How do I properly lay a tongue and groove plywood subfloor in a remodel?

Question

When laying T&G plywood subfloors, do all of the edges need to be supported around the perimeter of the room? If so, I may need to add some sort of blocking in my project and I'm not sure of the best approach since the walls are sitting over the joists and therefore there is nothing for the edges of the plywood to sit on.

Do I need to run board parallel to the joist with the wall on it so the entire edge of the plywood sits on top of it? I've heard that I should use "blocking" and also that it should be "continuous" but these are contradictory terms to me at the moment.

From what I've gathered, blocking would be boards inserted perpendicular to the joists and to the edge of the plywood that needs support and therefore not be "continuous". The other option would basically be sistering the joist with the wall on it which could prove difficult since there are pipes and wires in the way.

Could someone please clarify what needs to be done?

Answer 1

You defiantly need to support the subfloor around the perimeter.

Let's say this is your floor without plywood.

You want to put plywood down, but the edge of the sheet has nothing under it for support.

If you don't support the plywood, you'll end up with a "soft" spot around the perimeter. Stepping, or adding weight to this area will cause the plywood to flex. This flexing will cause the area to feel spongy, and could damage the flooring (depending on the type of flooring). To solve this problem, you have a couple options.

Blocking.

Installing blocking perpendicular to the joists will support the edge of the sheet.

The size and spacing of the blocking will be determined by local codes, so consult your local government for these requirements.

Additional Joist

Another option, is to install an additional joist to support the edge of the sheet.

This extra joist may not have to span the entire length, and may be able to be supported by blocking between the existing joists. Again, check with local building codes to determine what is required.

Answer 2

Continuous blocking means that there is a block between every joist, lined up with each other. It does not mean that the blocking is one piece of wood. It does not sound like what you need from the problem you are describing.

In general, no you don't need support all the edges of each panel - the edges interlock and can/should be glued - but if you are talking about the edge of the room where there's nothing to interlock with, yes, you have to support that one, not leave it hanging off the last joist.

If running an extra joist is impractical due to pipes and wires, run blocking (just in that bay) at a spacing less than or equal to the floor span rating of the plywood.