how to waterproof gap between cbu and shower pan?

[DIY DAD 2591]

Hi all, working on a bathroom gut and remodel. i have all but one more piece of cbu up and I'm almost ready to tape and mud the seams and apply redgard to the whole shower. my only question is what to do when it comes to the gap between the cement board and the shower pan nailing flange. in the pictures below the red line is the seam I'm talking about. removing the cement board and furring out the studs so the cement board can overlap the flange is not an option. listed below is the possible options i can see for this gap. what do all you fine folk recommend? anything i havnt listed?

option 1: silicone the gap and regard all the way down to it. will redgard adhere to silicone?

option 2: don't know if this is possible without removing all screws in cbu, but if possible, somehow pull the bottom of the cbu away from the studs and then put 6mill poly behind the cbu and drape into pan. then redgard all the cbu.

option 3: attach 6 inch wide waterproofing tape to the front of the cbu and drape it into the pan. redgard it all.

option 4: tape and mud this seam with thinset and alkali resistant tape like all my other cbu to cbu seams. redgard everything.

 

[DIY DAD 2591]

just came up with another idea, how about ditching the redgard and using a sheet membrane to cover the cbu that goes down over the gap and into the pan?

 

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[Cacher_Chick]

Hard to tell for sure in the photo, but is the CBU running down between the shower pan and the rough framing or is it cut flush at the seam?

The cbu should be flush with or slightly proud of the edge of the base. The CBU does not stop the passage of water through it. It is not an approved method, but you could overlap he seam with Kerdi-band and then Redguard the whole works. The approved method would be to Kerdi the CBU in it's entirety.

Once it is waterproof, the bottom row of tile should overlap the seam at least 3/4", or more depending on what size tile you are using.

 

[DIY DAD 2591]

Hard to tell for sure in the photo, but is the CBU running down between the shower pan and the rough framing or is it cut flush at the seam?

The cbu should be flush with or slightly proud of the edge of the base. The CBU does not stop the passage of water through it. It is not an approved method, but you could overlap he seam with Kerdi-band and then Redguard the whole works. The approved method would be to Kerdi the CBU in it's entirety.

Once it is waterproof, the bottom row of tile should overlap the seam at least 3/4", or more depending on what size tile you are using.

cbu does not go behind pan, will be using 10x14 ceramic tile. so the kerdi band would be attached to the bottom 3 inches of the cbu and then draped into the pan? do i need to attach band to pan or is that not necessary? does regard stick to kerdi band? thank you for your help

 

[Cacher_Chick]

The problem is that nothing really is meant to adhere to the composite shower pan. Redguard over Kerdi-band has not been tested, so I am unsure of how it would hold up over the long term. With a skim-coat of thinset on it, one would think that the bond would be good.

 

[Jadnashua]

I'd ditch the RedGard, and apply Kerdi to the whole thing. Schluter has some videos on how to make the pan/wall seam and keep it waterproof. Generally, they recommend you use a bead of KerdiFix to bond the membrane to the pan, and thinset on the rest of it on the cbu. Regardless of what you use, liberally wash the cbu down with a very wet sponge prior to either spreading thinset and Kerdi, or follow the instructions about doing a watered down primer coat of Redgard prior to applying your waterproofing...the stuff otherwise will just suck way too much out of either the thinset or the Redgard to produce a solid job.

If I were just starting this, I'd probably have used KerdiBoard verses cbu. It would have been easy to make a rabbit joint on the bottom and overlapped the tiling flange. The foam doesn't absorb moisture like cbu, and the whole thing is waterproof from the start.

Rinse all of the crud out of your pan and then cover it...the grit and stuff may already have scratched it up...you don't want to make it worse. Splattering thinset or RedGard on it will also be a pain, so a good cover with maybe a towel or blanket underneath would be good so you don't damage the surface.