Problems Sistering Floor Joists...


Old 03-19-07, 08:03 AM
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Problems Sistering Floor Joists...

So I am sistering some 2x8s to my 1st floor joists to firm them up and take some sag out of them. I am jacking the old joists up 1/4 of an inch in the middle and then putting 2 sections (I can't fit a full length sister joist in there) of 2x8s along side the old-nailing and screwing them to the old and also putting in 5 bolts with washers to clamp in on them (the joist is 12 feet long).

I did a few of them and everything was fine-then this last one I did the new joist didn't stay where it was-when I took the jack out the joist still sagged. I am wondering-what did I do wrong? Are more bolts necessary? If anyone with any info or experience in this could help me out and give me a play by play it would be appreciated. thanks.

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Old 03-19-07, 03:05 PM
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You can't sister joists successfully by cutting the sister and expecting it to maintain rigidity. The weakest point is the place where the sister is cut. Since you can't get in there with a full joist, you may want to consider installing posts and beam across the joisting, raising the beam off the posts via shims to accomplish your lift.
Old 03-20-07, 10:51 AM
Join Date: Feb 2006
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without a doubt, placing support at the beam midspan is the best solution. However, IF this is not possible and IF you cannot sister the full length of the joist, it would seem to me to be an acceptable solution to sister the joist in three (3) sections. The split between the sections would occur somewhere between 21% and 25% of span length for the first split and 75%-89% for the second split. The split points for the sister would roughly correspond to the points along the span of zero moment (and therefore zero bending stress). Be sure to glue / screw and bolt properly to get good force transfer between the old and new joist.

This analysis is based on beam calculations with y-support and some moment transfer at the beam ends.

Before you give this a try - let's get some feedback from others on the forum to see what they think.

Just out of curiousity, where did you have the split in the sister on your first attempt?
Old 03-21-07, 06:45 AM
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Originally Posted by rjanscha View Post

Just out of curiousity, where did you have the split in the sister on your first attempt?
Both of them are in the same spot-about 3 1/2 feet from the end of the joist (the entire joist is 11 1/2 feet long-so the other piece is 8 feet).

I actually think something else is wrong-cause I was able to get a full length sister in on one of the joists last night-and I had the same problem. I think when I jack up the joist it is not crowning the joist in the middle and eliminating the "bow" in it but instead just lifting the whole joist up. This house is a pain in my butt!


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