Hi,

I have a subfloor plywood over 16" joists OC, that measures exactly 9/16" width. I want to install 3/4" by 2.25" strip oak.
I believe that I need to add more plywood thickness to make the subfloor more sturdy, plus also based on info. here requiring at least 3/4" subfloor. I also noticed that the floor is not 100% level.

Can someone please provide me with details on what size plywood to add ? Or should I replace existing plywood, and put on a much thicker plywood piece ?

If I only need to add to existing plywood, how do I do it, in conjunction with also leveling the floor ? I've heard of adding 15# felt paper in low areas, and if its big enough to add shingles.

Any help would be greatly appreciated.

You subflooring is 5/8", unless the house was custom built and it did not follow national and local codes. It's possible you have 1/2" that had 3/4" particle board installed over it. If that is the case, you MUST add more plywood. IF the subflooring is plywood, is 5/8" thick and tongue and grooved on the long 8' sides, you do NOT need more plywood as 5/8" is the minimum acceptable by NOFMA. Go back and read this sticky. It will discuss subfloor requirements exactly.
particle-board-osb-subfloors-underlayment-types-t385.html



You could replace it but I would not. Replacement only if it's badly damaged or unusable. If your plywood is 5/8", you flatten it. If 1/2", then add 3/8" CDX plywood ( bare minimum ) 1/2" CDX plywood is prefered.



These topics have been discussed literally hundreds of times on this board over the last five years. I suggest you do a search and read the many responses. Thanks for the inquires.

Thanks, Gary. One more question.

The subfloor is all plywood.

If there are low spots, can I just fill them in with layers of 15#felt paper or should I use shingles if its very low ? I plan to use 15# felt paper below the hardwood strip(tongue & groove). So, I imagine myself adding a few layers of 15# felt paper in some specific areas where there is a low spot. Is this aproach Ok ?

thanks again for your help, and linked articles.

Yes, you can. Put a narrow strip in lowest part of the depression, followed by a wider strip, then another; so that you are building the entire low spot up so that it becomes flat when you apply the last, and largest, piece of the paper.