Cement board around shower base-should it sit on the flange or floating above it (<1/4" above it)

[Mini Me]

Most of the questions I am seeing are asking about the cement board sitting behind the flange. In my opinion that is inviting for trouble.

The other two major options are:

-sit on the flange like below and here are two sub options

a) sitting on the flange making contact

b) sitting above the flange no contact

-sitting in front of the flange like below (this is from the manual of my shower base , see the link below)

For completeness here are the specs (sizes) of my shower base, the manual says install the shower base in front of the flange but I am of the opinion (and my current situation is forcing me to do that) that it is better to install above the flange, sitting on it.

 

[wwhitney]

(a) is much better.

(b) is unacceptable, as the shower surround needs to be waterproof before any tile is installed, but (b) clearly has a gap between the cement board and the top of the flange.

BTW, cement board is not waterproof, so you need to either apply a waterproofing layer over the cement board, or a waterproofing layer underneath the cement board that laps over the flange. In which case I guess you could use detail (b) over the waterproofing layer (e.g. 30 lb felt, aquabar, etc).

Cheers, Wayne

 

[Sponsor]

 

[Mini Me]

Dont know what to say I have read opinions that a barrier is not needed as you want the cement board to be able to breath. I will have stone whool insulation behind the cement board

 

[wwhitney]

Those opinions are wrong. You don't want water to leave the envelope of the shower stall. If you have wood framing, you don't want it up against damp cement board. Cement board will absorb water and allow it to migrate through, as will grout. Tile will as well, but more slowly.

So your shower envelope should include an explicit moisture barrier. Either a sheet barrier applied to the framing behind the cement board (visqueen, aquabar, roofing felt), or (better) a barrier on top of the cement board, either fluid applied (RedGuard, etc) or sheet applied (Kerdi, etc). And that moisture barrier should have an intentional connection to the vertical flange on your tub/shower receptor: either just lapping, or better, a seal (e.g. Kerdi band).

Cheers, Wayne

 

[Mini Me]

yes that is the context, water proof with a redguard like product and no barrier on the other side
so we are on in agreement

 

[wwhitney]

OK, I mentioned that option initially when I said "a waterproofing layer over the cement board."

So you can just do (a) and not have an explicit connection between the waterproofing and the tub. That just relies on shingling to avoid water intrusion.

Or you could do (b) and put in an explicit waterproofing connection, like a strip of Kerdi band that is Kerdi-fixed to the tub and thinsetted to the cement board. Or if you are using liquid applied waterproofing and don't want to mix systems, manufacturers typically have a flexible mesh fabric that you can apply and then saturate with several layers of the liquid applied waterproofing.

Cheers, Wayne

 

[Mini Me]

re my question, this guy explains it very well ,

 

[Jadnashua]

The easiest way to handle this is to put some plastic on the wall beneath the cbu, and lap that over the tiling flange to direct any water that might get behind it into the tub/shower assembly. When using anything other than a very small tile (you want more than 1/2 of it supported on the cbu), the tile can overhang the tiling flange. If you want your cbu to go down lower, you'd have to shim out the walls enough so it doesn't bow out over that flange, or, if you're using something like Wediboard or Kerdiboard instead of cbu, it's fairly easy to cut a rabbit joint on the back of it so it fits. I suppose you could dado the stud so the flange sat flush, but that's a lot more work for not much gain. What you don't want is the bottom edge of the cbu sitting directly on the horizontal pan lip.

Personally, I'd probably use Kerdiboard or Wediboard on my walls rather than cbu and then a water management layer. They're easier to carry home, cut, and install than cbu and don't make carcinogenic dust when you cut them.

 

[Nebojsa]

Some more info.