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I'm going to install 3/4" solid red oak hardwood upstairs, but the subfloor is old, creaky, and needs to be replaced. The floor joists are spaced 24" OC (on center), and change direction in the middle of the room.

From what I have read, if I want to run wood flooring parallel to the joists, 24" OC is not sufficient, but what I'm not sure of is if I add additional bracing perpendicular to the existing joists, can I add the bracing at 24" OC, or should I add it 16" OC? In other words, will having a grid of 24" squares to support the OSB work, or should it be 24"x16" rectangles? Obviously 16" OC is 50% more expensive and time consuming.

I'm going to be laying down new, premium 3/4" OSB subfloor and then laying the oak flooring on top of that. I don't have the budget for plywood, and anything thicker than 3/4" subfloor will also not work.

I plan on laying the OSB with the 8' sides going top to bottom, and the finished floor will be laid running left to right. Direction of OSB is negotiable.

Floor joist drawing:

Floor Joists

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  • what material was the old subfloor
    – Traveler
    Feb 23, 2023 at 3:57
  • What size are the old joists? (Length, width, and height.) Feb 23, 2023 at 4:41
  • @AloysiusDefenestrate, the joists are 2x12, and the longest run (vertical in image) are 12'. Old subfloor was 3/4" OSB, installed with nails and minimal adhesive, probably rained on for a month when being built.
    – JPhi1618
    Feb 23, 2023 at 14:39
  • Why won't >3/4" work?
    – Huesmann
    Feb 24, 2023 at 13:48
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    Raising the floor could also bring my stairs out of code since the top step would be taller than all the others.
    – JPhi1618
    Feb 24, 2023 at 16:48

2 Answers 2

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23/32 (almost 3/4) T&G OSB is rated for subfloor under hardwood, if supported by at most 24in spaced joists.

The recommendation is to upgrade to 7/8, which you say is not possible in your situation.

The top part of the room would probably be fine, because the planks will run perpendicular to the joists.

The bottom part of the room would also probably be fine with 3/4 if you plan on adding blocking: a 24x24 grid provides better support than 24OC joists.

If you are not using T&G it is advisable to provide additional support-width at the seams, for instance by blocking at the seam and by sistering the joist at the seam with an additional 2x4 nailed to it (flush at the top of course). Cut & place the OSB so that the seams are supported by 1.5in, and not "half-on".

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    I will be using T&G OSB - glued and screwed. Thanks for the links and information.
    – JPhi1618
    Feb 23, 2023 at 14:41
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assuming 24" between joists is fine in the top part, 24" between blocking should be fine in the bottom part.

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