It does not just degrade in sunlight . It degrades in hot attics as well. Just a month ago I inspected and attic with a concrete tile roof. Due to penetrations they connected vents together running horizontally across the attic. One had such a large bow in it because it was not supported in the center and had filled with water. I barely tapped it with my foot stepping over it and first one end snapped at the y heading out the roof and when it fell the far end snapped off as well. Oil based products dry out over time and get brittle and lose there flexibility.

Cut a piece of vent pipe into 2 one foot lengths. Keep one in your garage and throw the other into the back yard for 6 months. After that time bring them both to your patio and hit each with a hammer. The one that has been in the back yard will shatter and the one from the garage the hammer will still bounce off

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Ted Menelly
Castle Home Inspection Services
www.inspectmycastle.com