Repairing Knothole Crack In Floor Joist

sequoia_2007August 16, 2012

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". So the repair 2x12's would be 67.5" long and be centered on the crack. I would apply construction adhesive to the mating surfaces and secure the sandwich together with 1/4" Simpson SDS screws or 5/16" GRK structural screws. How does this plan sound? What schedule (pattern) is recommended for the screws?

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

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Use 3/8" bolts instead of screws. And one board should be sufficient. Two is a bit much, but nothing wrong with doing two.

Also, use a lally column(the kind used in basements that have a screw adjustment on the top) to push up that joist just a bit before adding the repair.

Bolting pattern would be one on either side of the crack, about 6" away and about 4" up from the bottom. Two more about 4" each side of the crack and about 3&1/2" down from the top.

Then a bolt every 18" to each end, being careful to not place them on the same level as the neighboring bolts.

Don't remove the lally column for a couple days to let the construction adhesive to set well.

    Bookmark   August 16, 2012 at 9:24PM
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Thank you for taking the time to respond. The cracked knothole is within the first 1/3 of the joist span and there is no sagging of the joist so trying to jack it up would be a mistake and could case damage.

    Bookmark   August 17, 2012 at 11:00PM
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Your only problem is that a very small area of the bathroom floor might deflect slightly more than the rest of the floor and cause the tile to crack even though it has apparently not done so yet. I would put a long 2x4 against the lower side of the joist and put as many 10d nails in it as I could find.

I would also reinforce the other ones with low knots since they have little more capacity than the cracked one.

    Bookmark   August 18, 2012 at 9:51AM
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Sister with structural grade fir (no knots in vulnerable areas) with as much length as possible; best if one end had bearing, then jack the free end into place while you bolt them together. Use a glue if you wish, one that has minimal "creep" factor.

    Bookmark   August 20, 2012 at 1:19PM
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This floor is over designed for bending by a large factor so the crack doesn't reduce the load carrying capacity below what the code requires but it does allow that one joist to deflect more than the others when subjected to a very heavy load which is highly unlikely to occur in a bathroom.

It should be reinforced if you plan to add a whirlpool spa or use very large ceramic tiles.

    Bookmark   August 21, 2012 at 9:41AM
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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.

    Bookmark   August 22, 2012 at 6:55PM
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"He did the calculations and said my repair would meet code..."

"meeting code" os often not adequate for supporting tile floors.
The code allow a deflection of 1/360 of span.

Adequate for plaster and drywall (at solid wood spans) but not for anything larger than mosaic tile.

For 4-6 inch tiles, 1/480 is needed, and for stone or tiles above about 6 inches 1/720 is needed to prevent grout and tile cracking.

The I-joist manufacturers ran into trouble years ago by fabricating I-joists with spans of 14 feet and higher (rare for solid wood).

The allowed deflection of almost 1/2 inch resulted in 'bouncy' floors and unhappy customers (while still being structurally adequate to code).

    Bookmark   August 23, 2012 at 10:36AM
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Hi Brickeyee's,

His calculations are based on adding a 1" mortar bed with 3/8" tile. Our neighbors in the adjoining townhouse with the same floor plan did the same addition to their bathroom 16 years ago and it has held up perfectly. BTW, our floors are solid and not bouncy.

    Bookmark   August 23, 2012 at 9:51PM
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The building code should never be a standard or a goal for remedial work; it is a minimum standard for new construction and in the matter of acceptable deflection for large sized ceramic tile it should always be exceeded as has been pointed out already. I'm surprised an engineer would even mention the building code.

    Bookmark   August 30, 2012 at 8:43PM
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