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Per instructions of my 1/4" plywood underlayment that will be used for Vinyl loose-lay flooring, it states to run it parallel with the existing 3/4" T&G OSB flooring. I always assumed that running it perpendicular to the existing OSB would be better for strength and for flat (seams don't line up near as easily). What is the reasoning?

Also, how important is it to fill and holes in the OSB before laying and stapling the 1/4" plywood underlayment?

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  • Why would 1/4" plywood underlayment be used on top of 3/4" OSB for loose-lay vinyl? Is loose-lay vinyl adhered at the edges and at the seams? If the instructions are to lay the plywood in the same direction as the OSB, are there instructions to stagger the plywood to avoid the seams lining up? If not, maybe the seams should line up everywhere so that the thin plywood is not stressed by overlapping the sheets of the much stronger 3/4" OSB . . .
    – Jim Stewart
    Commented Dec 8, 2017 at 12:29

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Most subfloor and underlayment sheet goods are manufactured with greater strength/stiffness on one axis. It may have longer wood strands in that direction, or it may have a greater number of plies with grain in that direction. For this reason it should be oriented perpendicular to framing members.

You should stagger all joints. This may mean ripping a half sheet to begin an area, and may even mean cutting that rip into a quarter sheet. You wouldn't need to hit right at a 50% lap, but shoot for at least 12" or so on the long edge. Butt joints should occur over framing.

You don't typically need to fill flaws in subfloor, but we had a standard practice of taking a drum sander to swollen OSB subfloor if it had been rained on several times during house framing. The edges swell more than the field, and that creates humps. Small gaps such as at T&G joints don't require filling.

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  • Nice answer. I didn’t know that about stiffness per axis. How does OP determine which axis is stiffer? Is it typically marked? Usually the short axis? Or do you just have to inspect the constructuon of the plys? Also, what’s a T&G joint?
    – Stanwood
    Commented Jun 1, 2018 at 12:02
  • The bulk of the wood used in each sheet, whether plies or strands, will run lengthwise. I have never encountered a strength axis running across the width of a sheet.
    – isherwood
    Commented Jun 1, 2018 at 13:10
  • "T&G" is tongue-and-groove. 1/2" OSB will often have arrows or print indicating the allowed span direction. Plywood typically has an odd number of plies, and the greater number will be in the lengthwise orientation.
    – isherwood
    Commented Jun 1, 2018 at 15:28

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