A.Under asphalt shingles, #15 felt serves three purposes: it provides a temporary cover for the roof until the shingles go on; it protects the shingles from being damaged by any pitch that bakes out of the sheathing; and it provides a second line of defense if any shingles blow off. Considering the relatively low cost, it is cheap insurance for the last reason, even if the first two reasons don’t apply.
In cold climates, where you are uncertain if the ceiling insulation and roof ventilation is adequate, there is a strong likelihood of ice damming. In this case, install an ice and water shield along the roof edge, or two layers of #30 felt bedded in asphalt.
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