Marcus's $5,000 Question
The most expensive mistake I see homeowners make is paying for a $1,500 repair on a roof that clearly needs to be replaced. You're effectively paying 20% of a new roof's cost for a band-aid that will fail in 12 months. Here is my logical framework for making the call.
The 30% Rule
If the damage (missing shingles, rot, or leaks) covers more than 30% of the total roof surface, I stop talking about repairs. At that point, the labor to integrate patches into the existing system becomes almost as expensive as a full tear-off.
The Brittle Test
I perform a 'bend test' on your shingles. If I can't lift a shingle 45 degrees without it snapping or losing a handful of granules, the roof is 'brittle.' Brittle roofs cannot be repaired because the act of nailing in new shingles destroys the ones next to them.