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When I bought the house, it was a 4 bedroom, 1 bath rancher with basement laundry. We have since "converted" the smallest of the 4 bedrooms to an upstairs laundry, and took the closet out to accomodate a larger master bath on the other side of the wall.

In the very near future, we would like to split the current laundry room into a second bathroom, with tub, and a small laundry space. I am confident in doing all of the work DIY, but am unsure about the structural side of things. My floor joists are 2x8 beams 16 inches on center. What do I need to consider from a structural standpoint before proceeding with this plan?

Thanks in advance.

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    Is the added weight your main concern? Do you have other worries? Can you add additional support in the basement? – JPhi1618 Jan 17 '20 at 20:41
  • Added weight is my main concern. A tub full of water, with one or two kids in it has to be several hundred pounds at least lol. The basement ceiling is entirely opened up from a current re-wire project, so I can do whatever needs done there fairly easily. – clwhoops44 Jan 17 '20 at 20:47
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    2x8 is a small joist by modern standards but should still support a standard tub. What's the span? – isherwood Jan 17 '20 at 20:55
  • I think you're OK with your 2x8's, especially if the tub is going to sit across 4 joists (assuming the tub is 5 ft long). But like isherwood said, need to know the span of the joists to make a better guess, – SteveSh Jan 17 '20 at 22:16
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    It changes the load on the joists. Instead of the 300 lb - 500 lb+ weight of a filled tub being carried by 4 or 5 joists, it will be carried by at most 3. Understand the issues trying to put a full-size bath + laundry into a 12x12 area. You should be able to calculate the total load on your joists carrying the tub. Then go to a span table using 2x8 for your joist size and 15' for the span and see where your are. I think the design standard is 40 lbs/sq ft. – SteveSh Jan 18 '20 at 21:59
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It should be fine as-is, after-all a bath full of water weighs about as much as a waterbed, and there are no special requirements for framing under bathrooms. (unless you want to lower the floor)

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Structurally, in a word, you need to consider support. The joists alone will likely not be enough to support the added weight over time. They will start to sag. You may be able to get away with a single pillar support, and double up the spans. Or you may be able to put in two or even three supports in the area. It is hard to say without seeing the layout.

One way to consider it is this: how much weight will there be with both tubs full of water and the washer going? Doubtful that scenario would happen but it is what you need to plan for. Also, the floor under the washer needs to be very solid so that it doesn't vibrate a lot when it's running.

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    It really depends on span. Most homes from the 1950s to the 1980s had 2x8 or 2x10 joists, and they aren't all sway-backed. I'd consider 2x8s fine out to about 10 feet. – isherwood Jan 17 '20 at 21:45
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    Also depends on the joist spacing. Part of our house has 2x8 joists 12 foot span, but they are on 12" centres. 16" is more common. I did one house with 4 foot span -- wanted the look for the main room below: But I did the floor with 2" T&G pine. – Sherwood Botsford Jan 17 '20 at 22:40
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    over time? most tubs spend 90% of the time empty. – Jasen Jan 17 '20 at 22:53
  • @isherwood and crew, I was mistaken when I said 2x8s. I just measured them and they are 2x10s, spanning 14 feet. If the bathtub is laid parallel with the joists, so that it is being supported by only two, do you feel that is acceptable? – clwhoops44 Feb 13 '20 at 21:02
  • You'd think the tub is only being supported by two until you realize that the subfloor (and any X-bracing below) help transfer load to the adjacent joists, too. – isherwood Feb 13 '20 at 21:04

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