Underlayment is the protective layer installed between the roof covering and the roof deck. On a metal-roofing project, it should be selected for the system, climate, roof design, and installation method—not treated as an afterthought.
What underlayment does
It provides a secondary water-shedding layer during construction and after the finished roof is installed. It also separates the metal system from the deck and helps manage temporary exposure and incidental moisture.
Why old tar paper is not the whole answer
Traditional asphalt-saturated felt has been used for decades, but modern synthetic and high-temperature self-adhering products offer different performance characteristics. Valleys, eaves, penetrations, and other vulnerable areas may need specialized protection.
Match the product to the assembly
The correct choice depends on whether the old roof is removed, the deck material, expected temperatures, ventilation, roof pitch, and manufacturer requirements.
Ask what will be installed beneath your roof and why. A permanent roof deserves a complete, compatible assembly.