The answer to 'underlayment vs' is that underlayment and roofing felt are both important components of a roof system, but they have different functions and benefits. Underlayment is a water-resistant or waterproof layer installed directly on the roof deck, providing an extra barrier against leaks. Roofing felt, on the other hand, is a lightweight, asphalt-saturated material that helps protect the roof deck from moisture during the installation process.
Underlayment is typically made from materials like asphalt-saturated felt, rubberized asphalt, or synthetic polymers, and it provides a critical layer of protection against water penetration. It helps prevent water from seeping through the roof shingles and reaching the roof deck, which can lead to costly damage. Roofing felt, while also serving as a moisture barrier, is primarily used to provide a smooth, even surface for the shingles to be installed on.