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Discussion Forum

Add wood to rotting joist?

| Posted in General Discussion on September 12, 1999 06:37am

*
I’m in the process of fixing a bathroom in my 130 year old
house. I finally got the tub out and ripped up enough floor
to look at a joist. Directly under the tub there is some water
damage to both the planks and at least one joist. The planks I’m
intending to replace, but replacing the joist would be a huge
job. The joist was originally about 3″ by 8″. The top
2″ or so are rotten in places.

Is it reasonable to bolt or other wise attach new wood to to the top of the joist after I’ve removed the rotten material?
The floors feel solid, and the bottom six inches of the joist
are okay.

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Replies

  1. Guest_ | Sep 02, 1999 06:37am | #1

    *
    As you suggested, I'd cut out and rotted portion of the top of the joist to get to good wood. You could cut and fasten (use adhesive as well as nails/screws) a filler piece on top to replace the rotted section.

    This scabbed on piece, while bringing the joist top back to it's original height, won't come close to bringing the joist back to it's original strength.

    For added strength I'd consider sistering a piece of 2-by stock to one side, maybe even each side of the once-rotted joist. With the 3-by-rotted-8 it may be overkill, but without seeing the extent of the rot and damage, and without knowing what length of the joist is damaged and the joist spacing it may be the better way to go. Especially under a tub. The sistered piece doesn't neccessarily have to go from plate to plate. If desired you could, however, have the 2-by on one side bear on one plate, the 2-by on the other side bear on the other plate. Use adhesive and plenty of nails.

    Good luck...

  2. Frank_LaPiana | Sep 02, 1999 07:54am | #2

    *
    I had the same problem last year, but more than one joist. Three under the bathtub were water damaged and rotten.

    I sistered on *both* sides of the bad joists (removal and replacement was not an option). I ran the sister beams from the plate (second story) and back eight feet.

    I don't think adhesive is necessary; I did use screws though.

    Most headers (double beams over windows and doors) are just nailed together and that was always deemed sufficient.

  3. Guest_ | Sep 03, 1999 07:15am | #3

    *
    BTW, if you use screws use wood screws, NOT drywall screws.

    1. Guest_ | Sep 03, 1999 09:30am | #4

      *righto, NEVER use drywall screws for shear applications. - jb

  4. AmyS | Sep 03, 1999 07:40pm | #5

    *
    I was thinking more along the lines of lag bolts. I
    really don't want it moving around.

    Well, that solves one issue. Next: how to remove the
    ceramic tiles that were around the tub so I can find
    out how extensive the rot in the floorboards is. Sounds
    like no fun at all - and afterward, figuring out
    what to do with the floor. Remove all the tiles and put
    down other floor covering? (Including removing the asbestos
    tiles underneath? I'm already removing those under the tub to
    get at the wood floor.) Try to find matching ceramics?

    I guess that can wait until I have solid wood down.

  5. Guest_ | Sep 04, 1999 08:00am | #6

    *
    I'd vote for nails in shear, at least from the numbers I've seen and assuming you have access to a gun. Stagger them so as not to weaken the wood. A 16d can do something like 500 lbs in shear?

    1. Guest_ | Sep 08, 1999 07:47am | #7

      *I may be broken record on this, but if you can't conveniently clean out all of the dry rot, I have had good luck with the epoxy specifically formulated to repair dry rotted wood. It is as thin as water and will penetrate a ways into dried-out dry-rotted timbers. For deeper penetration, holes are drilled and the epoxy mixture is injected into the holes. I can't remember the vendor of this stuff, but you should be able to find it by doing a web search on "dry rot".

      1. Guest_ | Sep 08, 1999 08:42am | #8

        *I'd also prefer to see many fasteners (nails or screws) instead of just a few lag or carriage bolts in a situation like this.

  6. Fred_Matthews | Sep 08, 1999 10:17am | #9

    *
    I wouldn't count on epoxy for any structural value. Epoxy works great at replacing water/rot damaged, non-structural members. I would shore from below the joists, remove material down to competent wood, and sister on one or both sides and definitely use
    b glue and screws.

    I would use a good urea/formaldahyde like Weldwood Plastic Resin Glue. By using glue, and carrying the sistered member as far as possible in both directions, you are maximizing the transfer/sharing of load to the sistered joist.

  7. Guest_ | Sep 08, 1999 04:16pm | #10

    *
    I agree with Fred; glue (epoxy or UF, not PVA, which creeps), and screws, not nails. Might as well sister both sides now.

    The epoxy consolidant might still be a good idea in marginal areas, in case you didn't get all the rot, and it starts up again after the foor is closed. Abatron is the brand I use.

  8. AmyS | Sep 08, 1999 07:47pm | #11

    *
    Where might I buy the epoxy consolidant? Would it be available
    at the megahardware stores like Lowe's or Home Depot, or should
    I trot over to tiny little Wood World?

    A question about the rot: once I remove the rotten wood,
    and the source of the problem (incorrectly installed bathtub
    walls - ran all the way to the baseboard, and so did the
    water) shouldn't the rot cease? I mean, shouldn't it cease
    even if I didn't get all of it? I thought the water coming
    in was the primary cause.

  9. Guest_ | Sep 08, 1999 09:17pm | #12

    *
    As far as buying the consolidant, I doubt the chains would have it, but I don't have any nearby, so I don't know. Abatron usually advertises in FH, or for small quantities Lee Valley Tools carries it.

    As far as the rot goes, yeah, you'd like to think if you remove the bathtub problem, the rot problem goes with it. Depends on what type of rot it is (which depends in part where you are). Basically 'rot' is the action of wood destroying fungi and bacteria. If the spores are there, all they need are the right conditions to grow: the correct balance of oxygen, food (your wood), moisture , and warmth. Correcting the moisture problem is the best thing you can do, but some of these fungi, merulius lacrymans, for example, can generate water if it has food, and generate food if it has water, and it can get right into masonry walls as well, if there are any nearby.

    I don't mean to panic you, just to suggest that while the floor is open, and before you retile it or whatever, you do what you can to get rid of the fungi causing the rot (you can kill it with alcohol , for example, if you see any other mold starting) and do what you have to to keep from creating an environment where a new fungal attack can occur. Any basic wood technology book in your local library will have info on types of rot.

    1. Guest_ | Sep 09, 1999 07:03am | #13

      *I bought the epoxy mail order from:http://www.rotdoctor.com/ They have some interesting information on their web site, so you might want to take a look even if you don't buy their stuff. They do have a price list on their site, also. For the stuff I did, in addition to the epoxy and hardener, I bought the injection kit (basically a large hypodermic syringe with needle) and the solvent for cleaning the syringe after use.I don't know if most marine supply stores would have similar stuff or not. I know the West system epoxy materials were developed for marine use, but are used in a variety of other situations.Also, you may want to check out the article "Chemotherapy for rot" athttp://boatbuilding.com/content/rot.html

  10. Frank_LaPiana | Sep 09, 1999 07:17am | #14

    *
    Killing rot/preserving wood: I have read that glycol-based antifreeze will do the job. There's also something called "bora-care" that's borax/glycol. I didn't bother. My beams were water-damaged but not actively rotting. I can see the beams from underneath (my wife LOVES that in her kitchen!) and there's been no further deteriation. Serves her right for wanting to "re-arrange" the bathroom, hehe... Anyway a complete kitchen re-do is the next project.

    Oh, maybe someone knows why this was done.... when I tore up the old mud floor, there were ledges half-way down the beams. The cavity was filled with hundreds of pounds of cement pieces and cinder-like material. I can't figure out for the life of me why anyone did this.

  11. Guest_ | Sep 10, 1999 06:59am | #15

    *
    Nails do provide a better shear strength than screws. Bear in mind that too many fasteners of any kind can actuay compromise the strength of the supporting memeber. If I wer to use bolts ( I wouldn't, me & Mr. air nailer for this problem) I would through bolt, not lags.

    1. Guest_ | Sep 10, 1999 03:15pm | #16

      *Frank Dropping ledgers down was common around here. this was to get a little extra thickness to the mud base. Dont know why they wee dropped so far down. or why debris was in there. DIY project or incopentent contractor when done?

      1. Guest_ | Sep 11, 1999 09:46am | #17

        *Maaybe not so incompetant. . . perhaps he just needed a home for that debris. The man was ahead of his time, it was Thermal Mass (that's just down the road from Boston)-pm

  12. Frank_LaPiana | Sep 12, 1999 07:14am | #18

    *
    Re: ledges and debris....

    Well, since it was about 4 inches thick, I did think of thermal mass! I also thought maybe it had something to do with orgonism, or sound deadening.

    It was a professional mud base and tile floor though, quite nicely done.

  13. Guest_ | Sep 12, 1999 10:07am | #19

    *
    OK, back to nails vs. screws: I still have the impression that screws are mediocre in shear, both because they are hardened and because of their threading(?). Aren't nails req. for shear panel assemblies?

    Anyway, with a nail gun it's no contest which I'd pick. :) It is important not to get so carried away that you weaken the boards .. staggering and spacing are vital, so I hear.

    1. Guest_ | Sep 12, 1999 06:37pm | #21

      *I don't know where to put my hands on on a comparison of shear strength between nails and screws. But, drywall screws excepted, screws are plenty strong. And they do make them in bigger gauges, so you can size the screw to the shear strength required. The big advantage (and it is a big advantage), is that when the new material drys and shrinks away from the fastner, the screw threads are compromised less than the shank of a nail. As soon as the frictional force on the nail shank is weakened, so is the nails performance (although in this case, if an adhesive is used, this may not be that important). A gunnail does a better job in this regard than one that is hand nailed.Just my opinion; nails for speed, screws for piece of mind. Often use both in combination.

  14. AmyS | Sep 12, 1999 06:37pm | #20

    *
    I'm in the process of fixing a bathroom in my 130 year old
    house. I finally got the tub out and ripped up enough floor
    to look at a joist. Directly under the tub there is some water
    damage to both the planks and at least one joist. The planks I'm
    intending to replace, but replacing the joist would be a huge
    job. The joist was originally about 3" by 8". The top
    2" or so are rotten in places.

    Is it reasonable to bolt or other wise attach new wood to to the top of the joist after I've removed the rotten material?
    The floors feel solid, and the bottom six inches of the joist
    are okay.

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