This is the stat distribution of Roshi from the CN simulator:
For comparison this one is the stat distribution of a known tank (Kimimaro 3*):
On your question on why he dies too fast, I think that is normal, his stats really isn't for a tank. Actually I'm wondering why people expect Roshi to shrug of all the damage he takes. They expect too much out of the old man.
I'll share some of my observations. If Roshi is directly in front of 2-3 swordsmen, stacked with buffs, multi-hitting, has barrier boost; Roshi will die on Round 1. If Roshi is targeted by high damage Ninjutsu mysteries for consecutive rounds (Wind reset team); Roshi will die on Round 3.
You can do an easy solution by putting him in a place where he doesn't take much damage off standard attacks (maybe the bottom lane), else you will have to increase his survivability through shields.