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We had a contractor come in for remediation work for black mold removal. After removing the infected pieces of subfloor, drywall, wood, and shower liner, we had him prep the upper walls of the tub/shower combo for tiling rather than purchasing and installing a new fiberglass liner. We also had him lay hardie board on the subfloor to retile the floor.

Here's my issue: everything I've read, including the Hardie manufacure site, says to use mortar under the Hardie board. He didn't do this and instead used liquid nails.

I understand why mortar should have been used (not necessarily for adhesive qualities but filling in gaps and making solid subfloor for tile). My question is, do we need to rip out the Hardie board he installed, or can we lay mortar on what he laid and lay another layer of Hardie board?

In other words, if left in place, are any the pockets that might exist under he first layer going to affect any subsequent layers. If we rip out the existing Hardie board and lay new Hardie board, do we need to set the screws in different spots than where he installed the screws, or can we use his pattern for reinstallation?

If we have to install screws in different locations, will this affect the stability of the joists/subfloor?

Pardon my ignorance, this is our first experience tiling on a non-concrete surface (second story guest bath, OSB subfloor) as the foundation of one story homes in the Phoenix area tend to be concrete slabs. Also, we would rather remedy the situation ourselves rather than have him come back to fix the issue.
 

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I would remove it---That fellow was not much of a tile guy---

Clean the floor as well as you can---then trowel down thinset and add 1/4"Durrock---Hardi is not a favorite of mine----more likely to give you trouble than Durrock or Wonder Board.

Before you go any further--let us know what the floor structure is made of---floor joist size--spacing and unsupported length (beam to foundation--)

And type of subfloor---
 

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Here are my thoughts... If he did a proper glue job (meaning he left very few voids (not just a X pattern) then I would say leave it. I think you're right in that the mortar is used to fill voids preventing the hardi board from moving, causing cracked tiles.

Any movement of that hardi board is not good so I would apply some force to it at different spots and see if you can flex it in any way. If you see any movement then rip it out. If you feel it's solid then I don't think you would have any issues.

If it's a small area (say 4x5) then just rip it out to be safe and use Durock with thinset then screw in new spots according to manufacturer. You don't need to screw through the Durock into the joists. As long as you have a sound bond between the plywood (I'm assuming) subfloor and the joists, then screws through the Durock into the plywood should suffice.

"My question is, do we need to rip out the Hardie board he installed, or can we lay mortar on what he laid and lay another layer of Hardie board?"

how would attaching anything over a weak foundation be a good idea?
 

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If it were mine, I'd be replacing your OSB subfloor with treated plywood, screwed down, not nailed, since code here does not permit OSB or chipboard subfloors in bathrooms, toilets or laundry rooms. Then lay your cement board tiling base according to manufacturer instructions.
 
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