Just replaced about 1/8 of mine. It's metal (which is SUPPOSED to have a 50 year warranty, hahahahahaha) and a tree fell on it 7 years ago and it was patched. It's also the part of the roof where the septic outflow or whatever is called is and the power/cable/phone line entry which all needs boots or somesuch. They tried to repair it several times but it just wasn't taking.
I will probably replace the rest of it in a year or so - $12K+
