Subfloor plywood (t&g or no?)

This is a builder's forum discussion about a homeowner's own custom home project, not a broader trend or program that applies to houses in general. A homeowner framing a second floor with exposed wood decking (rough-sawn boards visible from below, rather than a finished ceiling) asked whether the plywood layer on top needs to be tongue-and-groove (t&g, meaning boards with interlocking edges) to stop the floor from feeling bouncy, given wider-than-usual spacing between the floor joists.

The advice that came back: tongue-and-groove plywood won't add meaningful stiffness once it sits on solid wood decking. What helps more is running the plywood sheets perpendicular to the boards below and gluing them down fully, which creates a much stiffer combined layer. Responders also raised practical concerns specific to freshly milled wood decking, like shrinkage as it dries, gaps opening between boards, cupping, and squeaking, and suggested details such as nailing boards near their center to allow for shrinkage, orienting any cupping downward, and possibly leaving small gaps between boards on purpose so shrinkage isn't noticeable later. One suggestion was to paint the underside of the plywood black above the board seams so any gaps that appear are less visible from below.

For most homeowners, this only matters if you're building or renovating with exposed wood floor decking as a ceiling finish. It doesn't apply to typical subfloor replacement or standard energy-upgrade work.