Acrylic Shower Pan and Cement Board Installation

**JesseBrud** · 08-10-2006, 08:55 AM

Hello,

I am finishing my basement -- in my bathroom, I'll be installing an acrylic shower pan with tiled walls in the shower stall. Once I get the pan set in the mortar bed and nailed (through the nailing flange) to the wall studs and everyting sets solidly - do I run my cement board down "over" the nailing flange on the shower pan? Or do I run it up to 1/8" of it and use silicone sealant in the gap? I've heard conflicting reports on this. Also - do I use some sort of moisture barrier between the studs and cement board (4 mil plastic sheeting, etc.)?

Thanks a lot.

**jadnashua** · 08-10-2006, 11:19 AM

put 6 mil plastic on the studs, run it down over the lip of the pan. Ideally, you shim the studs (or notch them) so the cbu can run down to within about 1/4" of the horizontal ledge in the pan. If you don't want to do that, go to the top of the flange. No need to silicon that junction since the plastic will protect the studs. Caulk the bottom of the tile to the pan instead of grouting and the corners. If you prefer not using grout, using an expansion trim like Rondec from www.schluter.com will work. That way, you could avoid caulk in the entire shower. Check out www.johnbridge.com for tiling questions. Note, the height of that flange and the size of the tile will make a difference. As long as more than 1/2 of the tile will be supported by the cbu, you don't have to run the cbu down, but if you are using small tiles, it becomes more important to run the cbu down so it has good support. You can use thinset under the tile on the flange to fill it in. Do NOT use mastic...use a thinset you mix up from the bagged powder.

**JesseBrud** · 08-10-2006, 11:36 AM

Jim - thanks a ton. That makes total sense.

**Calman** · 03-23-2009, 05:05 PM

Hey Guys!

I am sooooooooooooooooooo glad I found this thread, as it's identical to the query I was hoping to ask. We are in the middle of basement bathroom renos, where the looming prospect of installing our new acrylic shower pan (36"x32") is upon us. My main concern was the backing board and tile positioning over the lip (flange) of the shower pan.

I think the great response puts my mind at rest with our situation, but will keep you posted on our developments.

Kind Regards,
Cal

**Calman** · 03-23-2009, 07:21 PM

BTW, what's a cbu? (I'm guessing it's a term for backing board)

Cal

**jadnashua** · 03-23-2009, 08:16 PM

CBU is the generic name for cementious backer unit (aka cement board).

**Calman** · 03-29-2009, 02:15 PM

Thanks Guys! ... always wondered what that abbreviation meant.

I may post some images of our setup soon, as I will be purchasing this Easyboard, Thinset and tiles in the next couple of weeks, so I want to be absolutely sure that I am doing everything right, step by step.

Cheers!
Cal