joist sistering

I have a small cabin 20x24. The floor joist system is comprised of 2x8x10 joists resting on outside wall and a center beam. The ends of each joist are notched with the equivalent of a 2x4 tongue resting on the walls and center beam.
Joists are spaced 24" o.c. for the most part. Subfloor is 3/4" plywood.
I would like to reduce/eliminate floor deflection sufficiently that I will be able to install ceramic tiles with an appropriate backer board.
I have thought of installing additional 2x8x10 joists with all joists 12" o.c. Unfortunately the availability of good 2x8's just is not the case.
I'm looking for options:
1. Instead of 2x8's positioned between existing joists, could I use 2x6's?
2. Could I sister 2x6's with existing 2x8's?
3. Would sistering 2x4's with 2x8's provide sufficient stiffening?
Thanks.
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franz fripplfrappl wrote:

Answer to all probably, you might look at blocking to. Blocking between joists also help to reduce deflection. If you can sister existing then that would be the answer. All that you describe above would be better then what you have now.
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evodawg wrote: ...

W/ the likely exception of the 2x4 sistering option, I doubt it would make much difference altho I didn't go do the calculations to confirm my hunch.
My initial and still over-powering reaction was what is so hard about obtaining 2x8-12's altho they may be overkill.
What OP needs to do to get a guesstimate is to use one of the online beam deflection routines and get a feel for the differential deflection computed for the various options.
I also didn't do a calculation but I suspect the blocking option alone won't make a real big difference either owing to the fairly short span and relatively short vertical difference--it would be more effective for 2x10 and larger and longer spans. Will make some difference, true, but again my hunch is not as much as might be desirable.
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Is this over a crawl space? The joists are notched on the bottom edge and reduced in depth from 7.25" to 3.5"? That's an excessively large notch and it represents a potential failure point. Is there any splitting of the joists at the inside corner of the notches?

One simple option, depending on clearances, would be add two more girders (support beams), to reduce the joist span in half. But this would only be a good option if you could avoid further notching of the joists.
Cheers, Wayne
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Wayne Whitney wrote: ...

Actually, that's a most excellent idea, Wayne... :)
And, if it is crawl space, could simply pier it in a few places (every other or every third, say) and not need to worry about the height issue. Would need an adequate base for each pier, however, of course...
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On Tue, 27 May 2008 12:27:57 -0500, dpb wrote:

Shoring up the floor as described is OK but unfortunately I do not have the space nor do I care to clutter the already small 20x24 basement with extra support columns.
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On Tue, 27 May 2008 17:12:22 +0000, Wayne Whitney wrote:

There is a full basement in the building. In fact, a wood foundation basement.
I have not noticed any checking of notched ends.
There is some blocking midway between existing joists.
Building was built about 25 yrs ago. Most likely by a reader of Mother Earth News who smoked more than he read.
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As far as the notches go, if there is no splitting at the insidec corner of the notches, then there is not currently an imminent problem. But it is a poor condition you might wish to consider addressing.
As for stiffening the floor system, simplest and easiest would be to interleave 2x8x10s with existing ones, ending up with 2x8s at 12" o.c. and halving your deflections.
Interleaving 2x6s wouldn't work very well--a 2x8 is 3 times stiffer in bending than a 2x6 of the same grade and species. So assuming equal deflections of the two members, the 2x8s will take up 3/4 of the total load, and you'll only reduce the deflection by 25%.
Wit care, you could sister a 2x4 or 2x6 to the 2x8s. First, you should put them at the top edge or bottom edge of the existing joists. This will be stiffer than centering them. Given that the existing joists are notched on the bottom edge, you should choose the top edge for the sisters.
Second, you would need to somewhat unload the existing joists before attaching the sisters, using a temporary wall or jacks. Otherwise the initial nail slip in the connection between the two members means that you won't gain very much marginal stiffness in response to live load, which is what you want to avoid tiles cracking. By unloading the existing joists first, when you then reload the sisters with the dead load, the initial nail slip should be taken up by the dead load, so that for additional live load you will gain the full stiffness possible.
Lastly, as another sometimes poster here (BobK) has convinced me, you should use a construction adhesive between the new and old members, and attach them together with many small nails. E.g. 0.131" x 3" nails. This will allow the two members to work together as a composite section more effectively.
Hope this helps.
Cheers, Wayne
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On Tue, 27 May 2008 20:30:26 +0000, Wayne Whitney wrote:

Thank you for the ideas. By the time I finish this project, methinks it would have been better to raze and rebuild. There's nothing as frustrating as trying to undo and improve what someone else has done.
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"franz fripplfrappl" wrote in message

I've been saying something like that quite often lately digging out my crawl space and adding supports under the existing floor... It would have been so easy to do this stuff during construction. Why didn't they leave room to crawl in the crawl space? Arrrggg!!! (I wish I could magically lift up the house for a couple of days...)
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On Wed, 28 May 2008 04:31:38 -0700, Bill wrote:

Alternative would be to pray for a tornado to move the building to the next county. Start all over and build to correct all that had been forgotten.
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1) 10' isn't examply a "super long" span. You might actually run some tests whereby you actually measure the deflection. You might consider added another 1/2" to 3/4" of plywood on the top. This would give you "bullet proof" support in the 24" between the joists.
2) This isn't "standard" but you might consider adding a 2x4/10 to the bottom of your existing joists. This isn't "sistering" but actually converting the bottom of the joist into a "half I-beam." The connection between your added 2x4x10 and the existing joist has to be good. You might consider "glue and screws." The 2x4x10 has to be good stuff without significant knots. Just increasing the debth of the joists will reduce deflection by more than 1/3 (the strength increase with the equal of the height). With the extra wood, there might be even less deflection.

Compared to the 2x8's the 2x6's are useless.

Huh?
Huh?
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