I am remodeling a 2nd floor bathroom and was wondering how to contruct the shower pan. Specifically what needs to go underneath the shower pan liner? Can we use something like R-Matte sheathing (closed cell foam) between the plywood and the shower pan liner? This should provide a vapor barrier of sorts in case any moisture gets below the shower pan liner. I am worried though about the structural support of the sheathing because of all the weight that would be on top of it. I am also worried about not providing a slope to the shower pan as the sheathing can't be sloped. That may cause more problems than the vapor barrier (sheathing) would solve. Is a sloped shower pan important? Is it less important if the tile and grout are sealed with a sealant? I understand that there is a significant portion of water that actually makes to past the tile and grout down to the shower pan liner. True or flase? Any help is appreciated. Thanks.

I can't take credit for this but I can get you to a great step by step pro. How to Build a Shower Pan

While that is a fine website on how to build a shower pan, I was looking for answers to my specific questions.

1. Can I use R-Matte sheathing under the shower pan liner and are there structural concerns with using R-Matte sheathing?
2. Does it matter if the shower pan liner is not sloped to the drain?
3. Does sealing the tile/grout help with not having a sloped shower pan liner as it should help keep water from getting to the shower pan liner?

Thanks

I have to skip #1 since I have no first hand experience but can answer 2 & 3 since they are related. #2Yes you must slope your pan to the drain whole at 1/4" per foot, a flat pan will not work and your headaches will be endless. #3 Sealing grout helps grout look better but does not prevent water from reaching the liner, only a poor grout job or grout cracking will do that. Sealing does not cure or prevent a bad grout job. You must have a proper slope, this is just from a professional point of view, we would never do less.

Thanks ballengerb1.

So given the importance of a sloped shower pan liner, using only sheathing (R-Matte) between plywood subfloor and the shower pan liner can't be a great idea because there would be no slope to the shower pan. Understood.

Can you see any benefit to using sheathing? Since it is pliable, I would think it would not be a stable base for a floor, correct?

How about its vapor barrier benefits? What should you use between the plywood subfloor and the mortar layer that slopes the shower pan? Would roofing felt be a good vapor barrier?

Thanks

Isn't R Matte sheathing an insulation board? I don't think that is supposed to be used under shower pans. It is used for vertical exterior application, such as behind siding and brick. Look into Shulter Ditra membrane for the pan.

Yes, R-Matte sheathing is an insulation board and its primary application is interior and exterior vertical walls. I have looked at the Shulter products and it's an interesting system.

I have a person constructing my shower and he claims they use only a sheet of R-Matte between the shower pan and the plywood subfloor on construction jobs and it passes inspection. This is supposedly for vapor barrier purposes.

I'm arguing against using it because the shower pan does not end up sloped and because I can crush it between my fingers (not a stable material to be building a floor on).

I would think the shower pan liner itself would provide enough of a vapor barrier for the floor, hence no need for more protection under it.

If water vapor (or water for that matter) did happen to get under the shower pan liner then I'm not sure what you would do to protect the subfloor.

I would be questioning that too, slope to the drain should be accomplished with concrete or grout, then a layer of asphalt felt, then the waterproof membrane - just like in the link Ballenger gave you. I think the person that is building it for you is trying to cut corners. Even if it passes inspection, it is not the right way to do it. Did you pull a permit for this?