# Sealing Hardi joints



## bob_cntrctr (Jan 30, 2008)

OK, so stopping doing what we always do (mortar) long enough to ask about mastic in another thread got me thinking about sealing Hardi backer joints in shower applications.

Hardi's own installation recommendation for Hardi500 is fibreglass tape and mortar. BUT - read carefully - read the FAQ - its not to SEAL the joint. Their answer to "why do this" is :

"The tape will help disperse any movement of the substrate, therefore decreasing the probability of popping or cracking tiles along the seams."

That's all.

I couldn't count the number of posts on this forum about how grout (which is nothing more than mortar) does not (usually typed in all caps) perfectly seal tiles and you need a good moisture resistant backer because water will eventually get back there through the grout bla bla bla. So, "sealing" the backer board joints with the same thing isn't likely to be any better. Yet if someone were to suggest actually sealing with, for example, silicone, it'd be heresy.

Hmmm.....


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## CO762 (Feb 22, 2010)

on vertical joints, which way will the water run?
on horizontal joints, I waterproof the whole thing....but these in wet apps have a slope anyway. If they don't, they're a countertop....which I waterproof anyway. 

But I've since switched to denshield and I don't worry about horizontals. I still waterproof joints tho. Amazing what a $39 bucket of redguard and some drywall mesh will do.....


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## HS345 (Jan 20, 2008)

Neither the grout, or the backer board will do much of anything to stop moisture penetration. You need to have a vapor barrier behind (roofing felt, poly), or on top (surface applied/bonded) of the substrate.

Taping the seams of Hardi has nothing whatsoever to do with waterproofing.


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## PrecisionFloors (Jan 24, 2006)

I seal mine with Kerdi :whistling

Oh there I go stumping for Schluter again...must be the bot in me :no:


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## angus242 (Oct 20, 2007)

PrecisionFloors said:


> Oh there I go stumping for Schluter again...must be the bot in me


And here I thought you were being a sheep :whistling


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## jarvis design (May 6, 2008)

I can tell you from experience that taping CBU, while not at all waterproof, is essential. We always taped all seams and corners, well, execpt for this one job one of my guys did about 15 years ago. He didn't bother taping the seam of the 2 sheets he had running vertically at the back of the tub.
The seam went from the tub to the ceiling. About 6 months later I get a call - the tiles are cracked - right down the center, all the way from the tub to the ceiling. I ripped off the tile, saw their was no mesh - problem found - taped, retiled and grouted.

Now of course, I still use CBU, but, I cover it all in the orange koolaid stuff!

Plus, I use Spectralock grout which has the same absorption rate as porcelain tile (less than 0.5%)


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