Underlayment & Moisture Barriers
Felt underlayment, ice and water shield, vapor barriers, headlap requirements, and air barriers.
- Explain the primary purpose of roof underlayment as a secondary water barrier
- State the proper headlap for organic felt underlayment
- Describe the purpose and installation of ice and water shield membrane
- Differentiate between vapor barriers and air barriers in roofing systems
- Identify the typical R-value goal for roof insulation in Canada
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Underlayment Types & Applications
Purpose of Underlayment
The primary purpose of roof underlayment is to provide a secondary water barrier between the roofing material and the roof deck. If wind-driven rain penetrates the shingles or a shingle is damaged, the underlayment prevents water from reaching the sheathing and interior.
Felt Underlayment
Asphalt-saturated felt is the traditional underlayment material. It comes in two weights:
- #15 felt (15 lb per 100 sq ft) - standard weight for most residential applications
- #30 felt (30 lb per 100 sq ft) - heavier, more durable, required by some codes on low-slope applications
The proper headlap for organic felt underlayment is a minimum of 4 inches (100 mm). End laps should be a minimum of 6 inches. On slopes below 4:12, double-layer felt application is required with 19-inch headlaps.
Synthetic Underlayment
Modern synthetic underlayments are made from woven or spun polyethylene or polypropylene. They offer several advantages over felt:
Felt Underlayment
Lower cost per roll
Absorbs moisture - can wrinkle when wet
Tears easily during installation
Degrades with UV exposure in days
Synthetic Underlayment
Higher cost but covers more area
Does not absorb moisture
Stronger - resists tearing and foot traffic
UV-resistant for 30-180 days
The primary purpose of roof underlayment is to serve as a secondary water barrier. The minimum headlap for felt underlayment is 4 inches. Synthetic underlayments are stronger and moisture-resistant but cost more than traditional felt.