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I have shed floor that appears to be a sheet of 3/4 inch pressure treated plywood nailed to the joists. While the floor is "sturdy" in that it's holding up fine and shows no sign of rot, it feels extremely soft.

My steps feel springy and even my 350lb table saw can rock back and forth on the floor. There's just a lack of stiffness to the floor and things move and shake around as I walk on it.

Would adding another sheet of 3/4 inch plywood help make the floor more stiff?

Unfortunately I really don't have the time or resources to jack up the shed and potentially sister the joists, which would probably be the better solution.

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    Nailing another sheet would help. But the problem may be joist spacing. Are you able to deduce what that is? Commented Nov 19, 2023 at 3:35
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    I would laminate a thin sheet of mdf to a thin sheet of osb with construction adhesive and let it dry, then layer that on the floor. The locked layers will provide more resistance against sag than a thicker unibody.
    – dandavis
    Commented Nov 19, 2023 at 4:03
  • @RohitGupta Thanks, they're about 16 inches apart, however I don't know what size the joists are.
    – SBT23434
    Commented Nov 19, 2023 at 4:41
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    Are the joists actually soft, or is it plywood flexing to make it seem soft? Plywood sheathing has a "strength axis," where the short ends of the sheets should be fully supported by joists. Based on your picture it doesn't look correct. The joists run from the door's wall across to the opposite wall, no?
    – popham
    Commented Nov 19, 2023 at 4:46
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    Decent chance of it. Run it the other direction if you do, and use plenty of screws rather than nails so you can get it down really tight.
    – KMJ
    Commented Nov 19, 2023 at 4:54

1 Answer 1

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Normal plywood has the wood fibers in its outer plies oriented along the 8 ft length of the plywood. There exists specially marked plywood that defies this convention, but from your photo it's clear that the outer wood fibers are oriented from the door's wall to the opposite wall. This is the "strength axis," where it should be pointed in the perpendicular direction. That's assuming that the joists run across the narrower dimension from the door's wall to the opposite wall.

Incorrectly oriented plywood has only 21% of the stiffness of correctly oriented plywood. 3/4" plywood with a correctly oriented "strength axis" has a "flexural rigidity" of 440,000#-in²/ft, where stiffness is proportional to this "flexural rigidity." For incorrectly oriented plywood it's 91,500#-in²/ft. See page 52 from the AWC's Manual of Engineered Wood Construction, where "flexural rigidity" is the EI from Table M9.2-1.

That's the cause of your squishy floor. An additional layer of plywood will improve its stiffness by 500%, but only if it's correctly oriented. Get the strength axis correct or else you'll be wasting 80% of the next layer's stiffness.

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    Thank you popham, really appreciate you going into detail regarding the orientation. Answered above but your observation regarding the joist orientation is correct, it goes from the door to the opposite wall. Would regular 3/4" plywood suffice or would it be better to use tongue and groove? And should I attempt to stagger the seams as I lay this new layer perpendicular to the original floor?
    – SBT23434
    Commented Nov 19, 2023 at 5:15
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    @SBT23434, tongue and groove would be better, but I would not spend a fortune on it. You've got that underlying layer to stiffen the edges. You could use fasteners and/or a spash of wood glue along the seams to achieve similar stiffness to T&G. On staggering, I don't think it's necessary. If the plywood was doing more than carrying weight, then there's value in spreading around sheet edge fasteners and field fasteners so that the edge fasteners aren't all concentrated on single joists. With tongue and groove, maybe staggering gets better floor flatness? That could be a reason.
    – popham
    Commented Nov 19, 2023 at 6:23
  • Sorry one more question, when orienting the new layer should I aim to have the ends supported by joists, or would it be ok to have the seams be placed anywhere since there's a layer of plywood underneath them already?
    – SBT23434
    Commented Nov 19, 2023 at 12:36
  • Agree, obv w/ @popham, but here’s an additional thought: if you’re skinning the floor, it wouldn’t be a lot of additional work to cut the existing floor between the middle joist bay and add additional wood to those two joists. The new sistered joists would support the floor you just cut out, so not a lot of work there. Commented Nov 19, 2023 at 14:47
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    @SBT23434, comparing deflection of a "clamped-pinned" beam (under uniform loading) against two cantilevers meeting at midspan (under uniform loading), the two cantilevers have 35% of the stiffness. That would take you up to 56% of a 3/4" plywood's theoretical stiffness in spans where ends meet. That's worst case, though. Get the splice close to supports and the stiffness of the lower layer right beside supports will stiffen it nicely. Once you make one cut at the start, the joist layout will eliminate further cuts. If you miss by an inch and can't fasten one end to a joist, that would be ok.
    – popham
    Commented Nov 19, 2023 at 18:00

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