I have an OSB subfloor. It is tongue and groove, but for whatever reason, the floor is not perfectly level at the joints. When I put a 48" ruler across these joints, the ruler teeter totters. Any tips on how to get these level? I have a belt sander. Perhaps just running it along the seams is the answer? How much out of level can I tolerate? I've only exposed a few joints so far and only the "narrow end" of the 4 x 8 sheets. I don't know how much of the floor is like this. I plan to run the hardwood in the same direction the OSB is laid down.

Thank you

You should sand all the joints flat with your belt sander it does not take long and your floor will look better.