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Unread 09-18-2020, 12:26 AM   #1
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Add 1.5" on post tension concrete to lay flush with a shwr pan

I have a concrete subfloor scraped and clean in a small bathroom.

the job is installing a tub to shower conversion kit and a new floor to match.

It is a prefab kit, designed to be used as a zero threshold shower. Which requires raising the bath floor.

The pan height and the new floor height will be flush.

The new bath floor covering will be a water proof, floating vinyl plank.
It is not glued but floating and edgelocked - installed with a rubberized, pre-attached underlayment, on each plank
The shower pan has the same floor covering, but is also glued down.

The height I must raise the new plank floor to be flush with the pan is 1 1/2"

The new plank floor will butt against the pan and they recommend a silicone seal at the butt joint.
The planks that are parallel to the pan butt joint. Will be sealed with silicone sealed 12" out into the bath area to the raised subfloor from the distance of the edge of the pan.

TWO conditions concern me:
  1. The shower is open without a door. therefore a water threat exists at the butt joint sealed pan to new floor seam. (A length of 32" of seam is open to the shower.)
  2. The underlayment, that would seem to last longest is 3 layers of 1/2" Durock over thinset concrete subfloor.

NOTE: THE POST TENSIONED CONCRETE SUBFLOOR IN BATHROOM HAS NO VISIBLE CRACK LINES. (I don't see any movement along the countertop seem on the opposite wall also)

I have a 3/4" Durock laid now, laid loose. It does not crunch or flex as the pieces are cut in small rectangles to fit, as it is a small bathroom.

Please tell me why I should not layer Durock?
Is it subfloor movement? Even if there are no visible cracks?
It is a post tensioned concrete slab. Built in the late 1970's.
It should all move together as long as there are no cracks, yes?

My thoughts are:
  1. To test a 1/2" thinset bond on top of the concrete subfloor to a 1/2" Durock sheet.
  2. Tape the joints and thinset on top of that sheet a 1/4" thinset.
  3. Lay down another 1/2" Durock. Repeat the process to total height I need.
    (I may need to adjust the Durock to 1/4" if the thinset is too much.)
  4. Once complete, laydown floating wood plank vinyl floor.

Of course Durock has never tested this particular type of install and will not approve it.

So.... I come to you, Oh Wise and Noble Masters. What say ye?

Obi-Wan your my only hope!!!
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Last edited by cadtek; 09-19-2020 at 10:50 AM. Reason: Adding additional thoughts and images.
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Unread 09-19-2020, 05:20 AM   #2
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Hi Steven,

As you've noted, cement board is not approved as an over-lay on concrete floors, and three layers sounds like a disaster in the making. You can accomplish the same result with deck mud for about 1/10th the price and effort. We've got a slew of old threads discussing its use over concrete floors. We will, however, be glad to shepherd you through the process.
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Unread 09-19-2020, 10:54 AM   #3
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Thank you, I did an edit on the original post.

Thanks John
Would you please explain why, layering Durock in a small bathroom is a bad idea, for the kind of floating floor I am installing??
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Unread 09-20-2020, 01:39 PM   #4
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If I mud deck the whole bathroom floor to meet the pan height.Will it shrink even just a little?

What about expansion joints, doors and wall edges?

Seems like I need some very thin forms against the wall and doorways that will meet carpet.

Then use deck mud to create a low rising hump under the carpet so I wont stub my toe on the floor height difference?
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Unread 09-20-2020, 07:15 PM   #5
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Steven, you might check out Wedi board. I've never done it but seems like you can thinset it down to a concrete slab and it may come in 1 1/2 thick sheets. Someone correct me if I'm wrong.

I would mud the floor, myself. The beauty of deck mud is you can taper the floor from 1 1/2 to 1 1/4 if you need to and meet up with whatever you need to in the doorway. I'd mud it right up to the walls.

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