This isn’t an exact science and is largely based on a few factors: 1) The cost of a repair vs. a replacement, 2) The remaining service life of the roof if you only perform repairs, and 3) Your specific financial situation. If the repair would be $500, and replacement would be $15,000 with 7-10 years of service life remaining on the existing roof, then repair would make the most sense. If the repair would be $3,000, and replacement would be $6,000 and there is only 2 years of service life left, replacement makes the most sense. Keep in mind that when a roof is replaced, the repairs you paid for prior will be removed and replaced along with it.