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Old 08-15-2012, 09:01 PM   #1
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Repairing Knothole Crack In Floor Joist


While opening the ceiling in our garage to install Uponor Joist Trak under the subfloor of a bathroom above I noticed that a crack had developed in a 2x12 joist knothole. The unsupported joist span of the 2x12 is 19’ and the ends rest on 2x6 plates. The crack is located 60” (5’) from the wall. The crack extends up 3-1/2” on one side of the joist and 2” on the other side. There is no sagging of the joist and the crack does not extend past the knothole.

I was thinking of sistering two 2x12 boards, one on either side of the joist, that extend 300% of the depth of the 2x12, extending out from both sides of the crack. That would be 3 x 11-1/4 = 33.75” x 2 = 67.5”. I would tie the sandwich together with 1/4” Simpson SDS screws or 5/16” GRK structural screws. How does this plan sound? What schedule is recommended for the screws?

Thanks for taking the time to read my post and consider my questions.




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Old 08-15-2012, 09:10 PM   #2
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Repairing Knothole Crack In Floor Joist


Go buy a 2 X 12 X8, a small box of 3" ceramic coated deck screws and a tube of constrution adhesive.
Cut the 2 X 12 in half. Apply constrution adhesive on both pieces and screw in place, use plenty of screws.
Call it a day.

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Old 08-15-2012, 09:38 PM   #3
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Repairing Knothole Crack In Floor Joist


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Originally Posted by joecaption View Post
Go buy a 2 X 12 X8, a small box of 3" ceramic coated deck screws and a tube of constrution adhesive.
Cut the 2 X 12 in half. Apply constrution adhesive on both pieces and screw in place, use plenty of screws.
Call it a day.
Thank you for responding.

I have read where others have used construction adhesive and I would consider that, but I only want to use structural screws for the sistering. Just not sure of the screw schedule to use. I think I read somewhere to use 6” on center, but I imagine I have to stager them so they are not in direct line with the screws from the other side.
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Old 08-15-2012, 10:07 PM   #4
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Repairing Knothole Crack In Floor Joist


Your way over thinking this simple problum and trying to get around using the constrution adhesive that will add about 25% more strength and will cost less then $5.00.
I'd premark the board roughly where the knot is so I did not try and screw into it, and start adding screws top bottom and middle just slightly lower of higher so there there not all side by side in a straight line.
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Old 08-16-2012, 12:28 PM   #5
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Repairing Knothole Crack In Floor Joist


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Originally Posted by joecaption View Post
Your way over thinking this simple problum and trying to get around using the constrution adhesive that will add about 25% more strength and will cost less then $5.00.
I'd premark the board roughly where the knot is so I did not try and screw into it, and start adding screws top bottom and middle just slightly lower of higher so there there not all side by side in a straight line.

I am fine using the construction adhesive with the structural screws; I just need a schedule for the screws. I think I will try contacting my county planning department to see if the CA Building Code specifies a schedule for screws in this application.
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Old 08-16-2012, 12:45 PM   #6
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Repairing Knothole Crack In Floor Joist


Fire4effect - fwiw, i just had a structural engineer tell me to sister on 2x10s to my existing 2x6 joists. He slated the install for 2 12d nails spaced 16" apart, going the length of the board. In addition, I used construction adhesive.

From what I understand, its much better to go with nails rather than screws for sheer strength. I am sure the structural screws are great, but why bother when nails are known to be very strong, and take no time at all to install with a gun? I would sister the joists together with glue, clamp them tight, and nailgun the 12d's through.

I have also read that it can be good to nail in a "W" pattern across the joist to prevent cracks from forming, but my SE said it didnt matter. There are a million posts on here, and the internet in general for sistering joists. You will find a lot of different ways to do it, which leads me to believe that exact "patterns" dont matter very much.
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Old 08-17-2012, 10:08 PM   #7
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Repairing Knothole Crack In Floor Joist


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Fire4effect - fwiw, i just had a structural engineer tell me to sister on 2x10s to my existing 2x6 joists. He slated the install for 2 12d nails spaced 16" apart, going the length of the board. In addition, I used construction adhesive.

From what I understand, its much better to go with nails rather than screws for sheer strength. I am sure the structural screws are great, but why bother when nails are known to be very strong, and take no time at all to install with a gun? I would sister the joists together with glue, clamp them tight, and nailgun the 12d's through.

I have also read that it can be good to nail in a "W" pattern across the joist to prevent cracks from forming, but my SE said it didnt matter. There are a million posts on here, and the internet in general for sistering joists. You will find a lot of different ways to do it, which leads me to believe that exact "patterns" dont matter very much.
Thank you for taking the time to respond. Your comments are helpful, but I think I will stick with the structural screws. I checked the specs and the structural screws I want to use far exceed the shear of 16D nails, plus they draw the two pieces of lumber together when being secured.
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Old 08-22-2012, 08:29 PM   #8
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Repairing Knothole Crack In Floor Joist


I decided to contact a structural engineer to make sure my plan would meet code. He did the calculations and said my repair would meet code with his schedule for the 1/4” x 3” Simpson SDS screws. I installed the repair pieces yesterday and now I have piece of mind knowing I don’t have to worry about further deterioration of the joists.

Thanks everyone for your comments.




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