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rjschwar 05-13-2010 04:20 PM

Sistering Next to Cracked Joist
I have a cracked joist in my basement. My house was built in 1914, and one of the 2x10 joists above the basement, under the kitchen is cracked 3/4 of the way through where there is a big knot in the wood. My plan was to sister new 2x10s on each side of the crack, extending 4-5 feet on either side of the crack, but neither end resting on any support (foundation, or main center beam).

This is about a 15' span, and my question is will this be sufficient, or do I need to run the sistered joist the full length of the span, and rest it on the foundation on one side, and the load bearing center beam on the other?

I "might" be able to get the full 15' spanned, but wiring/piping/ducting is running all over the basement, and I think even getting 10' of sistered boards in there will be difficult, and if this is all that is needed I'm not going to bother with trying to go the full length.

Also, what should I use to fasten the joists together? I was going to use liquid nails as well as either bolts or nails, and I wasn't sure which would be best.

Lastly there is maybe a 2 inch crack (at another smaller knot) in this same joist at right near where the joist rests on the foundation. Is this something that should be addressed as well? For this one if anything needs to be done could I just rest the one side next to it on the foundation, and the other side extend 4ish feet past the crack?



tpolk 05-13-2010 04:22 PM

should be fine with thru bolts

Daniel Holzman 05-13-2010 05:13 PM

Personally I like bolts, I typically use 1/2 inch diameter through steel bolts with nuts and washers. Glue is not necessary, if the bolts are spaced about 6" OC, are predrilled, properly nutted off, and staggered (1 in upper third, next in lower third of joist). This is overkill, but I don't believe most codes are going to give the sizing and spacing for sistering joists using bolts, so this should be acceptable. Your overlap is OK, and it is not necessary to extend the sistered joist to the wall, unless there are other problems with the damaged joist.

By the way, some jurisdictions may require a building permit, you might want to check.

rjschwar 05-13-2010 05:19 PM

Thanks for the responses. Dumb question...what is OC? off center? I assume you are saying bolt upper 1/3rd, 6" over, bolt lower 1/3rd, 6" over upper 1/3rd again. Is this correct?

What about the crack at the end of the joist?



Jim F 05-13-2010 06:18 PM

OC is on center. It seems like a good idea to sister that area as well. Some on here have mentioned sandwiching a piece of plywood in between the sister and the original joist if bounce is an issue.

rjschwar 05-14-2010 09:44 AM

So I found some 1/2" 6" bolts, but they are threaded the full length of the bolt rather than just the last 1"-1.5". The ones that aren't threaded the full length don't quite fit. The 5" is too short, and the 6" is just a bit too long as I could hit the end of the thread. I couldn't find any 5.5". I'm sure I can from somewhere around here, but will the fully threaded bolts have the strength (shear?) necessary for this application?



Daniel Holzman 05-14-2010 12:25 PM

Fully threaded bolts are fine. In the event that you have to shorten them due to access issues, you can grind the end down on a bench grinder, then touch up the end of the bolt using a 1/2 inch die. Or you can cut half an inch off with a Sawzall, and use the die to fix the end.

If you don't need to shorten the bolt for access, leave it long, no problem. You can double nut the end to effectively eliminate the potential of a nut backing off due to vibration, or you can use a lock washer to accomplish the same thing.

And yes, I meant to stagger the bolts upper third, lower third, upper third etc. By the way, I usually use two bolts on either end, one in the upper third, one in the lower third. Never install a bolt closer than bolt diameters from the edge. And make sure you overdrill the hole slightly, the bolt should go in freely. Snug the bolts up using a ratchet, no need to overtighten. If you use a lock washer, when the washer is squashed flat, you are good.

As for the knot on the end, if it is not cracked through, it is probably not an issue. If it is really bad, sister a 4 footer on that end. The end only carries shear, no moment, so is generally much less critical than the center, where bending moment is maximum for a joist.

xxPaulCPxx 05-14-2010 09:37 PM

Honestly, that joist scares the bejeebees out of me... cracked in two places in an area with heavy appliances and lots of traffic.

If it's exposed wire and pipe you have, that's actually pretty easy to cut and repair. Ducting can be temporarily disconnected too.

If you really want to do this right, here is what I would do:

1st, Don't do this if there is a wall directly above, it will require straightening the floor above which will do harm to a wall that has settled in.

Get an accurate measurement of this beam - what are it's EXACT dimensions. Chances are it's closer to 10" than a modern 2x10. If that's the case, you should start with a 2x12. Whichever beam you get, buy it and bring it downstairs to dry out before you do anything else. Set it on something straight, flat, and dry (NOT the floor!), and make sure it has room 3/4" underneath for air to flow through. What you are doing is drying out the wood AND shrinking it. It won't shrink much lengthwise, but it will shrink on its width and depth axis. A $2 moisture meter from Harbor Freight will help you find when the wood hits >19% moisture, at this point it's stable. At this point, you can cut it to the exact dimension.

Cut the wires and pipes (shut them off first, silly goose!). When you reconnect them you can make sure they run through the center of the joist if any were notched. Pot a small box one each side of the joist for wires with a new wire connecting between, and use Sharkbite fittings to easily reconnect your copper water pipes.

You will have to do some creative jacking up of the low points, and some creative swearing as you angle the new joist into place.

After you set the joist and lower the jacks, you can toenail the joist to the floor above with screws.

When you are done, you will have a floor that will last another century.

mrgins 05-15-2010 04:24 PM

You never mentioned what is above this joist other than a kitchen. Options, depending on load rating are (in order of strength): another 2x10 just tacked to the split one, two 2x10's, as before with screws and glue or bolts. as before with plywood stiffener, microlam, steel flitch plate with 2x10 all bolted together.
Talk to an engineer if there is a heavy load bearing on this joist

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