Not sure how it happened but my inclination is dry Rot. I don't see much of any mold or mildew around and the room does not feel damp or smells moldy so I'm really not sure if fungus is the cause. I'm trying to finish the space and frame a wall under this joist but I'm afraid that I won't be able to nail into it since it looks like shredded paper. I looked up some techniques to strengthen floor joists so my main ask is:
What is this? How does it happen? And how can I repair it DIY? (Some people online recommend consulting a structural engineer, but I'd rather not if possible) I was thinking of sistering the joists, but with it being a pocket joist I'm not sure if that's ideal.
Added a second picture for what I think is the dry Rot, but it looks small and not dangerous?
Thank you so much for this response. This is the kind of details i needed, thank you so much again!
They definitely picked THE worst spot. I figured pocket joist were gonna be more complicated than your average joist lol.
No worries, my house is from around the same period, and there's always something needing fixing.
Like I said, I don't think you'll need a structural engineer or anything that drastic. The hardest part would be cutting the old joist back and joining it with a new head to sit in the pocket. Supporting the joist should be pretty easy, they'll just jack it up a little and frame a temporary support wall.
But yeah, I'd definitely would get someone to spray for termites, and probably call around to local builders to see if anyone has any experience with repairing pocket joist. If your worried about structural integrity, you might want to shift any heavy furniture from the floor above the joist. Good luck with everything, hopefully it's not too pricey!