Hello, Thanks for providing this forum.
I'm installing 5/16" solid Bruce Natural Reflections hardwood flooring in a 400 sq. ft. great room. The room had been carpeted about 25 years ago. The house was built in 1968 so the subfloor is 3/4 plank. During the carpet installation a layer of 5/16" plywood panels tightly butted together had been added as an underlayment over the planks in the great room.

My question is can I reuse this underlayment for the new hardwood floor? And if so, is it really necessary to cut 1/8" expansion joints between the tight panels, especially since this wasn't done during the carpet install, and the floor performed just fine. Cutting the joints seems simple enough, but I want to make sure I'm not introducing any new problems.

Thanks for your help.

Dana

Kerfing the plywood with a skil saw and carbide tipped blade will not hurt. Adjust the depth so it doesn't go too deep.
Check the existing floor for flatness with a six foot straight-edge. You need to have the floor flat to within the manufacture's tolerance to get a warrantee. Usually within a quarter inch in six feet will be OK.

Thanks for the advice. The floor is flat within mfgr, tolerance. Just want to make sure it stays that way