Free Underlayment Calculator — Rolls & Coverage (2026)

How to Use This Calculator

1. 1Enter total roof area.
2. 2Select the underlayment type you are using.
3. 3Click Calculate.

About This Material

Roofing underlayment is the secondary moisture barrier installed over the roof sheathing before shingles or other roofing materials are applied. There are three main types: 15-pound asphalt felt (the traditional choice, sold in rolls covering 400 sq ft, costs $15-25 per roll), 30-pound felt (heavier and more durable, 200 sq ft per roll, $20-35 per roll), and synthetic underlayment (modern non-woven polypropylene, 1,000 sq ft per roll, $50-100 per roll). Synthetic underlayment has largely replaced felt in professional roofing because it is lighter, stronger, does not absorb moisture, and can be left exposed longer during construction. In cold climates (zones 5-8), ice and water shield — a self-adhered rubberized asphalt membrane — is required along the eaves for at least the first 24 inches past the interior wall line. Most shingle manufacturers require underlayment for their warranty to be valid, and building codes universally mandate it as part of the roof assembly.

Installation Tips

• •Install horizontally, starting at the eave.
• •Overlap each course 2–4 inches over the one below.
• •Use ice and water shield in the first 2 courses in cold climates.
• •Fasten with cap nails or staples per manufacturer instructions.

Common Mistakes to Avoid

• ⚠Installing in the wrong direction (should go horizontal).
• ⚠Not using ice and water shield near eaves — required in climate zones with freezing.
• ⚠Using staples in high-wind areas instead of cap nails.

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