bouncing subfloor


Old 01-25-10, 09:46 AM
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bouncing subfloor

I replaced the dry rotten subfloor in my bathrom with 2x 1/2" plywood (screwed together now total of 1" subfloor) and put 1/4" hardie backer on top (with cement board screws).

Now I noticed that the floor bounces in one area when I walk over it and that`s the area when you get in or out of the shower.
What can I do to prevent this from happening? Use maybe 2" screws to screw both plywood layers and and hardi board together?
I plan on tiling the floor and don`t want to have cracked tiles after a while if I don`t fix that problem. It`s really not so much of a bouncing but I think it will be enough to damage the title and grout.

I put in 2x 1/2" plywood and then build a shower (3`x5`) on top of it which means I can`t get to the wood beams underneath anymore.

Last edited by maltschick; 01-25-10 at 10:13 AM.
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Old 01-25-10, 01:50 PM
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You can screw the plywood together all you want. If it's not secured to the joists, it will bounce forever. I would suggest a nail gun or 3 inch screws to reach the joists.
Old 01-25-10, 02:21 PM
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Also, the direction you ran the plywood can make a difference. It should go long direction of the plywood perpendicular to the joists. Then the second layer the same direction, but staggering the seams so they don't line up.

Unfortunately, even with proper installation, they don't make plywood the way they used to and two lauers will still be somewhat spongy. Did the joists have cross bracing? If you have access to below, you could add solid bracing, which would lock each joist to the two adjacent and provide some support to the bottom of the plywood.

Old 01-25-10, 03:34 PM
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The first layer of subflooring should have been at least 5/8" t&g plywood. I'm not sure where you got the idea to use 2 layers of 1/2" plywood, but I'd not install a tile floor on what you have under any circumstances without making some changes.

1/2" plywood is too bouncy for a subfloor. Additionally, with 1/2" subflooring, the screws you used to screw the plywood underlayment (2nd layer of plywood) will tend to strip out as the subfloor is too thin. Did you just use screws to install the second layer of plywood, or did you use a full spread glue as well? Did you screw the plywood sheets to each other, or just to the joists?

Here is what you should have. At a minimum the subfloor should be 5/8" t&g plywood glued and nailed/screwed to the joists. Leave proper gap for expansion. The subfloor should run perpendicular to the joists.

Your second layer of 1/2" plywood would be ok. For tile installations, the second layer of plywood should be screwed to the plywood subfloor only, not to the joists, to help isolate floor joist movement from the tile installation above. The screw pattern should be every 8" in the field and 6" around the perimeter of the plywood. This should also run perpendicular to the joists, and the seams should not line up with the subfloor below. Leave a 1/8" or so gap between the sheets to allow for expansion.

You don't say if you bedded the cement board in a fresh bed of thinset before you screwed it to the plywood subfloor. Every cement board manufacturer requires this. The thinset is there to fill any voids that exist between the plywood and cement board, and to fully support the cement board and limit any vertical (up and down) movement. Read the cement board manufacturers instructions as to proper screw pattern, spacing between the boards, mudding and taping, and gap around the perimeter of the room.
Old 01-27-10, 01:26 PM
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The only additional info I'd add to HeresJohnny is that if you're using marble or stone tiles for the floor, go with 3/4" on the second layer.

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