Sunburn, the loss of granular coating leaves the fiberglass and asphalt exposed and it degrades. Also, the fire resistance rating is for shingles with the fire resistant granules in place; no granules would likely make the shingles burn more easily.

If you can see the fiberglass strands, it is time to replace the roof.

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002