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Repairing Knothole Crack In Floor Joist


Sequoia's Avatar
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08-15-12, 07:08 PM   #1  
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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08-15-12, 07:16 PM   #2  
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... There is no sagging of the joist and the crack does not extend past the knothole.
I would consider leaving it as is.

What are you planning to support with the Joist Trak? How much does that weigh?

 
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08-15-12, 07:30 PM   #3  
Thank you for responding.

Joist trak are aluminum plates that 1/2” PEX tubing runs through to heat the subfloor with heated water. The bathroom above is small, but we do plan to add tile to the floor that will be heated by the plates..


 
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08-15-12, 07:52 PM   #4  
but we do plan to add tile to the floor that will be heated by the plates.
Is the room already a bathroom? Will you be adding some significant new load to the floor, such as a bathtub? What do you plan to do to the floor, above the joists and below the tile, before laying the tile?

My father, who was a timberman, would have culled that piece out. A shame it made it through the grading. That said, it is "checked" - cracked less than halfway through, and not due to load stress. If you'd feel better if you sister onto the piece, go ahead.


Last edited by Nashkat1; 08-18-12 at 04:09 PM. Reason: correct typo
 
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08-16-12, 10:38 AM   #5  
I am surprised any carpenter would have used 2x12’s with such large knots in this application. Check out the 2x12 next to the one with the cracked knot.

The bathroom above already has a tub and we will add tile to the floor which covers an area 3-1/2’ x 6’. I don’t think this is a major increase in floor load, but it is directly above the crack.

 
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08-18-12, 06:33 PM   #6  
The bathroom above already has a tub and we will add tile to the floor which covers an area 3-1/2’ x 6’. I don’t think this is a major increase in floor load, but it is directly above the crack.
You will be adding to the dead load which, in general, produces less stress on the framing than the live load does.

Looking up, in your pictures, we appear to be looking at plywood. What will you be putting over that to support and bed the tile? Will that stiffen the floor?

 
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08-22-12, 03:05 PM   #7  
I'd suggest drilling a crack-stopping hole at the very top of the crack, just in case it wants to grow at some point in the future. Particularly with the thermal expansion and contraction the joist tops will experience with in-floor heat. Then just bridge the existing cracked joist, and any others with large knots below their neutral axis, using single 2 x 6s (4' +/- long) glued and screwed to one side of the joists, flush at the bottoms. Simple, cheap, and good insurance that things will perform properly.

 
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08-22-12, 03:44 PM   #8  
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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08-22-12, 04:26 PM   #9  
Glad you got it resolved to your satisfaction, and thank you for letting us know - and for sharing how you did it.

 
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