On a well designed tile roof, the underlayment generally wears out before the tile. Asphaltic felt deterioration is caused in various ways. Exposure to the sun's ultraviolet rays deteriorates the felt's surface very quickly. This exposure often takes place after the roof felt has been installed but not yet covered with tiles. In some cases, months have passed before the felts are covered with tile.
Water running over or, much worse, ponding on the felts removes the asphalt's oils.
Dirt collecting on the felts behind horizontal batten boards placed directly on the felts will hold water. This will keep felts moist for extended periods of time, causing rapid deterioration. As the asphalt oxidizes from exposure to air, it becomes brittle. This is okay on a roof as long as it is not exposed to sunlight or handled since the old brittle asphalt felt will rip very easily. While an adhered waterproofing membrane may not be required in valleys and along eaves, it can be a cost-effective valuable addition to prolong the life of the roof.