The answer to 'lifespan of underlayment' is that the typical lifespan of roofing underlayment is 20-40 years, depending on the type of underlayment used and the climate conditions. Underlayment is the protective layer installed between the roof deck and the shingles, providing an extra barrier against water intrusion.
Asphalt-based felt paper underlayment typically lasts 20-25 years, while synthetic underlayment made of materials like polypropylene or polyethylene can last 30-40 years. The lifespan can be extended by proper installation and maintaining the roof in good condition. It's important to inspect the underlayment periodically and replace it if it becomes worn, cracked, or deteriorated.