Age and spread decide it. One failed detail on a sound 10 year old roof is a repair. The same leak on a 22 year old roof with brittle, granule bare shingles across slopes is money into a roof that will leak somewhere else next season. The working rule: if repairs in a short window approach a third to a half of replacement cost, or the shingle mat itself is failing across slopes rather than at one detail, replacement is usually the better money. Ask for both prices in writing, with the roof's age and condition stated.
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