Advice on support for replaced sections of OSB sub-floor

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Inapickle2

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I've begun a major DIY project (for me, anyway) re-modelling our main bathroom upstairs (first floor). Previously, the toilet, shower stall and sunk bathtub were installed on a tiered platform structure. The wife want's everything on the same level.

Having removed the platform structure, and the old 3/8" ply that covered the tiled floor sections, I’m down to the 5/8” OSB subfloor nailed on 10" I-Joists spaced at 16". The sub-floor was already pretty rough cut where the existing plumbing for the toilet, shower and tub came up and I’ve opened up those sections to figure out how best to plumb for the new utilities.

The existing water lines are Poly B and I was thinking about replacing it with Pex. However, having seen how the lines route through the joists and pass up through the wall frames, I’m not sure that would be possible without pulling up a lot more subfloor. Even accessing through the ceiling below would be difficult because of the structural layout.

So I’m thinking about replacing only what is necessary and accessible – specifically, one section of the sub-floor 3/4” lines to supply the new bathtub (standalone with pedestal faucet) and ‘above floor’ 1/2” branch lines in the walls that will supply the toilet, vanity and shower. Obviously there will be some reconfiguration of the drains also.

That said, another major consideration is how to provide an adequate support for the sections of sub-floor that will be replaced. I’m thinking it could be done by framing with ‘sister’ and cross (block) sections of I-Joist, but I’m not sure how best to go about that.

I’ve attached some photos to illustrate what I’m dealing with.

One is the toilet section. There I’ll need to add support on three sides, unless the edge that still has the existing ‘tongue’ intact is still usable and doesn’t need supporting underneath.

Toilet Section #1 resized.jpg

Toilet Section #1 resized.jpg

Toilet Section #3 resized.jpg


The other is the section where I’ll be re-plumbing for the new standalone bathtub. That will need added support on all four sides.

Bathtub Section resized.jpg

Would appreciate any advice on how to do this. I should perhaps mention that I’m going to applying Ditra XL directly on the OSB and tiling over that.

Cheers.
 

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jadnashua

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You want any pieces to attach to at least three joists whenever possible so it's not just hanging from the fasteners across an open section (i.e., you want the middle to be supported) along with ALL edges. If you can match up T&G, that suffices, but if not, you need either blocking between the supports, or you can glue and screw cleats underneath the seam and screw the pieces to it. With the I-joists, it's probably easier to use cleats rather than blocking.

Really old material might be an actual 5/8" thick, while most all of the new stuff is a 'nominal' 5/8", and is actually slightly under that thickness, so that can make repairs harder.

You'll need to clean off any leftover adhesive and wood pullout bits on the top of the I-joists so that when you install the new stuff, it doesn't force it to sit higher, and you can get good construction adhesive coverage.

Keep in mind that while Ditra is spec'ed for 5/8" subfloors at 16" OC, that is for a NEW floor, without all of those patches...today's 3/8" ply often doesn't lay flat (more like a Pringles), so most people prefer to go to at least 1/2" stuff on top of their basic subfloor layer.
 

Inapickle2

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You want any pieces to attach to at least three joists whenever possible so it's not just hanging from the fasteners across an open section (i.e., you want the middle to be supported) along with ALL edges. If you can match up T&G, that suffices, but if not, you need either blocking between the supports, or you can glue and screw cleats underneath the seam and screw the pieces to it. With the I-joists, it's probably easier to use cleats rather than blocking.

Great, thanks for the advice.

Really old material might be an actual 5/8" thick, while most all of the new stuff is a 'nominal' 5/8", and is actually slightly under that thickness, so that can make repairs harder.

My mistake -actually the OSB is 3/4" not 5/8".

You'll need to clean off any leftover adhesive and wood pullout bits on the top of the I-joists so that when you install the new stuff, it doesn't force it to sit higher, and you can get good construction adhesive coverage.

I don't think there was any adhesive. Once I'd removed the nails it came up very easily with no residual bits stuck to the joists, but I'll definitely apply some PL when I fix the new sections.

Keep in mind that while Ditra is spec'ed for 5/8" subfloors at 16" OC, that is for a NEW floor, without all of those patches...today's 3/8" ply often doesn't lay flat (more like a Pringles), so most people prefer to go to at least 1/2" stuff on top of their basic subfloor layer.

The tiles would sit just a bit too high with 1/2" ply over the OSB. A couple of years back I re-tiled a good portion of ground floor with Ditra XL over OSB; it matched up nicely with existing hard-wood floors and trims, and has held up very well. Requires a lot of preparation though. Thankfully this is a much smaller area.
 
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jadnashua

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Minimum industry standard for ceramic is 5/8" in good condition, but 3/4" gives you a bit more margin for damage along the way. You do want to use construction adhesive...it makes the subflooring MUCH stiffer and stronger with fewer chances of squeaks along the way.
 
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