Bouncy floor – Plywood under to stiffen?

[Bill Drury]

My family room floor has too much bounce for my taste. House is 4 years old. Span is 16 feet with 2×10’s on 12 inch centers. Cross bracing every four feet. The flor is fully exposed from basement below so I can do just about anything that would work.

Someone suggested screwing and glueing plywood to the bottom of the joists to stiffen. I’m about to begin a proejct to finish the basement and am considering taking the time & expense of attaching 3/4″ plywood instead of strapping to at least the center section of the floor. The contractor we talked with said he’s never seen that done but “worth a try”.

Anyone ever done/seen this before? Do you think this will work? Or should I just go with additional blocking and hope that works?

[Tim]

I built a house 7 years ago that has the identical span, and 12″OC. I don’t like the feel of it to much either, but I don’t know about this plywood idea. It would seem to add weight, and not support the joists in the direction they need it. Even 3/4 ” plywood flexes. Blocking seems like a better plan, 2X10 10 1/2″ pieces screwed to the joists and to the subfloor. Do your B’ment plans call for any partition walls under your F/room? Perpendicular to the joists, a wall would do the trick.

[Dodgeman]

There is cross bracing only every 4 feet. As a first step, I would add cross bracing at every joist. This ties them all together so movement is less noticable. It can make a big difference. I would use 1×4’s cut at the correct angle for your 12 inch centers, attach the top with liberal amounts of PL400 and glue/screw the bottoms. Solid blocking would be good too, but is usually difficult to install because of the uneven spacing and warping of the joists. For your 16 foot span, I would put cross bracing at the center and 4 feet either side of center.

The plywood attached to the bottom would add stiffness, but the cross bracing should be done anyway before the plywood is installed. The plywood attached at the bottom would turn the joists into a form of I beams, this is the method used by some prefab home manufacturers, and it does work.

Good Luck!

Dodgeman

[Author]

Posts