Manufactured products have failures over time. There are 2 primary types of fails: manufacturing related fails and wear out fails. If one plots failures vs time, the failure rate is typically high in the initial time period for a very short time (manufacturing related fails), decreases to a very low rate for a significant time, then starts to increase again rapidly as the product eventually wears out. A plot of this phenomena is often called a "bathtub curve" as it looks like the cross section of a bathtub.
The intent of a warranty is to get the consumer past the early fails and into the longer period of very few fails -- the bottom of the bathtub. Obviously, a manufacturer isn't going to extend the warranty to anywhere near where the wear-out related fails commence -- that's financial suicide. So, given that, one would expect a quality 15 year shingle to still be performing in the low fail period at 15 years, and in normal circumstances, well beyond that. (Just like one expects a car to be in the low fail rate area at 36K miles and well beyond.)
The intent of a warranty is to get the consumer past the early fails and into the longer period of very few fails -- the bottom of the bathtub. Obviously, a manufacturer isn't going to extend the warranty to anywhere near where the wear-out related fails commence -- that's financial suicide. So, given that, one would expect a quality 15 year shingle to still be performing in the low fail period at 15 years, and in normal circumstances, well beyond that. (Just like one expects a car to be in the low fail rate area at 36K miles and well beyond.)
