By reading the edges and the evidence rather than the tile. Underlayment shows its age at eaves, valleys, and penetrations, where a roofer can lift a course and read the felt's condition directly, and in the attic, where staining history is written on the deck. The tile's remaining life is measured in decades; the felt's is measured from its installation date and its exposure. An inspection that reports both numbers separately is the one that lets you plan a lift and relay on your calendar instead of the rain's.
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