An asphalt-saturated felt or other sheet material (may be self-adhering)
installed between the roof deck and roof covering, usually used in a steep-slope roof construction.
Underlayment is primarily used to separate the roof covering from the roof deck, shed water and provide secondary weather protection for the roof area of the building.
Underlayment is utilized in new roof installations
as well as repairs such as replacing a shingle.
Traditional roofing felt or modern underlayments are always a great idea, and they
may be called for in the written instructions as a warranty requirement.