The answer to 'difference between underlayment' is that underlayment and roofing felt are both types of protective layers installed beneath the roof shingles, but they have distinct features, benefits, and use cases. Underlayment is a synthetic or asphalt-based material that provides a waterproof barrier, while roofing felt is a paper-based material that helps prevent leaks and protects the roof deck.
Underlayment is generally more durable, longer-lasting, and better at resisting moisture and tearing compared to roofing felt. It can also provide additional insulation and help extend the life of the roof shingles. Roofing felt is a more affordable option, but it may need to be replaced more frequently as it can degrade over time.