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I have a wall opening that is 43" wide by 51" tall. I want to close it with drywall on either side (front and back). The other side is inset into the wall such that there won't be any stud support backing the OSB except around the edges of the board. The size of the OSB used to cover will be 48" wide by 55.5" (to reach the studs encircling the opening - unless it should be taller?). What is the minimum thickness OSB I should use to cover the opening? Bonus if there's a place in IBC that describes this scenario.

Full context: This used to be a deeper opening/shelf but it stuck into the bathroom on the other side and I wanted to make the bathroom bigger while maintaining at least a little bit of a niche on the other side for a picture. The bathroom (interior, where camera is looking from) side of the wall will be in the shower so of course it will be hardiebacker for tile and not drywall.

View from behind (where OSB will be mounted by screwing into studs surrounding hole)

View of the niche with a red border for the size of the OSB

View from the right

View from the side

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    Are you contradicting yourself with the "drywall on either side (front and back)" versus the "it will be hardiebacker for tile and not drywall"? I'm having trouble understanding your exact intentions. And the niche is in the adjacent room's wall surface, not in the bathroom shower's surface?
    – popham
    Feb 21 at 6:37
  • There's going to be tile installed on one side of this OSB?
    – popham
    Feb 21 at 6:46
  • That corner appears so useless that I'd probably block it off and frame a wall against the wall with the opening to support your finish.
    – isherwood
    Feb 21 at 14:02

1 Answer 1

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Use 19/32" OSB.

There's an IBC minimum 5 psf live load spec. The IRC (and therefore the IBC) has an H/180 deflection limit for interior walls. The TCNA Handbook specifies that

Walls intended for ceramic tile and stone installations shall meet applicable building code requirements,

so the IRC's deflection spec is what you need to design for (for your span length to OSB thickness ratio, the strength spec just gets double checked).

Taking 43" as H in the IRC's deflection spec, your maximum deflection is 43"/180 = 0.24". Taking 50#/[(43")(51")(1sf/144si)] = 3.3 psf as a design load, the IBC's 5 psf seems to apply, but I would double the 3.3 psf to account for a concentrated loading at center span.

You'll find OSB's performance requirements under NIST PS1-19. Table 8 has deflections for various loading. 19/32" OSB has a 40" span rating under roof load, so the table's "Roof - 40" case provides the deflection for your 43" span, 0.167" at 35 psf load. Rescaling the 0.167" for your 2(3.3psf) = 6.6 psf load provides a deflection value of (0.167")(6.6psf)/(35psf) = 0.031", and that's well below the 0.24" prescription from the IRC.

Just don't screw up the OSB's strength axis. Everybody screws up the strength axis.

There exists a Manual for Engineered Wood Construction with design values (under Chapter 9) for computing strengths and deflections based on thicknesses besides the special roof case that I used earlier. If you're desperate to shave 1/8" off of that 19/32" thickness, then I can compute that really quick.

Note that the deflection values from NIST PS1-19 are based on the plywood spanning across at least 3 rafters. Since you've got yours sticking past the opening edge quite a ways, I assumed that you could achieve similar stiffness. You should stagger your fasteners to include two studs along one edge of the opening to achieve this stiffness.

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    FYI, 19/32 OSB is colloquially referred to as 5/8", which may be what you see on the shelf tag.
    – isherwood
    Feb 21 at 14:03
  • Thanks for the fantastic and detailed response...clearly I was looking in the wrong places so thank you for pointing me in the right direction. If the strength axis is along the long edge of the board (assuming a 4x8 board), are you saying to mount it vertically or horizontally? Vertical would be simplest since 48" won't span the 51" gap.
    – Aaron_H
    Feb 21 at 18:24
  • @Aaron_H, dang. I'm supposed to notice stuff like that 51" versus the 48". I like it spanning the inconvenient way because of the rotational fixity provided by the pair of studs at the left in your image. Losing that rotational fixity at the one end alone increases the deflection by about 240%. Increasing length from 43" to 51" increases the deflection by about another 200%. That transforms the 0.031" to about 0.15", but it's still within spec. Sounds like shaving off that 1/8" probably wouldn't work out.
    – popham
    Feb 21 at 19:20

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