enter image description herei want to raise tji joists in second floor washroom. 9 1/2" tji joists need to be raised 3 1/4". We want the tub to be 5" higher that the rest of washroom floor. There is no lumber like 2x14. higher than 2x12 is engineered lumber. So is it ok to sister existing 2x10 TJI joists with 2x6 and 2x8 on top of each other to reach my desired height ? i just want to use 2 (2x6 + 2x8) joists rather than 1, 2x14 LSL. Thank you

  • 1
    Why not build on top of the floor? 2x4s shaved a bit will rise up 3 1/4.
    – crip659
    Dec 4, 2023 at 0:57
  • The tub and the standing shower will be on a raised sub floor. lets say, i shaved 2x4 and put it as you said. my concern, the full load gonna be on a lumber with 3 1/4 thickness ? i have concerns, what if the crack or i dont know is it safe ?
    – El Barto
    Dec 4, 2023 at 1:01
  • Just space them closely, or see options 2 or 3 in my answer. After all, without this, you'd be putting the full load on (probably) 3/4" plywood between the TJIs... I guess that leads to option 4 - stack of plywood or OSB sheets.
    – Ecnerwal
    Dec 4, 2023 at 1:05
  • @El Barto, the span calculator at awc.org/calculators/… goes down to 2x4. "Incised" is the pressure treatment cuts that we all see regionally except in Southern Pine areas. In a bathroom, wet service isn't necessary but isn't a bad idea. Your dead load is 10psf and live load is 40psf. As long as there's a layer of sheathing between the joist flange and the spacers, "concentrated loads" at the bearing points don't sound like a problem, but out of thoroughness you might call the TJI tech people.
    – popham
    Dec 4, 2023 at 3:04
  • @El Barto, and your deflection limit is L/360.
    – popham
    Dec 4, 2023 at 3:16

2 Answers 2

  • If the joists are existing and you don't need taller ones for loading purposes, just put some 2x4 across them at 90 degrees in the area where you want a raised platform for the tub, and you're up 3-1/2" (or you notch 1/4" if you really want 3-1/4")
  • Or: mount 2x4 joist hangers on your TJI's per TJI specs (typically need to fill the web where the joist hanger goes) and use deeply notched 2x8s at 90 degrees, cut to give 3-1/4" above the top of the TJI for a "more solid" option.
  • Or: use 2 2x4 laid flat and a strip of 1/4" plywood on top of the TJI in that area (not across them at 90°, on top of them at 0°).
  • Or put the sub-floor on level, then add 2 layers of 1-1/2" XPS foam, then throw on a layer of 1/4" cementboard. Don't forget to bed the tub and shower base in plaster or mortar (unless they are cast iron.) You can get XPS up to at least 100PSI compressive strength. The run of the big-box-store Fomular 150 is 15PSI minimum. So for a typical 30x60" tub area 27,000 lbs - it won't have any trouble holding up tub, people & water for a matter of a few hundred to 1000 pounds...
  • Or pour 3-1/4" of concrete on top of the subfloor there if you're a person who does not trust the load-bearing capacity of XPS foam. You might want to pay extra for lightweight aggregate, though, or the additional load could be an issue.
  • 1
    The 15 psi XPS could have up to 10% strain at the 15 psi by their spec. Under NIST PS 1-19 (Table 8), a 16" plywood floor span rating implies a maximum deflection of 0.044" under 100 psf. Scaling the 15 psi and 10% strain for the same 100 psf, the 3" XPS could contribute up to an extra (100psf)(3in)(0.10in/in)/[(15psi)(144si/sf)] = 0.014". That's an additional 32% beyond the floor sheathing's standard squishiness. Sounds plenty stiff. If the floor sheathing is installed at its exact span rating, then a second sheet will bring the XPS installation back into spec (0.044"/2 + 0.014" < 0.044").
    – popham
    Dec 4, 2023 at 10:05

a 2x3 can span the 18" between joists with no risk of cracking under normal floor loads or you can run them parallel on top of the existing joists, with some blocking to ensure that they stay standing on edge,

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