Tongue-and-groove plywood is superior to regular plywood for floor decks because it creates a stronger, more rigid subfloor, and helps eliminate sagging and "bounce" at the seams between the joists. But it is also more difficult to install because the tight-fitting joints don't always go together easily. Here's a simple way to close up those joints: Lay a 4-foot-long 2x4 across the joists to protect the plywood edge. Stand on top of the 2x4, then gently rap it with a sledgehammer. If the tongue-and-groove joint doesn't close up, the plywood might be slightly warped. Have a helper stand on the seam to align the tongue with the groove, then rap the 2x4 with the sledge. Once the joint is tight, drive in a couple of screws or nails to hold it, then slide the 2x4 down along the edge of the plywood sheet and repeat the process.