Which is fair--full cost to seller or split the cost?

Made an offer on a home for 263k in WV, knowing the roof is 20 years old. After inspection, the roof is leaking in two places and repairs might get another couple of years out of it. Other things in the inspection include the AC is insufficient for the third floor, a flue is broken on one of the fireplaces, and the plumbing appears to be Terra cotta so I need to have it scoped to see if it was ever replaced. My question is about the roof. Is it fair to ask for a full roof replacement? Should I offer to go half since I knew the age of the roof?