Couldn't you just cut the square edge plywood so that the edges butt up together on one joist? So you don't have to go through the trouble of additional blocking?

What about BETWEEN the joists? That's what we're talking about. Plywood is typically 4x8 sheets. Subflooring is laid across the joists so the top layer grain of the plywood is running at 90 degrees to the joists. Or put it this way. The plywood is installed with the long direction at right angles to the joists. Now whenever two panels come together, end to end, the 4' short side, they are to land on a joist. However, what about the sides between the joists? This is typically where T&G subflooring is locked together by the T&Gs, on the 8' long sides so that those spans between the joists are interlocked and do not flex. In a few homes that did not have this done properly, there was excessive flexing and noise created by not having T&Gs or blocking at these joints between the joists. Savvy?

It would be relatively easy to take up the affected floor and then toe-nail a brace between the joists where the seams land, no? Then all edges of the plywood would be screwed to a support.

That is called "adding blocking" and what I originally suggested if T&G plywood was not used.

It is easy, but there is no good reason to bother.

So you have to tear down the house and start over? Your argument is ridiculous, Gary. If sheet vinyl were going in, it would have some merit.

No, not really. In CA., residential buildings are inspected by the municipal building inspectors for code compliance. The inspections take place during all the phases of the building before the next phase can proceed. Therefore, if a subfloor was out of compliance, the builder would not be able to proceed with the building until the subfloor was brought into compliance. This is the process from grading through the completion. All buildings are inspected and signed off for code compliance PRIOR to the next phase. Non-compliance must be brought into compliance or the project is shut down.

I suppose I read this whole thing wrong. I thought it was some dude wanting to fix up his house.