This is a picture of my attic bathroom, the cast iron pipe goes out the roof. The house was built in 1928 in Florida with additions made in the 60's or 70's I believe. I've got a galv water line that comes up from my first story bathroom and then T's out from there with one of the lines from the T supplying my faucets and toilet and the other to my shut off valve for the water heater where it then T's off again from there and runs into that cast iron pipe in the picture. I thought it might be some kind of pressure relief line or something. Any idea what that would be for? I've asked around and no one seems to have a clue.