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I've stripped my 2nd floor bathroom to the studs and joists, installed new plumbing and electricity, and moved a few things around. The tub I'm putting in is acrylic, and weighs less than the cast iron tub I removed. But it holds 25 gallons more water. I'm also installing a glass shower enclosure with a tiled shower pan. And that should add about 200lbs. to the floor as well.

My joists span at least 10' but not more than 12'. They are 16" apart, except for the one parallel to a load-bearing wall, which is 24" away, originally designed like that to make room for the waste plumbing of the 1930s. schematic, 2nd floor bathroom My question is: what kind of weight can I expect this thing to support? My real concern is the tub. It weighs in at only 105 lbs. But full of up to 60 gallons of water, it could weigh as much as 585 lbs. Do you think the floor can handle it?

I should also add that instead of using 1/4" cement board on the floors, I've poured self-leveling cement. At six bags, it adds at least another 360 lbs. to the floor.

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  • You haven't told us what the joists are. :P
    – isherwood
    Dec 8, 2017 at 21:03
  • It's unfortunate that the tub and shower fall over mostly 1 joist. I'd be looking to double those two for at least 80% of their length. That would bring peace of mind.
    – isherwood
    Dec 8, 2017 at 21:04
  • Also the person in the tub. "ship in lock" displacement rules don't work in tubs. Dec 9, 2017 at 4:47
  • That's a tough one. I have no idea what kind of wood they're made of. I'm seriously considering having a contractor come in, open up the ceiling below and sister those two joists. I can sweat a pipe and join cable in a junction box. But structural stuff is beyond my skills. Thanks for your advice.
    – Ernie
    Dec 11, 2017 at 12:42

3 Answers 3

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If I assume you have 2x12 joists, then it's all good. If you want it better you can add full depth joist blocking perpendicular to the joists to try and help the floor out a little, but you don't have to.

If I assume you have 2x10 joists, then it's all good except for that joist under the shower/tub. It probably wouldn't catastrophically fail, but add a second joist next to it if possible. It'll also help prevent your new tile job from cracking by decreasing the live load deflection you'll get when filling the tub. If you absolutely can't add another joist there, you can add full depth joist blocking perpendicular to the joists to try and help the floor out a little.

If I assume you have 2x8 joists, that's bad. I hope you don't have 2x8 joists!

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  • They're 2 x 10". Plumbing in between the load-bearing wall and the joist won't permit for joist blocking, so I guess I'm looking at sistering it on the other side.
    – Ernie
    Dec 11, 2017 at 12:45
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I would add at least two additional full length properly supported, equally spaced 2x10 joist under the tub.

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We worry about two things for this type of bathroom: A) Will the joists hold everything up? and B) Will the joists deflect (bend) too much and cause the floor or ceiling to crack?

Obviously, the critical support is located under the tub/shower. However, there are other areas of concern too: 1) size and grade of joists, 2) bearing wall location opposite exterior wall, 3) finish floor material and finish ceiling material below.

1) The Code requires a residential floor to support a “live load” (people) of 40 lbs. per square foot plus all dead loads. The floor dead load (permanent material) seems to be about (from the top down): ceramic tile floor: 6 psf, underlayment: 2 psf, subfloor: 2.5 psf, joists (assuming 2x8 @ 16” oc: 1.5 psf, 6” batt insulation in floor: 1.5 psf, ceiling finish below (gypsum board): 2.5, plus misc. (lights, ducts, etc.): 1 psf, for a total of 17 psf. If you add that to 40 psf, you get 57 psf (plus the weight of the water).

2x6 (old sizes) joists @ 16” oc spanning 11’ (average of your guess) will support about 51 psf and 2x8’s @ 16” oc will support about 93 psf spanning 11’

As you can see, 2x6’s are close and 2x8’s are fine for the entire room, except for the added water (tub/shower area).

If the 60 gallons is correct, it would add about 500 lbs., not 585 lbs. However, adding 585 (tub water) plus 200 lbs. (shower water) plus 350 lbs. of people weight, you get about 1,135 lbs. of live load. For the 3’ x 8’-6” space of the tub/shower, that works out to about 44 psf. (Close to the 40 psf required by Code.)

Because the tub/shower area exceeds the allowable loads supported by 2x6 @ 16” oc, it will deflect and crack the tile floor and gypsum board ceiling below. I recommend adding 1 joist on each side of the existing joist under the tub. Make sure it fits tight against the subfloor, because new 2x6’s are smaller than your 1930 2x6’s.

If the span is 12’ and the floor is 2x6’s, then you’ll need to add 1 joist between each of the existing joists in the whole bathroom too. (2x8’s @ 16” oc spanning 12’ supports about 78 psf, which is fine.)

Oh, one more thing. There’s a lot of load on that bearing wall. I’d make sure there’s at least 1 stud directly under each end of those joists under the tub/shower area and you have adequate footing under each bearing wall too.

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  • Boy, this is complicated! Way beyond my skills. The joists are actually 2 x 10", although I'm not sure that is enough to do the trick. Your numbers are giving me a lot to think about, though. I'd hate to be soaking in my tub and then hear the creaking of wood right before I fall through to the kitchen below. Embarrassing, not to mention dangerous.
    – Ernie
    Dec 11, 2017 at 12:54
  • Oh, 2x10’s ??? If they are 24” or less on center, you’ll be fine. However, if the tub has claw feet, I’d put 2x blocking at about where they’ll rest on the floor.
    – Lee Sam
    Dec 11, 2017 at 22:45

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