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 at all possible, install 2x6 blocking at all those plywood seams. Then you will be able to properly install your 3/4" wood flooring at 90 degrees to the floor joists. If you cannot get under, then you should lay another layer of 1/2" plywood at opposing direction to the existing, making sure not to line up on any seams.

Hello Gary,

Thank you for the advice. As it is, I get get to most of the seams from underneath. What would be the best way to install the 2x6 blocking? Should I glue it to the underside of the seam then screw down from the top of the subfloor?

The idea is to provide support to those seams. Cut your blocks tight so you almost need to bang them in (called a pressure fit). Get them up tight to the subfloor seams. Some subfloor adhesive on the tops of the blocking is good. Then using #16 common nails, nail through one joist into the ends of the blocking. You will need to toe-nail the opposite side because the previous block will be in the way. You can use 3&1/2" screws if you prefer and it may be easier than toenailing, especially between joist bays. I use a pneumatic framing nailer which makes short work of nailing but hand nailing or screws are good as well. Once you're done below installing the blocking, go back up top on the subfloor and screw those plywood seams into the blocks you just installed. Make sure to do both sides of the seams/joints. A couple of screws per side should be fine. Now, you're ready to lay that floor without worrying about sags and squeeks.

Thanks Gary,

I assume you mean to install the blocking with the narrow side of the 2x6 up against the seam. Any harm in installed the blocking wide side up when heating ducts are encountered?

Yes, install the blocking "narrow side up". It is much stronger that way. If there is an occasional obstruction (heating duct), then go ahead and put it in flat. That's better than no blocking at all. Good luck

Thanks Gary, much appreciated. You saved me a ton of work.