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I'm remodeling a 2nd floor bathroom and after ripping up 2 layers of old tile and 4 inches of concrete I finally found the joist. My question is really sistering on new joist to the old ones. I need to gain 4 inches to be level with the flooring of the rest of the house. My idea was to sister on new joist 3 inches higher then the original floor joist to get the height I need then do 2 layers of OSB (1/2 inch each) to get my last inch to get the total of 4 inches needed.

The sistered joist would not run the full length of the floor joist and would just run the length of the room which is roughly 5 feet. Is it ok for me to sister on new floor joist to the old joist with the new joist being 3 inches higher ? I would be using a 2 by 10 onto the old 2 by 10.

So instead of the new joist being flush with the bottom of the old joist I would be leaving a 3 inch gap at the bottom of the new joist to I can gain the 3 inches of height. I would just shim or using blocking to get the new joist to the correct height and level then use GRK rugged structural screws to screw the new joist to the old one.

I had one friend suggest just ripping a 2 by 4 to the correct length and height and then just laying that directly ontop of the joist and screwing that to the of the joist. I also think it's a good idea but since I have everything opened I figured I might as well sister on new joist seeing the this is a 114 year old house to start with so some new wood wouldn't hurt.

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  • It should be okay. The problem is going to be measurements to make sure all the new pieces will be equal to each other on top.
    – crip659
    Commented Mar 29, 2023 at 18:34
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    Not just OK, but an ideal opportunity to ensure your new floor is very level! No need for shims or blocking, just nail/screw the new ones onto the sides of the old ones. Some blocking might be handy to hold the new joist in place while you get it leveled & the first couple of nails in. Other than that, not necessary
    – FreeMan
    Commented Mar 29, 2023 at 18:44
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    I'm about as far from an expert on structural wood and flooring as is possible, so just a comment (@isherwood or one of the others who knows this stuff will tell us whether it will or won't work): 4" = 3.5" + 1/2" - why not 2x4s on edge, perpendicular to the existing joists, with appropriate Simpson or other hardware to connect them together, + 1/2" plywood on top? Seems simpler to me than sistering and probably a lot less wood.
    – manassehkatz-Moving 2 Codidact
    Commented Mar 29, 2023 at 21:31
  • Look into Simpson Strong-Drive SDWS Framing Screws.
    – aquaticapetheory
    Commented Mar 29, 2023 at 22:20
  • What's your planned tile substrate? Just the OSB, or something like Ditra?
    – Huesmann
    Commented Mar 30, 2023 at 15:09

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The comments section really has this sewn up, so I’ll summarize.

2x10 beside an existing joist is a great way to add height. (Personally, I’d glue and screw 3/4 cdx t&g instead of osb, but that’s partially hatred of osb’s inconsistency.) When attaching, predrill the old joists: they’re likely to split if you just try and run screws through them.

Blocking would help stiffen the floor structure. Slightly ripped 2x4s would work well. (Ripped because there’s a chance your old joists are inconsistent in the wrong direction.)

I wouldn’t just stack a 2x4 on top of the existing joists. With blocking it would be structurally sound and stable, but you’d be carrying all of the old levels to your new floor, which probably isn’t ideal.

To get your sistered joists level, either use a laser or spend some time with a bubble stick. Pick the most important one (probably at the doorway) and work over from there.

One of the hardest parts of this job is not stepping through the first floor ceiling.

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  • It's a bathroom so the drains might drive the levels rather than the door especially if you have long drain runs under the floor.
    – jay613
    Commented Mar 29, 2023 at 23:48
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    You want the floor level and even with the floor it's being matched to, while maintaining proper slope for the plumbing, @jay613.
    – FreeMan
    Commented Mar 30, 2023 at 14:25

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