If you use the DITRA and cover the seems that should be waterproof so there really should not be moisture getting to the subfloor.

Of course the (relatively) small increase in price for treated plywood may be worth the peace of mind (are be a backup if water makes it through the floor's water proofer.

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The entire shower will be done with Kerdi. – Matthew Sep 5 '12 at 15:02

In such a high humidity environment, I'd remove the existing sub-floor and put in pressure treated plywood in the entire room. Not out of concern for spilled water but because it's a steam shower, so your overall bathroom is going to become a playground for mold.


Based on the comments to this answer, updated answer:

Since you're waterproofing the entire room, its not essential. However, it's not a BAD idea to do it anyway - and I'd do the entire room not just the floor under the pan - because an ounce of water-proofer is worth several thousand dollars of ripped out bathroom due to rot...

Just in case your kerdi leaks, that is.

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But you did not actually answer his question. Are you implying you would use pressure treated wood in the subfloor and not redgard it? – auujay Sep 5 '12 at 14:53
I've never heard of doing this. In order to pull this off I would need to add joists that line up with the dimensions of the shower. If the shower pan is done properly, I shouldn't expect water to get into the subfloor.... otherwise I'd be in big trouble (even if it was treated). – Matthew Sep 5 '12 at 15:00
How do you expect to prevent steam from getting everywhere? Unless you waterproof the entire room floor, walls, and ceiling, steam induced moisture is going to penetrate SOMEWHERE to wood. – The Evil Greebo Sep 5 '12 at 15:16
@TheEvilGreebo of course.... The entire room will be wrapped in Kerdi; walls, floor, ceiling... – Matthew Sep 5 '12 at 15:21

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