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Old 11-30-2015, 03:03 PM   #1
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Leveling subfloor, blocking


I will be shimming joists with 2' shims (ripped from 2x4s) to level a subfloor and also repairing some water damaged subfloor at the same time and have a couple questions.

I know I need to add blocking under the subfloor where I will need to cut the T&G plywood, to support the subfloor at the seams. Where I am replacing subfloor and adding shims on top of the joists near the walls, I have had to cut the subfloor up to the bottom plate of the wall.

Do I need to add blocking under the subflooring that is under the walls? (For clarity, if yes, I would have the following from bottom to top: blocking | 3/4" subfloor | 2x4 subplate for wall | 2x4 wall framing.) If the answer is yes, how would I attach the blocking? Just toe nail from the open side? I am hoping the answer is no and that I can add blocking about 6" out from the wall and then toenail blocking from both sides to support only the edge of the subfloor as close to the wall as possible.

If I do not need to add blocking under the walls and I add the blocking about 6" from the wall as described above, is that sufficient support for the new subfloor along the walls? (There would be 6" lip of subfloor between the blocking and the wall that would not be supported. The seams I'm asking about are where the cut subfloor seam runs perpendicular to the joists.)

Thanks! Sure I'm leaving out necessary details, my first post here, so please let me know if there is additional information I need to provide.
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Old 11-30-2015, 03:34 PM   #2
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The blocking under the seams is to prevent flexing of the subfloor that might cause squeaks or crack tile---so it is not necessary to have the blocking under the walls--only areas where there will be finished flooring.
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Old 11-30-2015, 04:21 PM   #3
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Quote:
Originally Posted by oh'mike View Post
The blocking under the seams is to prevent flexing of the subfloor that might cause squeaks or crack tile---so it is not necessary to have the blocking under the walls--only areas where there will be finished flooring.
Great, thank you. Is blocking 6" from the seam that runs along the wall acceptable? I will have solid hardwood floor over that 6" span, but hope it wouldn't flex too much over just 6". (Given that the joists are 16" OC, I would think 6" unsupported is fine but am not sure. I'm not sure if it is more important to toenail from both sides or to have the blocking all the way to the seam. I can't do both, though.)

I saw someone on youtube (
) who built square boxes from 2x4s, where one side of the box was slid under the subfloor seam with two of the sides being nailed into the floor joists. Is this the best way to handle this or is the 6" unsupported span acceptable? I have also thought about building an upside down U shape out of 2x4 and nailing the two vertical pieces into the joists, that would do essentially the same thing as in the youtube link.

Thanks again!
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Old 12-01-2015, 07:03 AM   #4
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Subfloor blocking is often simply screwed to the sub floor at the seam---not installed until the first sheet is in place---
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Old 12-01-2015, 08:03 AM   #5
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Quote:
Originally Posted by oh'mike View Post
Subfloor blocking is often simply screwed to the sub floor at the seam---not installed until the first sheet is in place---
The way I read this is that the blocking isn't attached to the joists anywhere, only the plywood subfloor, which I don't understand. Am I misunderstanding your response?

Edit: I think I understand what you're saying. Because I am reinstalling the plywood subfloor up against the wall where I had to cut the subfloor, I will have to install the blocking before that row is inserted. In any event, I was thinking that I needed to toenail from both sides but I won't have access to both sides of the blocks as one side will be covered by the subfloor/wall that I just cut the subfloor up against.

So, I either have to leave a 6" unsupported edge so that I can have access to nail from both sides or push all way to the edge of the subfloor and only nail from one side.

Last edited by clix; 12-01-2015 at 08:14 AM.
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Old 12-03-2015, 05:33 AM   #6
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I like to use screws. The piece is easier to hold and doesn't move around as much as hammering nails.

From one side is fine and I would put it closer to the wall than 6".
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