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rolls831 01-24-2009 12:26 PM

Cementboard and Silicone?
I recently installed hardibacker board in a walk in shower. Leaving 1/8" gap between, then filled with waterproof silicone. I plan to use mesh and thinset over the seems also. Is this overkill and is this a good process? Thanks

angus242 01-24-2009 01:30 PM


Originally Posted by rolls831 (Post 218348)
Leaving 1/8" gap between, then filled with waterproof silicone.

Gap where?

rolls831 01-24-2009 06:28 PM

Hi Angus242. Thanks for the reply. The gaps are between the sheets of backerboard where they meet, also where the board meets the shower pan. This is a full walk in shower 33x60.

Tom Struble 01-24-2009 09:50 PM

silicone is not the best sealant for porous materials better off with a urethane

Termite 01-24-2009 10:23 PM

If it isn't too late I'd remove the silicone and properly fill/pack the joint with thinset and of course mesh tape the seam. Honestly, there should not be a gap at all.

angus242 01-24-2009 11:10 PM

The even plane seams should be addressed with alkali-resistant tape and thinset, no gaps. The gap between the tile and shower pan will be caulked.
You should waterproof before tiling.

rolls831 01-25-2009 01:15 PM

1 Attachment(s)
I'm including a photo. The gaps are 1/8" as you can see they are filled with 100% silicone. I will apply mesh then thinset. after drying and smoothing, I plan to coat with RedGuard. I really don"t want to remove the silicone unless an expert says this will lead to a disaster.

angus242 01-25-2009 03:25 PM

I don't understand why you chose to leave gaps filled with silicone.

ANSI standard A108.11 states to use alkali-resistant tape over the seams and then fill with thinset. This would be true for all seams except to the floor, shower pan or tub deck.

Hardibacker installation instructions say to follow ANSI A108.11. It also states to tape and fill seams as above.

TCNA method W244C-07 also says to tape and fill with thinset.

None of the above methods say to have vertical gaps.

Furthermore, thinset will not adhere to silicone.

You've decided to use your own installation method. I don't see disastrous results but you've strayed from tested building procedures. The "overkill" isn't taping and using thinset on the seams, it was the use of silicone in all gaps.

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