Underlayment is important to ensure the durability of roofing assemblies. Compared to traditional felt or tar paper (which can serve its purpose perfectly fine in many locations and assembly types, especially above vented attics), the best modern underlayment products are more durable and tear-resistant. The products listed here have one or more beneficial attributes such as being self-adhered, highly durable, or able to withstand months of exposure before being covered with roofing; or having an integrated ventilation mat (especially important below metal roofs). 

Highly vapor permeable products are often the best choice and can improve the resilience of a roof. Be sure to know your building science (or find someone who does!) if specifying or installing a vapor barrier underlayment. Ice shields are typically vapor barriers (i.e. Class I vapor retarders) and are best used only on the areas of a roof where required by building codes, including eaves and valleys.

Fully adhered underlayment is a best practice for airtightness and long-term rainwater management, and such products include "air barrier" as a product type.

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