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britepepper 12-01-2011 02:56 PM

Support for new subfloor
I'm in the middle of a second floor bathroom remodel. The subfloor had to be removed to replace plumbing drains and install pex.

I've cut the 1" thick subfloor flush with the wall bottom plate. On the side running parallel to the 2X10 joists I planned on attaching a new (and shorter) 2X10 to the existing joist. Is gluing and bolting/nailing a board sufficient to support the floor? Or do the ends of the new joist need to be supported just like the existing joist, i.e. on top of the beams?

I have access to the beam on one side, but getting to the other one would involve a lot of demo I'd like to avoid.

I appreciate any advice...

JazMan 12-01-2011 03:31 PM

Post a few pics, need to see where the joists are in relations to the walls. Try to post at least 3 pics. One to show each end and the 3rd showing the center area.


britepepper 12-01-2011 04:41 PM

4 Attachment(s)
Attachment 41871

Attachment 41870

Attachment 41869

Attachment 41872

I hope you can make sense of these. The drawing shows the existing joist in green. It extends to the left under the wall about 4 feet. It is supported on the right by the blue beam. Yellow is the edge of the old subfloor. The floor joists are spaced 24" O.C.

My plan for the new floor is two layers of plywood (3/4" and 3/8") followed by floor warming mats, SLC, Ditra and tile.

JazMan 12-01-2011 07:05 PM

Hi Pepper,

Is the 1" subfloor actually 1" plywood or something else? What is the exact thickness?


Is gluing and bolting/nailing a board sufficient to support the floor?
Yes that's fine. It doesn't even have to be 2x10. 2x8 or even 2x6 will work if it helps avoiding obstacles.

However it looks like you'd have a tuff time installing blocking between the joists where new ply meets old. So, you'll probably have to install new multiple sheets to equal the old, then install an underlayment of min. 3/8" to overlap all seams. This is a must cuz you do not have the t&g anymore, good idea anyway.


britepepper 12-01-2011 09:15 PM

1 Attachment(s)
Thanks Jaz...

The subfloor is one piece of plywood 1 1/8" thick so my two layers of new plywood should bring it up to the same level. I'd like to avoid adding another 3/8" on top of that if at all possible. What do you think about something like this:

Attachment 41883

The red layer is 3/4 ply that would go underneath and be screwed to both the old subfloor and the new layers of subfloor binding that seam together. It would be notched into the 2X6 on one side and on top of a block on the other. I think I could fit this in between all the drain pipes and water lines.

Do you think that would be stiff enough to avoid adding the 3/8 underlayment? I should probably add that this corner is where the shower will be....mud bed with Kerdi.

JazMan 12-02-2011 10:53 AM

If you can support the ends I'd say OK. However it's still not the recommended way cuz both seams would be at the same place. Your original plan including Ditra should work.


britepepper 12-02-2011 03:33 PM

OK....thanks for taking the time to respond to my post.

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