materials he will provide receipts for and be reimbursed by the owner. you get to bill mistakes if you can cover them with them with plausible excuses. when you start scratching the surface no telling what you are going to find, but the longer you work in a building the better idea you have. for instance the water damage to the beam support under the toilet is predictable but dreaded and one would rather not know but is probably the source of the problem (sinking toilet) and must be addressed. I know someone who paid for it themselves, but their rent is at least half yours and they have been on 6th st over 30-40 years now. $20-$25 cash an hour is not terrible pay, sounds like he is doing a little better than that.