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I have followed a couple of the threads where sistered joists were discussed and I have a more general question: how do you know when sistering is needed?
Stepping back, my place is owner built (not me), probably to code from other evidence. Joist size, span and spacing fit the tables. Then some other owner drilled a lot of holes in the lower 2in for wiring. Obviously someone thought strength had been compromised because they installed a screw post and a 2×8 improperly sistered to one joist. Plus there are several short sections of semi-bearing stub walls where the table says they are unneeded.
I am going to be doing a major reno this summer and if the joists should be strengthened, I’ll do it. But how can I tell if it is truly needed? I have thought of pulling the stub walls and the post to see if deflection changes but is there any kind of rule of thumb like “1 bad hole mid span = 2 bad holes outer thirds = gotta sister”?
I also noticed the warning “don’t glue and screw, bolt.” Before I read that I figured I could plate both sides of the holes with 4 foot strips of 3/4 ply glued and deck screwed. Assumed it would act more or less a web joist. Bad thinking?
So I’ll ask the question again, in the absence of the obvious (breaks, rot, careless plumbers,) how do you know when sistering is needed?
Bruce
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Don't do this to yourself, Bruce. There are people who still love and care very much for you. If you do this, they might be gone forever, and you will only have yourself to blame.
*Sistering is needed when the wood breaks and not a moment before.I don't know how close your house is to failure and I don't care to inquire.Find a code book for your locality and make sure each joist and all joists as a whole meet code. This might meen replacing joists or perhaps doing a reasonable job of sistering, or even doing nothing.It is possible that even with the holes and the other defects the house is safe and too code.
*If you remove the stub walls and the floor feels too spongy you might want to sister. Also if during remodeling something very heavy ie. cast iron whorl pool tub,will be placed there. You probably need to sister. Aside from these I say listen to George.
*The simple answer is have an engineer look at. There is no simple rule-of-thumb for this. Things to consider are that comprimising one joist may not be a problem. The load will spread to adjacent joists. If all the joist have holes in them, then it may be a problem. The deflection is proportional to the moment of inertia (engineer's term here), which is proportional to the depth of the joist cubed. It also depends on where the hole is located - middle of the depth, top or bottom.