Roof Replacement?

I have had the opportunity the past 2 weeks to inspect several roofs for myself and other investors.
The roof is one of the main items to inspect as you are evaluating a property. How do you know if the roof needs to be replaced?
Look for evidence of loss of granules, cracking, cupping or clawing. With advanced deterioration, shingles may be torn off. As the asphalt gets older, it becomes more brittle.
On new shingles, the width of the slots between the tabs is typically a quarter of an inch, roughly speaking. As the shingles get old, they shrink, and the slots can be up to one inch wide or more toward the end of the shingle life.
On other trick is to look in the slots, or cutouts, for evidence of premature wear. There are some shingles that were known to wear prematurely in this area.