What Is Roof Underlayment? (Felt vs. Synthetic vs. Ice & Water)
Learn the differences between traditional 15/30 lb asphalt felt, modern woven synthetic underlayment, and self-adhering ice & water shield membranes.
Key Article Takeaways
- •Underlayment is the secondary waterproof barrier installed directly over plywood roof sheathing before primary shingles or metal panels are laid.
- •Synthetic underlayment is 10x stronger than 15 lb felt, tear-resistant under contractor boots, and impervious to moisture rot.
- •Building codes require self-adhering Ice & Water Shield along eaves in cold climates (24 inches inside warm wall boundary) to prevent ice dam leaks.
Three Primary Types of Roofing Underlayment
Underlayment acts as a crucial moisture seal protecting plywood sheathing from wind-driven rain, ice dam backups, and missing shingle blow-offs.
| Underlayment Type | Material Composition | Key Advantage | Cost Per Square |
|---|---|---|---|
| Asphalt Felt (15/30 lb) | Paper saturated with asphalt | Low initial material price | $12 - $20 / SQ |
| Non-Woven Synthetic | Polypropylene / Polyethylene | Extreme tear strength & UV stability | $25 - $45 / SQ |
| Ice & Water Shield | Rubberized Asphalt + Poly film | Self-seals around nail penetrations | $75 - $130 / SQ |
Frequently Asked Questions
Do I need Ice & Water Shield on my entire roof?
In most residential regions, Ice & Water Shield is required only at eaves, valleys, sidewalls, and roof penetrations. Low-slope roofs (under 4/12 pitch) may require double underlayment or full ice shield coverage.
Apply These Calculations to Your Project
Use our free, contractor-grade calculators to generate instant material takeoffs and cost estimates.
Estimate 3-tab, architectural, or luxury shingle bundles, starter strip courses, hip & ridge caps, coil nails, and synthetic underlayment.
Itemize materials, labor, tear-off, synthetic underlayment, flashing kits, ventilation, dumpster fees, and custom contractor rates.