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Old 12-23-2013, 04:43 PM   #1
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sistering floor joists

Hello all my name is Travis and I'm new to the forum.
I am looking for advice/guidance on sistering up some of my floor joists...

1 joist has a crack that formed at a knot and has run amuck, rest of joists in the area look good. There is a small amount of sagging caused by cracked joist and I am in the process of slowly jacking it up now.

Additionally the floors throughout the house seem pretty bouncy. The joists are 2*8, 16' o.c., and span 13' (basically 13' from bearing wall to 4*8 steel beam then 13' to bearing wall).

I was hoping to sister the existing joists with new 2*8 d-fir, but I will have to put each new joist in as two pieces 9' and 4' or I can do some finaglin and maybe put em in as 10' and 3'. Basically I will have to butt the 10' and 3' pieces together but this way one end will land on bearing wall and one end on beam).

I plan on glueing, nailing, and bolting each new joist to the original. My question is will doing it this way add anymore strength to the overall floor above and beyond if I just sistered the existing joists with a 9 or 10 foot board splitting the difference on the ends and not reaching the support points? I will be removing the drop ceiling in the basement to do this work and will remove all ductwork, wiring, bracing,etc. necessary, however the way the previous owners chopped up the basement with the rooms they built prohibits me from manuevering anything longer than 10' and even at that length I will have to be doing a little dry wall work.

I am just trying to think ahead to the future when the wife tells me she is tired of carpet and wants tile which is out of the question with the joist size, spacing, and span right now. Thank you for your time and insight. Travis


Last edited by oh'mike; 12-24-2013 at 07:38 AM. Reason: added spaces
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Old 12-23-2013, 05:43 PM   #2
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Need pictures.


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Old 12-23-2013, 05:47 PM   #3
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It would be far better if they ran the fill span.
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Old 12-23-2013, 06:43 PM   #4
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You don't necessarily need to span the whole distance if the sistered member is glued and screwed correctly. But you might want to consult a structural engineer since 13' span on 2x8 is probably over the allowable span for that size member. Forget using two pieces
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Old 12-23-2013, 09:35 PM   #5
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i have sistered 2x8x14 floor joists, bearing on both ends.

we need pictures .
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Old 12-23-2013, 09:56 PM   #6
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You get most of the strength from sistered joists by properly fastening the sistered board to the old board. Since the maximum moment on a uniformly loaded beam occurs at the center of the beam, it is most important that the sistered board cover the center of the beam, so it is best to center a short board about the center. It is possible to compute the horizontal shear, and develop a fastening schedule, but this is almost never done. Most carpenters used tried and true fastening methods, typically 10d or 16d nails spaced 6 inches OC, staggered top and bottom. Bolts can also be used, but this is not necessary.


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