Underlayment goes down before the shingles or metal panels and acts as the roof's secondary water barrier, so it needs to be ordered by coverage area rather than by the footprint of the house — a roof's actual surface area is always larger than its footprint once pitch is factored in. This calculator takes that roof area and adds an overlap allowance, since each course of underlayment overlaps the one below it by several inches, which means the effective area you're covering is larger than the raw square footage.

Roll coverage varies by product: traditional 30 lb felt typically covers around 200 sq ft per roll, while modern synthetic underlayment usually covers closer to 400 sq ft per roll because the rolls are wider and longer. Always check the coverage rating printed on the specific product you're buying and adjust the roll coverage field accordingly.