What's the function of felt underlay for shingles?
Shingles (standard three tab type) have felt (tar paper?) installed on the
roof first, and then the shingles are installed. What is the function of
the felt? I understand there are 2 types of felt, perforated and
non-perforated. Is perforated felt for vapour movement? What happens if
the felt is not installed?
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