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Discussion Forum

Calculating tub volume/weight on floor

IanD | Posted in General Discussion on November 29, 2005 08:06am

I’m a homeowner and occasional poster to this forum (spend more time over on Knots), so I bow to the collective wisdom of Breaktime.

We are starting to brainstorm some ideas for a bathroom remodel which include upgrading from the standard 60″ bathtub we currently have (I posted previously about building a custom tub on this forum; this is kind of a follow up to that query). According to my calculator, my current tub (54″ X 24″ X 12″ ID) has a capacity of 9 cubic feet which converts to about 67 gallons. If I recall my high school science, a gallon of water weighs approx. 8 lbs., so a filled tub with bather should weigh in at around 850-900 lbs. Spread over 11.25 sq. ft., that’s about 80 lbs./sq. ft. Am I figuring this right?

If we go to a larger/deeper tub how do I figure out what, if anything, I need to do to beef up the floor to support it? Let’s assume a 69″ X 27″ X 16″ (ID) tub which, if I’m figuring this correctly, would have a capacity of about 130 gals. That’s an extra 500 lbs. of water. Though spread over the larger area, it still pushes the load to 107 lbs./sq. ft.

I haven’t looked under the house yet to see what’s there, but is my current framing (circa 1987) likely to be sufficient for that kind of load? What should I look for under there? And what should I consider adding to it if what I find is insufficient?

Sorry if these sound like questions from an ignorant newby; they are. I throw my ignorance on the mercy of the forum.

TIA,
Ian

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Replies

  1. xosder11 | Nov 29, 2005 09:18pm | #1

    When you say "under there"  do mean in the crawl space under the first floor.  If you can access it, you should measure the depth of the framing Likely 2" x 10" (actual 1 1/2" x 9 1/4") or 2" x 8" (1 1/2" x 71/4") and spacing from center to center.  Also you would need to know the span of the joists and what they are bearing on like foundation or carrying beam.  These are questions that most posters would likely have. 

    Good news is this.  If it is in fact on the first floor over a crawl space, and your existing floor does need some beefing up, it should be relatively simple to carry the added load to the earth, as opposed to say a second floor bathroom over a finished first floor.

    The answer to your next question of how to beef it up would rely on the answers to these questions



    Edited 11/29/2005 1:20 pm ET by xosder11

    1. Lateapex911 | Nov 29, 2005 09:51pm | #4

      I am in the same situation. I am installing a Kohler whirlpool, and the manufacturers specs call for a load of 1600 lbs.

      In my case, the joists are 2 x 12s on 12" centers, and the span is 5'...less than the length of the tub!

      To my thinking, only half the tub remains in need of support, as 2 of the 4 (total) pads are right over the supporting wall beneath. So thats 800 pounds left to be distributed, and i was going to rest it on (2) 2 x 4s assembled as a beam and placed on edge to distribute the load to the joists over the 48" width of the tub. (I allowed enough room for this by designing my deck height high enough)

      Am I on the right track?Jake Gulick

      [email protected]

      CarriageHouse Design

      Black Rock, CT

      1. xosder11 | Nov 29, 2005 10:48pm | #5

        sounds good to me, but I'm not an engineer. Does the supporting wall center over a carrying beam or does that bearing wall transfer load to some kind of footing? I am a home designer, and even on the simplest of projects I consult a structural engineer. Never assume anything. A couple hundred bucks for a consult and analysis could save you headaches down the road. Tubs and bathrooms are one area to pay close attention to deflection

        Edited 11/29/2005 3:18 pm ET by xosder11

  2. DavidThomas | Nov 29, 2005 09:47pm | #2

    my current tub (54" X 24" X 12" ID) in at around 850-900 lbs.

    Sort of. Yes, 9 cubic feet of air would fit, but the overflow outlet is typically about 2-3 inches below the rim, giving a max water depth of 9-10 inches. A cubic foot of water is 62.4 (7.48 gal/cf * 8.34 lb/gal) so I get 470 water weight. Plus the weight of your upper body (NOT your legs) for maybe another 100 pounds. Plus the tub weight - 80 or so for arylic/fiberglass or enamaled steel(don't!). 200 or a bit more for cast-iron. For a total of 770 even with a cast iron tub. Spread over 11.25 sq. ft., that's about 60 lbs./sq. ft.

    For the larger/deeper tub of 69" X 27" X 16" (ID) tub, I get 805 water weight (overflow is usually further down on a deeper tub) + 100 for one of you, 100 for a second person (that's the point, right?) plus 150 tub weight (they are never cast iron). For a total of 1150 or 72 psf (72" x 32" footprint?). Not much difference.

    If you are at all worried, just glue and screw a 2x4 along the top and along the bottom of the affected joists. Make sure to block between them at the bearing ends. i.e. create a 2x4 rectangle around the perimeter of each joist, on one side. Won't be in the span tables but would be vastly stronger and stiffer than the existing joists.

    David Thomas   Overlooking Cook Inlet in Kenai, Alaska
    1. durabond5 | Nov 30, 2005 01:32am | #7

      Why don't you count the weight of your legs?

      1. JohnSprung | Nov 30, 2005 03:12am | #9

        They displace water, and are of almost exactly the same density. 

         

        -- J.S.

         

        1. durabond5 | Nov 30, 2005 05:13am | #10

          Well then what about the rest of your body?

        2. jeffwoodwork | Nov 30, 2005 05:48am | #11

          I also wonder why not count the leg weight?  They disperse weight ok I'll bite , but that weight is dipersed into the water which is in the tub which is on the floor.  Maybe that guy had a terrible accident lost his legs there for doesn't need to count that weight.

          1. IanD | Nov 30, 2005 06:44am | #12

            The way I understand it, you count the weight the same; doesn't matter whether it's legs or water, the stuff filling the volume still puts the same downward pressure on the floor. So then what you have to add is the weight of the body above water because it's displacing air which is putting such a negligble amount of downward pressure on the floor that we don't even include it in the calculation.Ian

          2. durabond5 | Nov 30, 2005 06:49am | #13

            Right. If you were floating upon the surface, it is still weight in the tub.

    2. PeteVa | Nov 30, 2005 01:42am | #8

      Heck with leg weight, where do you get 200 lb cast iron tubs? I just installed 2 on a second floor that went 470 each. 200 pounders would have been a breeze instead of aching fingers and backs.

  3. User avater
    Taylor | Nov 29, 2005 09:49pm | #3

    The specs for the tub (weight, capacity) should be available from the tub manufacturer.

    For my 32"x60" CI tub, I figured 1300 lbs for tub filled with water and two people. You can't fit it all in, but a good engineer overbuilds.

    The tub is held up by 2 joists, each of which can hold up the tub on their own. That includes the carrying capacity of the joist hangers.

    Once the tub is installed.....fill it with water and see if the floor holds. Better to have a total failure with an empty tub than one with someone in it. This was recommended by an engineer.

    Engineers think in terms of total failure.

  4. JohnSprung | Nov 29, 2005 11:38pm | #6

    Thinking of it in pounds per square foot is the way to do it if the tub is uniformly supported under its whole area.  But most are supported at their edges, and should instead be figured as point loads.  Check the maufacturer's web site for the numbers to use. 

     

     

    -- J.S.

     

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