đ Underlayment Calculator
Calculate how many rolls of roofing underlayment you need. Supports felt and synthetic types with overlap adjustments.
How to Use This Calculator
Enter your roof area and select the underlayment type. Synthetic underlayment covers more per roll and is more durable. 30 lb felt is heavier duty than 15 lb. Adjust the overlap based on your local code requirements or manufacturer recommendations.
Understanding Your Results
Roll count is rounded up â you cannot buy partial rolls. Synthetic underlayment is more expensive per roll but covers 2-3x more area, often making it cost-competitive with felt on larger roofs. Always follow manufacturer installation guidelines for overlap and fastening patterns.
Frequently Asked Questions
How much underlayment do I need?
Divide your roof area by the coverage per roll (after accounting for overlap). A standard roll of 15lb felt covers about 400 sq ft with standard 2-inch overlap. Synthetic underlayment rolls cover 1,000+ sq ft.
What is better: felt or synthetic underlayment?
Synthetic underlayment is more durable, tear-resistant, and lighter than felt. It also lays flatter and resists moisture better. However, felt is less expensive and has been the standard for decades. Synthetic is preferred for most modern installations.
How much overlap for underlayment?
Standard overlap is 2-4 inches on horizontal seams and 6 inches on vertical seams. 15lb felt uses 2-inch overlap, 30lb felt uses 4-inch overlap, and synthetic varies by manufacturer (typically 3-4 inches).