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?
Shredded paper and little dried mud spots. That's old termite damage. I'm not sure how to fix your structural issue, but you'll need to see how much damage there actually is. Does the wood around that area feel solid or soft?
Still feels fairly solid, although near the shredding it kinda sounds hallow, I could be wrong through, I might not be hearing it right. The other joists don't have any damage whatsoever, they sound solid. This is a 50's house, so it feels like whatever damage happened it was years ago. I'm not sure where to go from here, sistering two joists sounds like it can help structurally, but I'm just not sure if that's the right move... Plus I have pocket joists so I'm not sure what kind of madness is involved taking the old one out and putting the new one in
Def termites...... you're probably going to have to see how it looks behind that wall. Hopefully it's just a small spot from a long time ago, but I'd be looking at all the joist near by.
Joists nearby sound solid, no visible termite damage that I can see. Do you think the joist is still somewhat structurally sound? Any recommendations on supporting this joist or helping prevent future termites?
I would call an exterminator to come and spray for termites to make sure. It's just something you need to do every 5-10 years.
If the joist isn't sagging under weight, I would just reinforce it with a sister joint. They're pretty simple and there's lots of guides online. If the joist is sagging you're going to have to call a pro.
The problem is that it looked like the damage extends all the way into the wall, so it's hard to see how long of a support joist you would need. I would be doing my best to see if you can cut out some of that wall and see if you can see where the damage ends.
Edit: sorry looked at the pic again, did not realize you had said pocket joist.... that's different
You would have to do something like frame a temporary support wall to hold the weight of the joist to repair the head of the joist. Kinda as you would for framing a new window in a load bearing wall.
To be honest, the termites couldn't have picked a worse place. You're probably going to have to hire someone to fix that pocket. I don't think you need a structural engineer for this, I would just start by talking to some local builders.