The longevity of asphalt felt, the most popular type of underlayment, is 20-30 years. However, its lifespan can be halved in extremely prolonged situations. On the other hand, new rubberized or synthetic asphalt underlayment can last for as long as 35 years.
While it is feasible to repair smaller areas by patching them, it is best to replace the felt fully to ensure that your roof lasts as long as possible and that no further damage occurs to your home.
When severe rain and storms cause moisture to become trapped between the shingle and the wood of your roof decking, it acts as a water repellent. Humidity is awful for roofs since it can lead to leaks, rot, and even death. The felt prevents water from seeping into your home as a barrier between the wood and the elements.
Modern torch-on roof felt is incredibly robust and, critically, resistant to UV radiation and heat, despite the material's formerly poor reputation for longevity.