Roof Underlayment Guide: Felt vs Synthetic and How Many Rolls You Need
Introduction
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.
For a complete suite of roofing estimating tools, visit the roofing calculators hub .
Felt vs. Synthetic Underlayment
| Feature | Traditional Felt | Synthetic |
|---|---|---|
| Coverage per roll | ~200 sq ft | ~400 sq ft |
| Weight | Heavier | Lighter |
| Durability | Lower | Higher |
| Foot traffic | Tears more easily | Handles traffic better |
| Cost per roll | Lower | Higher |
Traditional 30 lb felt has been the standard for decades and remains a lower-cost option. Synthetic underlayment is lighter, tears less, handles foot traffic better, and usually comes in larger rolls — commonly covering around 400 sq ft per roll.
For help with this calculation, use the Roof Underlayment Calculator .
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.
Formula:
Roof area = Footprint area × Pitch multiplier
For pitch multipliers, see the shingles guide .
Accounting for Overlap
Underlayment doesn’t cover the roof edge-to-edge with zero waste — each course overlaps the one below it.
| Overlap Type | Typical Amount |
|---|---|
| Horizontal (side) overlap | 4–6 inches per course |
| End laps (rolls meeting) | 6–12 inches |
Formula:
Area with overlap = Roof area × (1 + Overlap allowance) Rolls needed = Area with overlap ÷ Coverage per roll (round up)
Example:
2,000 sq ft roof area, synthetic underlayment (400 sq ft/roll), 6% overlap.
Area with overlap: 2,000 × 1.06 = 2,120 sq ft Rolls: 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
Using minimum-code overlap in high-wind regions instead of manufacturer’s recommendation
Important Limitations
Estimates Are Not Construction Specifications
Underlayment type and installation must comply with local building codes and manufacturer specifications.
Frequently Asked Questions
How much overlap does roofing underlayment need?
Horizontal overlaps are typically 4–6 inches, while end laps should overlap at least 6–12 inches.
How many squares of underlayment do I need for a roof?
You need enough to cover the full roof area plus overlap, generally about 6–10% more than the raw roof square footage.
Is synthetic underlayment better than felt?
Synthetic is lighter, tears less, handles foot traffic better, and usually comes in larger rolls. Felt remains a lower-cost option that still meets code in most areas.
Ready to calculate your underlayment? Use the free Roof Underlayment Calculator .
Explore More Roofing Resources
This guide covered the basics of underlayment estimation. For a complete suite of estimating tools, visit our roofing calculators hub — where you’ll find dedicated calculators for shingles, cost, pitch, nails, underlayment, drip edge, gutters, metal panels, and ventilation. Each calculator is paired with a full guide walking you through the math, common mistakes, and project-specific considerations.