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I have some old termite damage to the end of a 2x12 floor joist. One end is sitting on a metal beam running along the centerline of my two story house (that sits on the foundation walls). The other end is sitting on the sill plate. No termite damage to sill plate, a small bit on the rim joist, but the end of the floor joist is basically eaten through for about 2 inches horizontally. So solution is to sister a new 2x12 joist, correct? Do I need to sister a full 16 foot joist, or can I get away with an 8 foot or 10 foot span (due to limitations in my vehicle capacity...). Plan is to apply 1-2 tubes of PL premium, knock sister into place and fasten with three 3" #10 screws on vertical lines every 12". For the rim joist, figured I can just install a piece of blocking over the sill plate, essentially sistering that part of the rim joist?

Photos added. Ignore the spray foam to keep pests out. Obviously not a solution.

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  • How about a picture so we can see what we're commenting on.
    – JACK
    Jan 9, 2022 at 14:56
  • Done see above. The span is 16 ft
    – Chris
    Jan 9, 2022 at 17:59

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The elements you’re referring to do two different tasks: The floor joists primarily hold the building “up” and the rim joists primarily hold the building “down”, especially if you’re located in a high wind area or seismically active area. (I know I’ll get criticized for my rim joist remark because it supports the wall above it, but it’s not critical to the structural design.)

Without doing any calculations, I can tell you that the floor joists need about 2” of bearing. Assuming that’s a 5 1/2” sole plate and you subtract 1 1/2” for the rim joist and subtract 2” that you indicated was probably lost to the termites, then you have about 2” left for bearing.

I’d check by using a pick to make sure you have adequate bearing at each plate. (I’d check the plate too.)

The rim joists are less important structurally. It’s often used to nail the ends of the joists into it, but is not required, (blocking is required if the joists are 2x12 or larger.)

However, if you live in a high wind area or seismically active area, then it helps create that shear nailing flange needed to hold the building in line. (The sole plate is far more important and must be checked to make sure it’s adequate to attach to the bolts.)

Sistering joists and replacing rim joists is a lot of work. Before I started that project I’d check these other issues.

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  • Added photo. I cut away the foam to expose the rotted damage. I actually think this was more water rot than the termites. Part of the joist isn’t sitting on the rim at all but part of it is. There’s no visible damage to the sill plate.
    – Chris
    Jan 9, 2022 at 18:16
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You might want to look at this question, Sistering Floor Joists, where my son had a similar problem to the one you're facing.

We ended by using a sister along ~8 feet running out from the "bad" end. We would have liked to sister (or replace) the entire 2x10's, but there was no room to work a 16' 2x10 up along side of the existing joist.

If you have the space to it, I would sister the entire joist, from sill plate to the beam.

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