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Unread 12-28-2017, 07:02 PM   #1
terabitdan
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Join Date: Dec 2017
Posts: 3
Tile on built-up concrete

Hello everyone, long time reader of the forum looking for advice on how much plywood underpayment is needed for Ditra and porcelain tile.

We’re converting an attached garage into living space. The floor is built up from the concrete pad with:

Plastic vapor barrier
2” XPS Foamular 250 rated at 25 psi
5/8” OSB
2x4 stringers 16” on center, with 2x4 bracing every 4’
23/32 OSB placed parallel to stringers.

Each layer is directly connected to the one below, with masonry anchors from the plywood into the concrete below.

Originally planned on a second layer of 3/4” + 1/4” plywood to match floor height in the room next to it. Then Ditra duo and 3/8” porcelain tile.

After doing some reading on the strength of plywood or OSB placed parallel to joints made me wonder if 3/4” underlayment screwed into the subfloor will be strong enough for the tile. There is some slight bounce now.

1” plywood is hard to get and expensive, but appears to have more bending stiffness than 3/4” and 5/8” combined, correcting the incorrectly installed subfloor.

Which should I go with? From a height perspective, it’s slightly over an inch to the top of the family rooms underlayment.

Thanks,

Dan

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Unread 12-29-2017, 03:32 AM   #2
JerseyDIYguy
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Is there a way to lay plywood perpendicular to your stringers? You should get less bounce with ply laid at 90 degrees to stringers/joists.

Also, 1/4" plywood is a poor choice for a subfloor. It's not stable enough for tile.
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Unread 12-29-2017, 07:59 AM   #3
terabitdan
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The underlayment will be installed perpendicular to the stringers.

My underlayment options are to add Option 1: 1” ACX plywood, Option 2: 3/4” ACX with 1/4” for filler or Option 3: 3/4” with 5/8” Or ???

I can’t remove the existing subfloor.


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Unread 12-29-2017, 08:56 AM   #4
Redlands Okie
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As long as the 1/4 inch option had the 1/4 inch under the 3/4 inch you might be ok. But option 2 would be my last choice of the three.
Two layers of 5/8 would also work and perhaps less waste if you need thicker than the 1 inch
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Unread 12-29-2017, 04:09 PM   #5
terabitdan
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Thanks for your help. I decided to go with the 1” plywood. In the APA specification it is listed as a higher stiffness than the 3/4 + 5/8 combined and I hope will be less work than two layers.


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