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Old 12-27-2008, 08:55 AM   #1
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Floor load w/ bathtub


I'm going to be installing a bathtub on the second floor of my house in a newly built bathroom. The floor has 2x10 joits, 16oc. The span is about 12 feet. The tub will be about four feet from the rim.

I want to put in a 300 pound cast iron tub with claw feet. Now I know with water and humans, this presents a pretty good load. And I know that it needs to be analyized from a structural engineering standpoint. But when I present this info to my 75 year old friend who has worked construction all his life, he says it would definetly not present a problem. He has never heard of joists cracking from a floor load, let a alone a bathtub crashing through the floor.

So my question is.... codes and book-hazards aside, has anyone out there really actually witnessed a bathtub crashing through a floor? If so, what were the circumstances?

Anyone see that "Deconstruction" episode where they drilled a bazillion large holes in the joists? They were trying to demonstrate how dangerous it is to over-bore in a floor joist (and I totally agree it is... not to mention the legality) but I was surprised by how much boring the joists actually took... with a full tub load on top.

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Old 12-27-2008, 11:46 AM   #2
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Floor load w/ bathtub


Although i haven't witnessed a tub falling through the floor, I do sell all sorts of tubs including cast iron stand alone tubs. Are you able to access the joists from the main floor or are the covered by drywall already? If accessible, it would be wise to double up those joists with more 2x10s if possible, providing electrical, plumbing and heating aren't in the way. There is a chance....but remember i said "a chance" that the weight of the tub+water+human could possibly cause the floor to sag as you are looking at probably around 700 pounds on the same spot forever. That is a big weight load. You're friend could be possibly right but you don't need to witness a tub falling through the floor to say it never happens. We recommend any and all safety measures when installing these types of tubs. of course it would be different if you has a load bearing wall below but that's non existent. I hope this helps you out and gl with the reno.

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Old 12-27-2008, 12:21 PM   #3
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Floor load w/ bathtub


Your friend is right, to a point. The Deconstruction episode was also right, to a point.

Where they both fall short is that they're thinking in terms of structural failure, which is rarely an issue. You have to do quite a bit to force a structural failure, and your situation is nowhere near that.

The problem is that the added weight (or damage to the joist in the case of the TV episode) increases the deflection (sag) of the floor system, and by loading the joists to their outer limits the floor that was once solid and sturdy will now react when someone walks across it...Often like a trampoline, there will be a perceivable bounce in the floor. That's bad for tile and other finishes.

Furthermore, the claw feet are 4 tiny concentrated load points on the plywood floor sheathing, as opposed to the evenly distributed loads of other tubs. Those loads would possibly deflect the floor sheathing somewhat, and would likely be enough to crack a tile.

I'd take the aforementioned advice and consider doubling up the joists immediately under the tub, which will solve all your problems. Blocking between the joists under the claw feet would be a good idea as well.
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Old 12-29-2008, 11:26 PM   #4
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Floor load w/ bathtub


Thanks for the replies guys. Unfortunately the lower level is finished, so doubling up the joists will be impossible. But I may be able to take up the floor where the tub will go. Is there anything else I can do that does not require doubling the lumber the entire length of the joist span? I understand that blocking will help to distribute the point loads, but will it help at all with overall support?
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Old 12-30-2008, 12:39 AM   #5
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Blocking helps joists share each others' load, but won't really make a huge difference in the overall performance of the floor. It never hurts though.
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Old 12-30-2008, 11:07 AM   #6
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Floor load w/ bathtub


Approaching it from the top requires lots of patience as "new" wood tends to be slightly bigger than older wood (moisture shrinks over time). You may need to measure and not assume that all 2x10's are equal. Otherwise, you may damage the finished section below.

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