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1/2” Sheathing as subfloor over T&G flooring

Wunderbar | Posted in Expert Exchange Q&A on

I’m renovating my second story on an old farmhouse. Currently there’s only 3/4” fir flooring installing over the 24 o/c joists without subfloor.

I’ve got a stack of 1/2”ply sheathing left over so was going to install that over the top of the t&g and then new hardwood 3/4” will go over that. I was going to screw down into the joists to stiffen up the floor. Do I need to worry about slip and not attach plywood to the joists or would it be better for the floor system since it’s 24 o/c joists with only 3” t&g flooring as subfloor. 


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  1. gusfhb | | #1

    OK, I have done it with CDX
    Under hardwood it is fine, but anything else the voids might be problematic.
    Allows you to run the hardwood any way you choose where it would have to be at right angle to the flooring if done right over.
    I think I would tighten up the existing floor first if only for squeaks

  2. Expert Member
    BILL WICHERS | | #2

    It used to be common to use diagonal 1x planks over the joists, then 3/4" plywood over the planks, making an overall subfloor assembly around 1.5" thick. That top plywood layer was nailed down. This made a very solid floor, but nowadays everyone just uses plywood or OSB alone. I don't see a problem with what you want to do, but I would secure that plywood layer down to the structure in the usual way. If you want a slip layer, put that between the plywood and the finished hardwood floor.


  3. Expert Member
    Michael Maines | | #3

    I'd have no no concerns whatsoever about what you propose to do. I would glue the plywood to the fir to reduce the chances of squeaks, and I would fasten the plywood with screws or ring-shank nails (preferably screws) to the joists, with at least 1 1/4" penetration. Squeaking occurs from vertical movement, not horizontal movement, so a slip sheet wouldn't do anything for you.

  4. Expert Member
    DCcontrarian | | #4

    On the second floor you have to think about increasing the floor thickness that much, it throws off the rise of the stairs. You'll need to raise each tread a bit to get it right again.

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