Underlayment over 5/8" Plywood?


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Old 04-27-06, 12:15 PM
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Underlayment over 5/8" Plywood?

I've presently got a 5/8 plywood subfloor over 16" OC floor joists. My home was built in 1958 and they did not use tongue & groove plywood. I've securely screwed the subfloor to the joists to eliminate all squeaks but I've noticed some flexing at the plywood joints that run perpendicular to the floor joists. Approximately 1/8" flexing between the plywood panels in many spots, less to no flexing in others. I plan to install 3/4" x 3 1/4" solid prefinished oak perpendicular to the floor joists. Should I install a 1/4" plywood underlayment first or can the flooring be installed over the 5/8" plywood without concern? In other words, will the flex between the joists result in damage to the floor? If so, what thickness underlayment?
 
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Old 04-28-06, 04:20 PM
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I would prefer seeing another 1/2" plywood installed across the grain of the existing subflooring. But check the height of the other flooring in the house. By adding the 1/2" plus 3/4" of flooring a simple transition piece may not be feasible. If you have the space, install the 1/2" first. Others may chime in and say installing the 3/4 directly over the 5/8 will be OK, and it may, but any flexing you have now, won't go away.
 
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Old 04-28-06, 04:37 PM
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I'll chime in.

Because he is going to be laying that full 3/4 inch thick...t & g across the joists, instead of with the joists, I'd say he'll be just fine, over the 5/8, irregardless that he has those deflections between the joists. He won't get any deflections going cross-joist, with each board t & g'd...IMO.

A quality house will have 1 1/4 inches of flooring...3/4 + 1/2". And ontop of that, there might be just linoleum. In essence, the structural of the floor is 1 1/4". In his case he will be only 1/8 inch short of that.

I have seen some houses built where they use 3/4 t & g 4 X 8 sheets, and let it go at that. They may only put 3/16 inch lauan down on top of that for when they lay the linoleum.
 
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Old 04-28-06, 04:47 PM
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In that case, just make sure you lay your flooring across the floor joisting as DaVeBoy says to ensure proper support without deflection.
 
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Old 04-29-06, 04:01 AM
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Robanov:
The reason your 3/4" sub floor is not T & G is that it was not available then. The flexing you are experiencing is quite common in that age of plywood. You could really do it either way, either put your flooring on the existing sub floor or put down a 1/4" sheathing to level things out. Its really your choice. Personally I would prpbably put down a 1/4" sheet before the 3/4 flooring. Just my thoughts.
 
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Old 04-29-06, 09:56 AM
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If you have access to the underside, I would add 2X blocking between the joists where the plywood seams are located. Then the plywood edges could be screwed down to minimize the flex. Another thing to consider is widening the seam if there is no space between the plywood sheets. They should have been layed with 1/8" spacing, but if they're touching, you might get some squeaks.
With that done, if the floor height accepts it, I would lay 1/4" plywood perpendicular to the subfloor.
 
 

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