Underlayment is used under asphalt shingles for a variety of reasons, such as providing:
- backup for water-shedding protection of the deck if shingles fail from wind-driven rain. The lower the slope, the more important underlayment is, since water flows more easily under shingles on low slopes.
- a protective barrier to the elements between the time the old shingles have been torn off and prior to the new shingle being applied.
- an agent to hide minor imperfections of the decking material and reduce "picture framing" of deck panels.