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Unread 04-12-2007, 01:11 PM   #1
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Join Date: Apr 2007
Location: Manitoba, Canada
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underlay requirements over insulated concrete? (plywood/OSB/foam board)

Hello all,

First of all a big thanks to all you pros out there for sharing your knowledge and experience with us DIY'ers, it's greatly appreciated and saves us much $$, time, and aggravation!

I'm in the process of building/tiling a bathroom in my basement and I need to know if I should put down metal mesh on the plywood subfloor for the additional stiffness and/or adhesion.

What I have in place right now over my concrete floor is:
-1" rigid XPS type 3 foam board
-then 5/8" T+G OSB secured with 2-3/4" tapcon screws
-then 5/8" ext plywood over the OSB screwed in place with 1-1/4 wood
screws (not drywall). Besides needing some SLC in a few areas, the floor
seems solid.

The concrete is fairly level, with no water problems.

I am installing the True Comfort (Canadian Home Depot) wire heating system which will need to be embedded in mortar.

So I'm wondering do I still need to put down the mesh before the wires and ceramic or can I go ahead and install directly over a burned-in plywood?

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Unread 04-12-2007, 04:44 PM   #2
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Hi Doug, Welcome aboard.

Not familiar with the system, but it sounds like your floor is strong enough for just about anything.

No mesh (lath). I would go with a membrane such as Schluter Ditra or Noble Company Nobleseal (not sure if it's distributed in Canada).


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Unread 04-12-2007, 04:48 PM   #3
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Hi Doug, you're not related to my friend Douglas are you?

I'm not familiar with that system but I would use the SLC first. If you put SLC on top of the ply you'll need metal or plastic mesh raising the height by about 3/8"

Maybe others are familiar with it.
Jim Buckley

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Unread 04-12-2007, 05:00 PM   #4
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Location: Nashua, NH
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SLC manufacturers require lath on a wood subfloor. Check the specs about minimum thickness above the highest point. With the electric heating wires, you might want to consider the plastic lath verses metal. I think the metal will be okay only if you don't walk on the stuff before you get it covered with the slc. Metal lath has sharp edges, and you could damage the wires if you walked on them. Safer to use the plastic lath. The combination of the lath and wires will mean you should plan on making sure you cover the wires at the high point deep enough. Don't remember the thickness above the heating elements that is recommended - you might want to check the specs on the stuff you are planning to use.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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