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I'm looking into finishing my attic and turn it into a man cave. I had someone look at the joist and they said it was up to code for adding a floor. There is already 1/2-in. OSB subfloor in place, but the person said I need 3/4-in. OSB. With lumber prices the way they are, can I save some money and just add another layer of 1/2-in. OSB? Or should I just rip it up and put down 3/4-in.? I just plan on putting in carpet for the floor.

Thanks for the help.

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2 Answers 2

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You could add 3/4" sheets of plywood/OSB to the subfloor if you wanted.

The code requires a 3/4" subfloor as a minimum, but there is no maximum until you exceed the physical limits of the structure. There are, of course, practical limits, as 12" of subflooring is ridiculous, but adding a 1/2" layer to an existing 1/2" layer to have a 1" thick subfloor will not give you any issues at inspection time. You won't be flagged for overbuilding the structure.

The extra 1/4" will come in handy should you (or a future owner) ever decide to go with a more picky flooring like tile or floating laminate flooring.

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    As a subtle point on "You won't be flagged for overbuilding", overbuilding the floor does matter, because generally dead weight goes into the framing calculations. Here it is negligible, even if tiled, so likely not a factor. Adding 2in of concrete for sound insulation could require a new load calculation.
    – P2000
    Mar 18, 2022 at 18:06
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Yes, you can add another 1/2" layer. It was common in the US during the 70s and 80s to use one layer of 1/2" plywood and a layer of 5/8" particle board. Since particle board is crap, two layers of OSB is much better. In fact, I've replaced the particle board in such floors with OSB. The results were very nice.

Just be sure to fasten it down well to avoid noise, and stagger all joints. Consider a zigzag of construction adhesive between joists.

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  • I have the construction you mentioned (house built in 1978, has 1/2 plywood + 7/16 particle board. Particle board is awful, deals w moisture terribly, so it's all coming out. I'm leaving the plywood in there. I was wondering if you can add a layer of mass loaded vinyl on top of plywood, and install 1 layer of floating OSB on top, before adding some Quietboard(stiff) underlayment before installing LVP. Is there any issue with just floating the OSB?
    – Mark S
    Jul 4, 2023 at 22:11
  • I don't have experience with that. New questions shouldn't be presented in comments, though. Post a proper question and you'll get better help.
    – isherwood
    Jul 5, 2023 at 13:12

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