Partial joist sistering?

hearthside_im_all_inJune 2, 2009

I am trying to level the floor of my second floor bath in preparation for tile. The joist that runs under the subfloor has sagged over the years due to plumbers hacking at it. We had a contractor install a post and beam in the kitchen that now supports the sagging joist. Structurally the upstairs bath floor is solid, however it does follow the sag. The joist runs under the bath and continues into an adjacent attic void that would be a major pain to access. I'm thinking of sistering to the portion of the joist under the bathroom in order to level the floor instead of sistering to the entire length of the joist. Will this work or am I missing something?

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It worked for me over 20 yrs ago in a bedroom. Due to kneewalls and a 45 degree slope on the ceiling I couldn't go all the way to the end. Floor is solid and level.

    Bookmark   June 3, 2009 at 7:54PM
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It can work, depending on the reason for the sag in the first place.

It also takes a pretty decent number of nails to make sure the two pieces of wood act together uniformly.

The first thing you should do is measure the existing joists and consult a span table to see if they were adequate in the first place.

    Bookmark   June 4, 2009 at 3:30PM
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Thanks folks, for the feedback. It's good to hear that I'm on the right track. The contractor who installed the beam in the kitchen below (we had to replace our ceiling, which led us to identify the sag initially) was fairly confident that the sag was due to the couple of cuts made in the joist to run plumbing back in the day. The house was built in the 1920's and the rest of the second floor is level, so hopefully that was cause. It sounds like we should check the spans to be on the safe side. We got a good view of the bottom of the joists during that project, but unfortunately didn't realize we'd need to work on the upstairs floor too - too bad, now we can only work on the joists from one direction, above.

What advice do you have about hardware? I don't have a compressor, so I'll be pounding in nails old school. Some of the research I've seen suggests using bolts. Are there pros/cons I should know about when deciding to use nails versus bolts (or a combination?) for this project?

Thanks again!

    Bookmark   June 4, 2009 at 6:34PM
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Screw and bolt is the way to go. We had plaster ceiling so nailing would have resulted in a lot of cracks.

    Bookmark   June 4, 2009 at 9:48PM
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you can use screws and bolts, but the spacing does not change all that much.

Bolts larger than 3/8 do not help much.
If you space larger bolts out based on the bolt strength you will exceed the compressive strength of the wood.

Nails from a nail gun are less damaging to plaster then using a hammer.

    Bookmark   June 5, 2009 at 12:52PM
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I'm not quite clear on how the hardware type would affect plaster - the sistering would be covered by the subfloor, so you're putting the final surface on after sistering is complete. Are you talking about the lower surface of the joists on the opposite side from the sister, i.e. potential damage to the kitchen ceiling below? Good point - didn't think about how there could be some vibrations.

    Bookmark   June 5, 2009 at 7:21PM
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