Why synthetic, not felt.
Standard 30-pound asphalt-impregnated felt was the residential-roof underlayment standard for decades. In Paradise Valley's climate, it is no longer the right specification. Felt UV-degrades through tile gaps and around penetrations within a few summers; by year ten it has lost meaningful waterproofing capacity. Synthetic high-temperature underlayments — woven or non-woven polymer-based — are designed for sustained high-heat exposure and outperform felt by two to three times in this climate.