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I am considering getting a soaking tub (pdf here) for my second floor master bathroom. However, I am unsure of how much weight the floor and walls below can support.

The house itself is wood frame pier and beam, built in 1885, with an addition in 1941. The bathroom is roughly 8 ft by 9 ft, and straddles the line between the two, 4 ft in the 1885 half, 4 ft in the 1941 half. The room has 2x10 joists running long-ways under a double layer of 3/4 inch tongue-and-groove subfloor, with an easily accessible 3 ft space underneath (installing the plumbing will be easy!). The walls of the 1941 half are 2x4s at roughly 16 inch spacing, and the walls of the 1885 half has 4x4s at 24in spacing. The 8 ft sides are both exterior walls, and the 9 ft sides are both interior. The 1941 9 ft wall spans a room, otherwise all the walls continue down to the foundation. As far as I can tell, there is no sign of rot, water, or insect damage.

The new bathtub will sit in the corner, with the long edge against the 9 ft side of the 1941 half. It's a two person tub with an "average fill" of 75 gallons, though by my calculations it could hold almost twice that. It will be half-sunk into the floor, with three 9 ft long 2x10 joists supporting it underneath and two 9 ft 2x10 joists (under the floor) holding up the frame. The upper and lower joists rest on top of 2x10s that are directly attached to the outside walls. The upper and one of the lower joists are part of the original 1885 building; the others are new lumber.

I want to make sure that what I'm doing is not only safe, but would pass a building inspection; I don't want to spend a lot of time and money drawing up plans and applying for permits if I don't have a hope of succeeding. Will my floor hold the tub with ample safety margins?

  • Apologies in advance if I've provided too much information - I'm not sure how much is relevant, and wanted to be thorough. – ArmanX May 21 '15 at 22:08
  • Why would you need a permit to install a bath tub? Do you live in England or something? – Tyler Durden May 22 '15 at 22:24
  • No - Texas, USA, actually, but letter-of-the-law in my city says if you move a pipe, you need a permit. I'm doing a lot of other work too, so I figure I should. – ArmanX May 22 '15 at 23:21
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If what you're saying is the tub will be on top of 2x10 at 16" on center spacing, with a span of 9 feet between bearing points that continue down to the foundation, you'll be fine. A drawing of the complex framing you seem to have would help out immensely. I still recommend doing the drawings and getting permits: I have a lot of experience framing houses, but I'm not an engineer. You can find span tables online that can help you determine weight allowance for framing members.

FYI, the 'average fill' is probably based on the displacement of two people in the tub; even if you could fit more water in, you'll probably want to only fill it halfway before two people get in.

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    People have approximately the same density as water, so from a weight perspective if the tub is full to the brim it doesn't make much difference if there's anyone in it. I suspect the "75 gallons" is there for water usage calculations, not weight. The framing must be strong enough to support a full tub. – Hank May 22 '15 at 17:31
  • Which is why I specifically said displacement... the framing, if it is what I understood it to be, is plenty capable of handling the weight of the tub (and people, and more), which will probably never be 'full to the brim' anyway because most tubs have an overflow a few inches down from the top. Do you think it wouldn't be? – DrewJordan May 22 '15 at 17:48
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No apologies needed. To determine the structural strength of that side of the house, get 50 pound bags of concrete and just keep adding them until something cracks or buckles. Then count the bags and fix the broken part of the house. So, for example, if it took 250 bags before it broke, then it means you can add an additional 12,500 pounds of dead weight.

All kidding aside, you could build the tub and fill it up with gold bricks and the house would still hold it up.

  • As much as I like your answer, I don't think it'll be the chapter option. All that cement would be expensive! – ArmanX May 22 '15 at 23:36

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