new subfloor


Old 12-08-10, 04:15 AM
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new subfloor

I am getting ready to instal 1/4" plywood over my existing 3/4" t+g floor in my kitchen. Then I am putting 1/4 backer then tile. My question is when I secure the 1/4 ply, what type of screws should I use? Backer screws? Drywall screws? As always, I appreciate your help,
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Old 12-08-10, 05:40 AM
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Usually 1/4" ply is stapled to the sub floor and used as an underlayment for vinyl. I don't know that it would provide any benefit under the cement board and tile. If you need a more solid sub floor I'd suggest using 1/2" plywood......... but I'm a painter not a carpenter
Old 12-12-10, 04:39 PM
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Good call Mark, 1/4" ply is not recommended anywhere in a tile installation sandwich.

Actually a plywood underlayment may not be necessary at all, although anything over 3/8" will add strength to the subfloor. A single 3/4" t&g properly installed meets specs as the subfloor when it's covered with a concrete backer board or membrane such as Ditra. Of course the more plywood the better so add as much as possible.

The subfloor system must meet L360 deflection criteria for both the joists and subfloor though. What is the on center spacing of the joists? What is the unsupported span of the joists? Do you know the species and grade of the joists? Let us know so we can verify if your floor can support tiles.

When it's time to install the backer board you must spread fresh thin set mortar under it and fasten with the proper fasteners. Drywall screws are not the right ones to use, you should use the screws made for the boards you chose or roofing nails. The boards get taped too.

Old 12-13-10, 05:06 AM
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I may be reading the OP wrong, but are they talking about covering 3/4" "hardwood" T&G?? They said "floor" not "subfloor". If it is only 3/4" subflooring I can understand further underlayment prior to cbu. I would worry about build up if an existing floor is there. Just adding fuel to the fire.
Old 12-13-10, 05:36 AM
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I was thinking T&G subfloor like you find in the older houses...... but if there's a subfloor, hardwood, plywood, cbu and tile - you might need a ladder to step up into that room

It would be nice to have it clarified

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