Synthetic vs Felt Underlayment Economics

Why this matters

A 30 square (3,000 sq ft) reroof uses approximately 6 rolls of 30 lb felt at a modest per roll, or 3 rolls of synthetic underlayment at a modest per roll. Material cost difference: roughly a modest per roof. Labor cost difference: synthetic is 40 to 60 percent faster to install because rolls are larger, lighter, and tear less. Warranty difference: synthetic carries the manufacturer's 25 to 50 year UV exposure rating versus felt's 30 to 90 day rating. The contractor who quotes felt by default in 2026 is leaving margin on the table; the contractor who quotes synthetic by default but doesn't explain why is failing the customer education conversation. Knowing the actual cost-benefit math lets you size the choice to each job correctly.

What underlayment does

Underlayment sits between the roof deck and the finished roofing (shingles, metal, tile). It serves three functions:

1. Secondary water barrier: when wind drives water under the primary roofing, underlayment prevents leak into the deck
2. Deck protection during install: between tear-off and shingle install, the deck must be weather-protected
3. Slip plane: allows minor thermal expansion of the finished roofing against the deck

Code (IRC R905) requires underlayment for asphalt-shingle, metal-shingle, and most other steep-slope coverings.

The four underlayment families

Performance comparison

Tear resistance

A worker stepping on felt during install routinely tears it; a foot on synthetic does not. Tear resistance affects waste percentage and the integrity of the installed system.

UV exposure

Felt degrades quickly in sunlight; coverage must follow within 30 to 90 days depending on manufacturer. Synthetic is rated for extended exposure - some manufacturers warrant up to 12 months exposed.

This matters for two scenarios:

• Reroof completed in stages over weeks (synthetic permits this)
• New construction with delayed roofing install (synthetic permits this)

A felt-underlayment roof exposed for 6 months in summer sun is failing before the shingles ever go on. Synthetic in the same exposure is intact.

Water resistance

Both shed water effectively. The differences are at the laps and at penetrations:

• Felt laps depend on overlap and small staples / cap nails for seal
• Synthetic laps come with manufacturer-printed lap lines; some have integrated adhesive strips
• Synthetic resists wind-driven rain better at laps

Weight

A 30-square reroof using felt requires moving roughly 9,000 lb of underlayment up to the roof. Same job with synthetic requires roughly 1,200 lb. The ladder time difference adds up.

Installation differences

Felt

• Roll size: 36 inches wide x 144 ft typical
• Installation: stretched flat, fastened with roofing staples or cap nails every 4 to 8 inches on edges, every 12 inches in field
• Laps: 2 inch side, 6 inch end overlap minimum (or per local code)
• Cap-nail spacing varies; verify with code and manufacturer instruction

Synthetic

• Roll size: 48 inches wide x 250 ft typical (some rolls 60 inches x 286 ft)
• Installation: stretched flat, fastened with cap nails per manufacturer pattern (typically 6 inch lap, 12 inch field)
• Pre-printed lap lines guide alignment
• Some manufacturers allow plastic-cap-only fasteners (no metal); others require metal cap nails
• Lap requirements per manufacturer; commonly 4 inch sides, 6 to 12 inch ends

Self-adhered (ice and water shield)

• Installed at vulnerable areas only: eaves (per IRC R905.1.2 in cold climates), valleys, around penetrations
• Peel-and-stick application: peel backing, press to deck, roll for adhesion
• More expensive per square foot; used selectively

Code-driven application

IRC R905.1.2 requires ice and water shield at eaves in regions with historic ice damming (typically 36 inches in from the wall line above interior heated space). State and local amendments may extend this.

For everything outside the ice-and-water-shield zone, underlayment is either felt (15 lb or 30 lb) or approved synthetic per ASTM D8257.

When felt is the right choice

• Customer is extremely budget-driven and accepts the labor / waste tradeoff
• Local code requires felt (rare, but some jurisdictions specify)
• Customer has only felt on hand and refuses to upgrade
• Cold-weather install where adhesive on synthetic may not stick reliably (some synthetics)
• Specific historic-preservation projects requiring felt

For most other situations, synthetic is the better choice.

When synthetic is clearly worth the upgrade

• Reroof in mid-summer (UV exposure tolerance)
• Reroof in autumn or winter (felt wrinkles in temperature swings)
• Multi-day installs (synthetic stays clean and intact during weather delays)
• Steep slope where walkability matters
• High-end projects where the customer expects modern materials
• Insurance-funded work where the carrier permits or specifies synthetic

Customer conversation

A typical conversation:

• "What underlayment are you proposing?" (customer asks)
• "Synthetic. Costs $X more than the basic felt option. Installs faster, lasts longer if we get rain during the project, and most premium shingle warranties require it."
• (Customer asks the difference) "Felt is what your father's roof had; works fine when covered immediately. Synthetic is what every code-driven modern project uses; tolerates weather delays better and grips like sandpaper for the crew."
• Move on.

References

• ASTM D226 (Asphalt-saturated organic felt for use in waterproofing).
• ASTM D4869 (Asphalt-saturated organic felt used in steep slope roofing).
• ASTM D8257 (Synthetic underlayment for steep slope roofing).
• IRC 2021, Section R905.1.2 (Ice and water barriers).
• IRC 2021, Section R905.2.5 (Underlayment for asphalt shingles).
• NRCA Roofing Manual: Steep-Slope Roof Systems, current edition.
• Manufacturer installation instructions: GAF FeltBuster, CertainTeed RoofRunner, Owens Corning Deck Defense, Tyvek Protec.
• Manuall internal: Roofing Synthetic Underlayment, Roofing Asphalt Shingle Replacement.