How to reinforce floor for cast iron bathtub?

yadax3May 16, 2007


I'd like to replace the acrylic alcove tub in our hall bathroom with a nice cast iron tub but I don't know if the floor can withstand the weight.

We live in a typical single-story ranch built in 1993. We have plywood subfloors with a (dirt bottom) crawl-space underneath. I don't know the size/spacing of the floor joists without crawling under the house but I'm sure their pretty standard.

Since we plan to DIY our bathroom remodel, I'd rather not have to hire an engineer to tell me if my floor is strong enough to support a >350 lb. tub filled with water.

Should we play it safe and reinforce the floor? If so, what is the best method?

Thank you in advance for your expert advice.

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Assuming the framing underneath has not rotted or been cut up by a knuckle-dragging plumber, any house built to code in 1993 will have more than enough strength for any tub on the market. The weight of the water in in it is much more than the tub itself. I wouldn't worry at all about it.

    Bookmark   May 16, 2007 at 4:37PM
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Good news. Thanks Matt!

    Bookmark   May 16, 2007 at 5:50PM
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You are describing a pier and beam foundation. If the floor joists are at least 2 X 10 on 16" to 24" centers with a 3/4" thick sub floor you should have no problems. You should be able to check the floor joist situation without crawling around under the house. Water weighs 8 lbs per gallon so 20 gallons in a tub would weigh 160 lbs. The tub will weigh around 500 lbs.

    Bookmark   May 17, 2007 at 9:07AM
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matt, piggybacking here: do you know anything about code for *second floors* in 1978? I'm thinking I have to rule out a cast iron tub on the second floor, but if by chance we could use one, I'd much prefer to.

    Bookmark   May 17, 2007 at 12:00PM
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flyleft: same advice applies -- as long as the framing below was done to code and has not been compromised by rot or brain-dead plumbers, you should be fine. A simple test is to put a bucket of water on the floor and jump up and down around it. If you get significant movement in the water, it may need to be stiffened.

Remember that just two or three good-sized people (say 200 lb. apiece) are going to put far more load per square inch than any tub.

    Bookmark   May 17, 2007 at 1:26PM
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matt, thanks so much. Seems rather intuitive, doesn't it? Oh well, I'm thankful you were kind enough to write it out.

    Bookmark   May 17, 2007 at 1:28PM
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