What Is Roofing Underlayment and Why Does It Matter?
Roofing underlayment is a water-resistant or waterproof barrier installed directly on the roof deck (sheathing) before the primary roofing material (shingles, metal panels, tiles). It serves as the last line of defense against water intrusion if the primary roof covering is ever breached by wind-driven rain, ice dams, or storm damage. It also protects the roof deck during construction before shingles are installed.
The Three Main Types of Roofing Underlayment
1. #15 Asphalt Felt (Tar Paper)
The traditional underlayment, used for over 100 years. Made from natural fibers (paper or fiberglass) saturated with asphalt. #15 felt weighs approximately 15 lbs per 100 sq ft (1 square).
- Coverage per roll: ~400 sq ft (4 squares), with 2-inch side laps
- Best for: Low-to-moderate slope roofs with asphalt shingles
- Cost: $0.10–$0.20 per sq ft
- Limitation: Tears easily when wet, buckles in heat, limited UV exposure time (60 days)
2. #30 Asphalt Felt
The heavier version, weighing ~30 lbs per square. Provides greater tear resistance and better water holdout than #15.
- Coverage per roll: ~200 sq ft (2 squares)
- Best for: Steeper slopes (6:12 and above), metal roofing, and wood shakes
- Cost: $0.18–$0.30 per sq ft
- Required by code: Most codes require #30 under wood shakes and on slopes above 7:12
3. Synthetic Roofing Underlayment
Made from woven or spunbond polypropylene or polyethylene. This modern product has largely replaced felt paper on new construction because of its superior performance in nearly every category.
- Coverage per roll: 1,000–10,000 sq ft depending on brand and width
- Best for: All roof types, especially metal roofing, tile, and high-wind regions
- Cost: $0.08–$0.25 per sq ft (more coverage per roll = lower installed cost)
- Advantages: 4x stronger than felt, UV exposure rated 6–12 months, lighter weight, slip-resistant surface, won't buckle or wrinkle
Direct Comparison Table
| Property | #15 Felt | #30 Felt | Synthetic |
|---|---|---|---|
| Tear Resistance | Low | Moderate | Very High |
| Water Resistance | Moderate | Good | Excellent |
| UV Exposure Time | 30–60 days | 30–60 days | 6–12 months |
| Slip Resistance | Poor (when wet) | Moderate | Excellent |
| Weight per Square | ~15 lbs | ~30 lbs | ~1–3 lbs |
| Cost per Sq Ft | $0.10–$0.20 | $0.18–$0.30 | $0.08–$0.25 |
| Code Compliance | Widely accepted | Required for steep/shakes | Accepted everywhere |
How to Calculate Underlayment Rolls Needed
- Calculate your total sloped roof area in square feet using our Roofing Calculator.
- Add 10% waste for overlaps, valleys, and ridge cuts.
- Divide by the coverage per roll of your chosen underlayment product.
- Round up to the next whole roll.
Example: A 1,800 sq ft roof area. With 10% waste: 1,800 × 1.10 = 1,980 sq ft needed. Using #30 felt (200 sq ft/roll): 1,980 / 200 = 9.9 → order 10 rolls.
Installation Tips
- Always start at the eave (bottom edge) and work up the slope, overlapping each course over the one below.
- Horizontal laps must be a minimum of 2 inches (6 inches in valleys and hips).
- End laps (where two rolls meet side by side) must overlap at least 4 inches.
- In high-wind zones (over 110 mph design wind speed), use cap nails or cap staples rather than standard roofing nails for underlayment.
- Install ice and water shield at all eaves (minimum 24 inches past the interior wall line), valleys, and penetrations before installing standard underlayment.