Roof Underlayment: Synthetic vs Felt vs Peel-and-Stick

Quick answer

Underlayment is the waterproof layer between the decking (plywood) and the shingles. Synthetic underlayment has replaced 15lb felt as the modern standard and is what any quality install uses today. Peel-and-stick (self-adhering) goes on top of that at eaves, valleys, and around penetrations - and on entire roofs in high-wind or wildfire zones. 15lb felt is the cheap pick, fine on a budget job but inferior in every measurable way.

15lb & 30lb felt - the old standard

Asphalt-saturated paper. What every roof had from the 1920s to the early 2010s. Cheap, easy to work with, and adequate. Also tears in wind, wrinkles in heat, holds moisture, and gives you ~6 hours of protection if the install is interrupted by rain.

• Cost: $0.10 - $0.20/sqft material; $0.25 - $0.50/sqft installed
• Weight: Heavy on a roll; awkward to carry
• UV exposure: Maximum a few days before degrading
• Still appropriate for: Tight-budget jobs, sheds, garages, code minimum

Synthetic underlayment - the modern standard

Woven polypropylene or polyester. Lighter, tougher, doesn't tear or wrinkle, UV-stable for 30 - 180 days depending on product, and grippy enough to walk on without slipping. Names you'll see: GAF Deck-Armor, Owens Corning Deck Defense, CertainTeed RoofRunner, Titanium PSU30, Sharkskin.

• Cost: $0.15 - $0.40/sqft material; $0.35 - $0.65/sqft installed
• UV exposure: 30 - 180 days (lets roofers stage the job)
• Watch out: The cheap synthetic (off-brand, no UL listing) is barely better than felt. Specify a known brand.

Net cost difference vs felt on a typical roof: $150 - $400. Worth every dollar.

Peel-and-stick (self-adhering / ice & water shield)

Rubberized asphalt with an adhesive back, applied directly to the deck. Creates a fully sealed waterproof barrier that bonds to the decking and self-seals around nail penetrations. The ultimate underlayment - and the most expensive.

• Cost: $1.00 - $2.50/sqft material; $1.50 - $3.50/sqft installed
• Brands: Grace Ice & Water Shield, CertainTeed WinterGuard, GAF StormGuard, Owens Corning WeatherLock
• Required by code at: Eaves in cold climates (per the IRC in regions with average January temp <25°F)
• Recommended at: All valleys, around skylights, chimneys, vents, low-slope sections

For details on where this layer actually goes and why, see where ice & water shield actually goes.

What code requires

The 2024 International Residential Code (R905) requires a minimum of one layer of underlayment on every roof. In areas with recorded average January temp ≤ 25°F, ice & water shield is required from the eave to a point 24" inside the heated wall line. Local amendments may extend that requirement (whole- deck peel-and-stick is common in high-wind coastal counties).

What to look for on your estimate

Three line items that signal a quality install:

• "Synthetic underlayment" by name (brand should be specified, not just "synthetic")
• "Ice & water shield at eaves, valleys, and penetrations" with linear feet or sqft specified
• Hand-nailed or with plastic-cap nails on the underlayment (staples are inferior)

Estimates that just say "felt paper" or "underlayment" without specifics are usually planning on the cheapest 15lb felt. Ask. The savings to the contractor is real - the savings to you, almost zero.

The takeaway

On any roof you plan to keep for more than 10 years, synthetic underlayment with code-required (and ideally more) peel-and-stick is the right spec. The total upgrade cost vs felt is in the low hundreds; the downside protection is significant. Run the calculator with upgraded underlayment in the spec to see the actual delta on your roof.

Sources: International Residential Code R905 · NRCA