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Floor joists strong enough for tub?

WillieWonka | Posted in General Discussion on December 30, 2003 06:44am

Here is a question that is more or less gonna rely on your opinion vs. hard numbers.

Here is the situation: Old house with solid oak full dimension floor joists. Currently am remodeling the bathroom which called for installing a new whirlpool tub in parallel with the floor joists. The floor joists are oak, and vary in range from 2.5″ to a full 3″ thick and are either 8″ or 10″ deep, I forget which. The joists span 12′ from one end until supported on the other end by an 2″x3″ (full dimension) oak framed bearing wall (which transfers the load down to a steel I-beam in the basement).

Installation of the tub calls for the tub base to run parallel to the floor joists and for the base to span two of those joists (they are about 12″ to 15″ o.c, I forget which). The one end of the tub starts about 15″ from the end of the joists and then of course spans the length of the joists by 5′.

The tub specs say when filled with water (sufficient to operate the jets) that it weighs 604lbs. Add someone who is 200 to 300 lbs and you can approach about 800 to 900 lbs being carried by two joists. Will it work, can two joists that size carry such a load?

Note: When I remodelled the dining room (underneath where the tub is) these same joists had diagonal “checks” or “splits” in them sufficient enough to cause me to sister a 2×8 (or 2×10,again I forget the depth right now) to each one with glue and lotsa nails.

Opinions?

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Replies

  1. OneofmanyBobs | Dec 30, 2003 06:53pm | #1

    If they're 8 inch, I wouldn't do it in my house.  If 10 inch, marginal.  I still probably wouldn't.  You're adding another 50 PSF of load.  You also don't know whether the joists line up directly over a stud or not.  If not aligned, I would not trust the top plate.  Not designed for that .  For that kind of major item in an old house, I'd pull up the subfloor and reinforce the joists with steel, open the bearing walls and make sure they properly supported the joists.  Not what you wanted to hear, but I would not want a half ton of water, the tub and a big hairy naked guy falling through my floor.

    1. WillieWonka | Dec 30, 2003 07:04pm | #2

      Yeah, big naked hairy guy....yeesh, what a sight. Actually when I do my kitchen remodel which is next on the list I have access to the bearing wall. I can redo that wall to make it thicker studs or even put in a steel beam.

    2. fdampier5 | Dec 31, 2003 03:02am | #5

      He said, Oak not western white woods or spruce etc.  in addition they are twice as thick as a normal 2x would be.. (yes thickness does matter)    Old timberframes like this are surprisingly tough. I guess I'd like to see it before passing final judgement but if we are talking 24 inches on center and he sistered some additional lumber in place because of checks  (which really just look ugly but do not affect strength at all)  I'd say go for it..

        I can check my timberframing book to ensure but please realize that dired oak is tremendously strong relative to the green wet soft woods built with today..   I doubt even an engineer wouldn't have enough expertese to properly make ajudgement (unless he has a lot of experiance with timberframes..)

            

      1. OneofmanyBobs | Dec 31, 2003 03:34am | #7

        Yep.  He did say oak, but he also said he sistered joists in a different room because of checks.  Full dimension clear 4x10 oak, no problem.  Checked 3x8 oak with knots or holes on the tension side, no way.  Without actually seeing the joists, my opinion (and its definitely just an opinion) would be to err on the side of caution and not put a half ton of extra weight on it.  If you open everything up so you can see the structure, then you can talk facts.  I've seen timber structures I would not hesitate to drive a truck over.  Also ones I would not walk on.  Its a bathroom.  How do you know some plumber didn't notch the joists for something 100 years ago?  Likewise, dry rot from an old water problem.  It may be fine, or may not.  If its my house or a customer's, I wouldn't do anything unless I knew for sure.

        1. fdampier5 | Dec 31, 2003 03:46am | #8

          I just  scanned it but didn't he say they were like 12 to fifteen inches apart?

             Oak is mighty tough wood, dried oak is ten times tougher!   checks look ugly but too many books I've read do not downgrade a wood because of checks..   I've never read anything that says a check affects strength (at least not by anyone knowledgable) 

                I've got 6x6 oak beams and they have an astonishingly high load capablity.  (I was able to span 12 feet with 48 on center and still meet live load requirements  with car decking.. )  I really should go look up the numbers and give you the exact numbers rather than relying on memory that is several years old.

            As for your point about opening things up,  probably not a bad idea.  Should be a way short of total demolition..

          1. JohnSprung | Dec 31, 2003 04:59am | #9

            I'd definitely want to open it up enough to inspect and measure.  Unless it's a claw foot tub, that work will be hidden in the final installation, so retaining or matching the surface shouldn't be an issue.

            Can you find the fundamental engineering numbers for oak, Fb, E, Fc perpendicular and parallel, Fv (horizontal shear)? 

            -- J.S.

          2. WillieWonka | Dec 31, 2003 06:31am | #11

            Thanks guys for all the replies. Now that I am at home I took the liberty to get more precise measurements for you, so let's see how this changes the opinions expressed thus far.

            The house is old, 1930's oak framed. The oak is super super dry, so dry sometimes it's really splintery (microscopic ones) and while it's heavy and solid, it feels like "light and airy" because it's just dang dry.

            The two joists I mentioned are 18 o.c. They are 7 1/2" deep, 3" thick and again span 12' end to end.  The tub in question is a fiberglass tub but the whirlpool style. I stand in it and jump and down and I feel no give in the floor other than the normal vibration you'd feel doing such a thing. The subfloor is solid 3/4" T/G old hardwood flooring that has been covered with luan. The only weakness in the subfloor is where the tub drain is and really there is no weight being pressed on that area. The area the actual water filled tub sits on is the solid T/G hardwood planks. So in essence these planks in my head help carry the load to other adjacent joists as opposed to just the two I"m most concerned about.

            The two joists in question were sistered with 2x8x12 pine boards. I sistered these joists due to checks as previoiusly mentioned, there were diagonal checks, a number of them. Under this bathroomis the dining room. When I remodelled the dining room that is when I sistered these joists. All joists were exposed. I was concerned because in the process of remodelling, one of the oak joists (elsewhere in the DR, away from the bathroom) literally broke in half to where is sagged about 2 or 3 inches. Had to jack it up and sistered it, so while I was at it I looked to see if any others looked like they might split and that is when I noticed the two joists in question as having diagonal splits making me fear they'd break like the other one did. Maybe they wouldn't have, I was just being cautious.

            With th sistering of the joists in question, that makes the joists equivalent to 4 1/2" thick.  How does this change everyones opinion?

          3. Piffin | Dec 31, 2003 07:49am | #12

            With 18"oc instead of 12-15"oc and diagonal checks, and 3x8 instead of 3xmaybe ten, I suppose I should be less confident.

            But you are right that a floor operates as a system. It is not only those two joists that bear the load even tho they get the most abuse from it. I would average out the layout distance to calculae that from.

            I would not count any extra brownie points for the sistering you have done. That is insurance for the checks. But supposing that you had oak joists 4.5" x 8", this would give mne less confidence than hearing that the joists are 10" high. You gain far more from the vertical section.

            .

            Excellence is its own reward!

          4. woodroe | Dec 31, 2003 08:16am | #13

            Piffin is correct in what he says about the vertical section. But I think you will be just fine. There is a bad joist in many units of lumber. If you had said all your joists were of questionable quality that would be a different story. But you have the equilavent of 2x8's 9" on center. Now if your oak has a span rating similar to Doug. Fir or even SPF you will be fine.

          5. User avater
            BossHog | Dec 31, 2003 03:35pm | #14

            I still don't like them. The 7.5" deep and 12' long part is what bothers me.

            Jumping up and down might give you some measure of vibration, but not long term deflection. Vibration isn't much of an issue under a tub.Until America, door to door, takes every handgun, this is what you’re gonna have. It’s pathetic. It really is pathetic. It’s sad. We’re living in the Dark Ages over there. [Sylvester Stallone]

          6. fdampier5 | Jan 01, 2004 12:07am | #16

            Sure no problem, give me a little time since the house is half timberframe and a giant storage closet in the rest..   (oh and somehow we're trying to live in it)

          7. fdampier5 | Jan 01, 2004 12:49am | #17

            As you know it is nearly impossible to tell the species of white oak just from the timbers and thus   I'll try to give you some averages,, OK? 

              specific gravity varies from.58 to .88

              Modulus of rupture from 8000 to 18,400  (most north of 10,000)

            Modulus of elasticity (in millions of PSI)1.00 to 2.05 average seems to be around 1.6

              work to max. load (in pds per cu in. )  10.7 to 19.2

            shear parrallel to grain 1350 to 2650 (PSI)

             side hardness (load perpendicular to grain)  1110  1620 (pounds)

             Compression parrallel to grain  (max crushing strength) 3020 to 8900 (PSI)

            Impact bending 35 to 46 (inches)    While some of the numbers of the best Doug fir approach Oak,  majority of common oaks, swampy white, white, swampy chestnut etc.  are all significantly better..   As for western white woods or other common woods they are dramatically lower..

             In addition Oak can achieve those numbers in 200 years or less while it may take hundreds of years more (if not a thousand years)   for Fir to achieve numbers like those

              Further Tedd Benson in his book gives a rating of 2106 pounds per beam for a 7 inch beam uniform load and 1839 for a two point load..   If he had all that weight on one joist it should be striong enough two joists adds to the safety factor and since we can properly speard that load out between a total of four joists   unless the fat ols guy taking a bath weighs over 8,400 pounds there shouldn't be too much to worry about..  Just for information I figure that 3x7's over a 12 foot span should only carry  5640 pounds while western white woods should have about 3610 pounds.

            Edited 12/31/2003 5:01:45 PM ET by frenchy

      2. User avater
        RobKress | Dec 31, 2003 06:22am | #10

        I agree and can concur.  Oak framing is just like none other.  My cabin in PA was built by the local idiots there over a few truck loads of beer.  We call it the club house that got out of control (1800 sq ft with a full kitchen and 2 bath).  Anyway, it was built with rough cut oak and not to any code or standards at all (like I said, they were drunk and lost their level).  Anyway, a big tree fell on it a couple of years ago (75 ft tall tulip poplar) and did no damage at all.  Direct hit.  The only thing that needed fixing (besides all the work that was done before me.... everything) was some dry wall screws that popped out.

        I say no problem.

        Rob Kress

        1. fdampier5 | Jan 01, 2004 01:03am | #18

          I just looked 'em up and that floor should carry over 8000 pounds with safety..  (that's just over 4 joists)  western white woods the same size  wouldn't get to half of that number!

          1. WillieWonka | Jan 01, 2004 05:30am | #19

            So what are you saying???? That I"m prob ok with my oak joists after all??

          2. fdampier5 | Jan 01, 2004 10:02pm | #20

            Well if you weigh less than  8000 pounds with the tub, water and your fat ol' butt  ;-)  you should be peachy.. again if you have any real concern you could easily haul stuff up there and set it in place and do the bounce test...

              let's see you could grab four big plastic garbage cans and fill them with water, or you could make a simple plywood box about the size of the tub, line it with plastic and fill it with water..   five really fat friends, or ten skinny ones.. some combination of the above..

              I'm havin' fun figuring out ways to test what I'm pretty certain would be just fine.. but then I'm a belt and suspenders kind of guy.,

          3. WillieWonka | Jan 02, 2004 09:27am | #21

            Well my wife has 3 friends that are really really really, hmmmmmm, shall I say.....FAT!!!!! So much so that I run away from them :) One of them would literally fill the tub leaving no room for the other 2, that's a little scary. I suppose I could try and get them to stand on top of one another, acrobat style, but believe me, I'll be doin some serious praying they don't fall thru the floor.

          4. fdampier5 | Jan 02, 2004 06:54pm | #22

            If they did you'd solve two problems at once.. loss of er,.. amply endowed female friends of your wife (to be replaced by skinny scantily clad young things hopefully)  and a new access hole to inspect the remainderof the joists..;-)

  2. User avater
    BossHog | Dec 30, 2003 09:34pm | #3

    "Here is a question that is more or less gonna rely on your opinion vs. hard numbers."

    I think you're right. But I also think I wouldn't do it without beefing up the joists somehow.

  3. Piffin | Dec 31, 2003 02:44am | #4

    Oak three inches thick?

    No sweat

    .

    Excellence is its own reward!

  4. woodroe | Dec 31, 2003 03:13am | #6

    My gut says no problem. Are you replacing the subfloor with 3/4" plywood? If so this gives additional strength by distributing weight to the adjacent joists. Bridging helps too.

  5. andybuildz | Dec 31, 2003 06:22pm | #15

    I did it in my last house and was heavier than that with all the marble and mud job under the platform it sat on.

    I used 2x12's 12 OC dbl'd up...Five years and no problem at all. Sooo if I were you I'd get some lam beams or a flitch plate  between the dbl'd up 2x's

     any chance of using dbl 2x12" dbl'd up and lowering the ceiling below it a cpl O inches?

    A flitch plate tween a few joists'd work IMO if you can't lower the ceiling below.

    Be well built ; )~

                             andy

    My life is my practice!

    http://CLIFFORDRENOVATIONS.COM

  6. sungod | Jan 03, 2004 01:25am | #23

    I dont know if it applies to your tub but, for all other tubs you forgot something.  "ALL tubs need a good ledger on the wall to hold the rim"

    Did you ever see a tub pull away from the wall tile.  They forgot to nail a ledger on the wall to support the rim.

    1. WillieWonka | Jan 03, 2004 07:47am | #24

      Except that this is a fiberglass tub, and ya don't dare support the tub on the rim. My tub is on a big mud base.

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