Underlayment is applied over the entire roof deck before installing the shingles. Note that underlayment is installed over the ice and water barrier. It is a tough, water-resistant material that adds another layer of defense against water. Underlayment is available in felt or synthetic materials, and the selection depends on the specific job.
Another important function of underlayment is to allow moisture to escape from the building, similar to “wicking.” Moist air in the attic may pass through wood decking. If that moisture condenses on the backside of the roof shingles, the underlayment is moisture barrier to prevent it from coming back in contact with the wood decking.
SEI Roofing primarily uses: Shingle Dog 15 Underlayment
Here are articles about underlayments:
Underlayment, Part 1 – InterNACHI – Purpose of Underlayment
Underlayment, Part 2 – InterNACHI – Types of Underlayment
Underlayment, Part 3 – InterNACHI – Types (cont’d)
Underlayment, Part 4 – InterNACHI – Synthetics
Underlayment, Part 5 – InterNACHI – Factors Affecting Underlayment