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  1. #1
    Join Date
    Jun 2007

    Default kind of foundation for cast iron bathtub

    We want to install a Kohler cast iron tub. We will have a 1/2-inch plywood subfloor after taking up the existing vinyl, etc. Installation instructions for the tub sound like 1/2-in plywood will be adequate. But, since the tub (316 lbs w/out water and person!) will rest on its 4 little feet on that subfloor, will 1/2-inch plywood actually be enough to support this weight? I do not want the tub to "sink" or flex and cause a crack in the interface to the tile surround above the tub. If anyone has had experience installing a cast iron tub, would appreciate any advice.
    Many thanks!

  2. #2
    Join Date
    Dec 2010
    Houston Texas

    Default Re: kind of foundation for cast iron bathtub

    What are you fixin' to do for flooring outside the tub?
    if you had tile in mind 1/2" plywood alone is far too thin for tile. 5/8" is the bare minimum and most pro's prefer 3/4" installed properly. If you were planning on natural stone tiles, you'll need a minimum of 1.25" of plywood properly installed.

  3. #3
    Join Date
    Jul 2010

    Default Re: kind of foundation for cast iron bathtub

    Assuming that your bathroom is on joists, you may want to double them in the area under where the tub is going to rest. This is a job for a framer. Make sure that the 4 tub legs will rest exactly over joists. See old postings for more information and details.

    Then lay the 3/4" or thicker subfloor, fasten it as directed and finish your rough plumbing.

    Lay the finished floor of your choice, connect the plumbing, fill the tub and enjoy the fruits of your labor.

  4. #4
    Join Date
    Oct 2008

    Default Re: kind of foundation for cast iron bathtub

    I have one of these tubs on 3/4" (5/8" actual) subfloor. The subfloor has 1/4" luan over it and then the vinyl. I ran the vinyl floor wall-to-wall to give some floor protection under the tub and set the tub on the vinyl.

    I weigh 400# myself and then add the cast iron tub on that. No problems at all.

    You can run extra joists. I could not because below this I have a lot of utilities running through my joists. I did, however, have my contractor connect the joists across to each other with blocking and carried this to at least one additional joist on either side of where the tub sits. We thought it would help distribute the load.


  5. #5
    Join Date
    May 2011

    Default Re: kind of foundation for cast iron bathtub

    Your design the load for the tub full of water. SO you need ot know the volume of the tub. A person is about the same density as water, although they could be standing up in the water ... so I'd add another 500lbs to that calculation.

    SO using some rough numbers, I calculated 30 cubic feet from a 30" wid, 60" long 24" deep tub. Water weighs 62lb/cuft. SO 1800lb for the water + 320lb tun + 500lbs for people. So you're at around 2600lbs!!!

    Best to double check the volume calculations... but it's probably not far off.

    Tubs need MAJOR reinforcement. You should located them when possible near a load bearing wall or you will have sagging of the floor joists over time... which can crack tile floors.

    On my 90 year old house, both of my tubs not surprisingly are located adjacent to the load bearing walls. Its' also a easier location to run piping to and from the basement up those walls.
    1925 Two-Story Stucco Beaux Arts Neoclassical

  6. #6
    Join Date
    Jan 2011

    Default Re: kind of foundation for cast iron bathtub

    For a standard Kohler 60" cast iron tub you are looking at approx 1,000 lbs total weight, tub 325, water 332 (8.3x40 gal) and one fat body 250

    I install lots of cast iron tubs and we always reinforce and/or rebuild the framing because it's rarely adequate for the weight.

    1/2" ply sub won't cut it for much of anything. rip it up, evaluate the floor framing and go from there. Don't take any short cuts.

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