Roof Underlayment Guide: Felt vs Synthetic and How Many Rolls You Need
Underlayment goes down before the shingles or metal panels and serves as the roof’s secondary water barrier — the last line of defense if wind-driven rain or ice gets past the primary roofing material. Because it’s ordered by coverage area rather than by house footprint, and because coverage varies by product type, underlayment is one of the easier materials to under-order if you’re not accounting for overlap correctly. This guide covers both the material choice and the math.
Felt vs. Synthetic Underlayment
Traditional 30 lb felt has been the standard for decades and remains a lower-cost option that still meets code in most areas. It typically covers around 200 sq ft per roll.
Synthetic underlayment is lighter, tears less, handles foot traffic better during installation, and usually comes in larger rolls — commonly covering around 400 sq ft per roll because the material is both wider and longer than felt rolls. Many roofers have shifted to synthetic as the default for its durability and ease of handling, even though it typically costs more per roll.
Why You Calculate Off Roof Area, Not Footprint
Just like shingles, underlayment needs to be ordered based on the roof’s actual sloped surface area — always larger than the house’s flat footprint once pitch is factored in. Skipping this conversion and ordering off footprint square footage is a common way to come up short on delivery day.
Accounting for Overlap
Underlayment doesn’t cover the roof edge-to-edge with zero waste — each course overlaps the one below it. Horizontal (side) overlaps typically run 4–6 inches per course, and end laps, where two rolls meet along the same row, should overlap at least 6–12 inches depending on the manufacturer’s instructions.
Because of this overlap, the effective area you’re covering is larger than the raw roof square footage — a 6% overlap allowance is a reasonable default for most roofs, though steeper or more complex rooflines may need more.
Worked Example
A 2,000 sq ft roof area, synthetic underlayment (400 sq ft/roll), 6% overlap allowance.
Area with overlap: 2,000 × 1.06 = 2,120 sq ft.
Rolls needed: 2,120 ÷ 400 = 5.3 → round up to 6 rolls.
Common Mistakes to Avoid
- Ordering underlayment based on house footprint instead of actual sloped roof area.
- Not adding an overlap allowance, which typically leaves the job 5–10% short.
- Mixing up felt and synthetic coverage-per-roll figures when switching products mid-estimate.
- Using minimum-code overlap in high-wind or ice-dam-prone regions instead of the manufacturer’s enhanced recommendation.
Related Guidence
Frequently Asked Questions
How much overlap does roofing underlayment need?
Horizontal (side) overlaps are typically 4–6 inches per course, while end laps where two rolls meet along the same row should overlap at least 6–12 inches, depending on the manufacturer’s instructions.
How many squares of underlayment do I need for a roof?
You need enough underlayment to cover the full roof area plus overlap, generally about 6–10% more than the raw roof square footage — use the roof area (not house footprint) as your starting number.
Is synthetic underlayment better than felt?
Synthetic underlayment is lighter, tears less, handles foot traffic better, and usually comes in larger rolls with wider coverage than traditional felt, though felt remains a lower-cost option that still meets code in most areas.
Try the Roof Underlayment Calculator
Ready to run your own numbers? Use the free Roof Underlayment Calculator on our site — just enter your measurements and get an instant, accurate estimate.
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