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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?

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You defiantly need to support the subfloor around the perimeter.

Let's say this is your floor without plywood.

Joists

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

Subfloor

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.

Blocking

Blocking with Subfloor

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.

Extra Joist

Extra Joist with Subfloor

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.

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  • Awesome pictures and answer – Joe Phillips Jan 20 '14 at 18:27
  • Does blocking have to be the full size of the joist when it's simply used to hold up a subfloor or do you think you can get away with a 2x4 or 2x6? – Joe Phillips Jan 21 '14 at 2:04
  • You'd have to check local codes and/or with an engineer. You might be able to get by with a 2x4, but I'd probably use full depth blocking if I was doing it. – Tester101 Jan 21 '14 at 11:19
  • Yeah, I'd use full depth if I could but there are so many things in the way (the heater is right below) so that is nearly impossible. I'll try to stick 2x6's in where I can but may have to end up using 2x4's in some spots. I'm using 16oc or less so maybe I can just add a few extra in the 2x4 spots – Joe Phillips Jan 21 '14 at 16:39
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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.

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  • So if I understand you, I can run blocking every ~24 inches (since it's sturd-i-floor rated for 24oc)? That's good news. Do you happen to have any additional documentation on it? I would like to read more about the technique – Joe Phillips Jan 19 '14 at 6:19
  • Also, can my blocking just be dimensional 2x4's (since it's only spanning 16 inches)... also, can I use joist hangers to attach them between the joists? I have joists sistered under most of the room so there is 3" of wood I'd have to drive a nail through before reaching the blocking... – Joe Phillips Jan 19 '14 at 6:21
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    @JoePhilllips - You can use the steel joist hangers to put your blocking in place. It can be a bit of a pain nailing the part of the hanger that goes into the 16" length of block because it likes to bounce around. One option you have is to make sure your block fits nice and then take it down and attach the hangers to its ends and then place it up in between the joists for hanging in place. Another option is to consider using screws to attach the hangers to the blocks and joists as it eliminates the bounciness of hammering nails. – Michael Karas♦ Jan 19 '14 at 15:54
  • @MichaelKaras Thanks for the tips. I have a palm nailer which may help in this situation. Sounds like I have a plan to move forward :) – Joe Phillips Jan 19 '14 at 16:59

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