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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.

enter image description here

SBT23434
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1 Answers1

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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.

popham
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