When planning a roof replacement or repair, understanding what goes under shingles is essential for long-term durability and protection from weather. The layers beneath the shingles—underlayment, moisture barriers, drainage components, and ventilation—work together to keep the roof watertight, resist ice dams, and extend the life of the roofing system. This guide explains the purpose of each layer, the materials commonly used, installation best practices, and maintenance tips to ensure a reliable roof over your head.
What Is Shingle Roof Underlayment
Underlayment is a protective layer installed directly beneath the shingles. It serves as a secondary barrier against moisture that might penetrate the shingles, such as wind-driven rain or shingle granules. Underlayment also provides a slip-resistant surface for installers and helps seal around fasteners. There are two main types: synthetic underlayment and asphalt-saturated felt. Synthetic options tend to offer higher tensile strength, better slip resistance, and improved performance in cold weather, while asphalt felt is traditional and often more economical. For many modern installations, synthetic underlayment is the preferred choice due to its durability and longer lifespan.
Common Underlayment Materials
Choosing the right underlayment depends on climate, roof pitch, and roof covering. Here are the most common materials:
- Synthetic Underlayment: Made from polymers, these sheets are lightweight, tear-resistant, and provide excellent moisture resistance. They also tend to be more durable in temperature extremes and are less prone to drying out and cracking.
- Roofing Felt (Asphalt-Saturated Felt): Traditional option that offers cost savings. It provides an initial moisture barrier but is less durable in prolonged exposure to the elements and may be more susceptible to tearing.
- Self-Adhering Membranes: Also known as peel-and-stick underlayment, these provide superior water resistance, especially along eaves and at protrusions. They are particularly effective in cold climates or areas prone to wind-driven rain.
In areas with heavy snowfall or ice dam risk, higher-performance underlayments with higher moisture resistance are recommended. The installation must cover all roof sections without gaps, and overlaps should meet manufacturer guidelines to ensure a continuous barrier.
Water Barriers And Ice Dam Protection
Water barriers are critical in regions with significant precipitation or where ice dam formation is a concern. Some roofs include an additional moisture barrier or ice and water shield installed along the eaves, valleys, and around penetrations. These membranes are self-sealing and highly resistant to water penetration, even when ice forms behind them. They prevent water from seeping under shingles during freeze-thaw cycles and help prevent shingle lift caused by wind. The placement is typically at the lowest edge of the roof, in valleys, along roof penetrations (vents, chimneys), and at any eave-to-ridge transitions where water flow is most intense.
Roofing Felt Versus Moisture Barriers
Historically, asphalt-saturated roofing felt provided the primary moisture barrier. Modern practice often combines felt with a more robust underlayment layer or uses a self-adhering membrane for enhanced protection. The key difference is durability and reliability in harsh conditions. Roofing felt is still used in many installations for cost efficiency and compatibility with certain shingle brands, while synthetic underlayment paired with a separate ice and water shield offers superior protection in vulnerable areas. Builders select the combination based on local building codes, climate, and the roof design.
Ventilation And Drainage Considerations
Ventilation under the roof deck plays a crucial role in moisture management and energy efficiency. Proper ventilation reduces condensation, which can lead to rot and mold growth and compromise the underlying structure. Attic ventilation should align with the roof’s design and climate. Adequate intake and exhaust ventilation helps maintain a stable roof deck temperature, reducing the risk of ice dams and extending shingle life. Inadequate ventilation can cause trapped moisture, leading to premature aging of underlayment and sheathing. Additionally, proper drainage and slope ensure that water flows toward gutters without pooling behind the shingles.
Proper Installation Steps
Correct installation of underlayment and moisture barriers is essential for maximum performance. The following steps outline a standard approach used by professionals:
- Inspect the roof deck for damage and repair any warped or rotten boards before laying underlayment.
- Install underlayment starting at the eaves and work upward, with horizontal laps of the recommended width. Maintain manufacturer-specified overlaps to ensure water shedding.
- In areas of high wind or near roof penetrations, apply self-adhering membranes as a secondary line of defense or use leak barrier membranes in accordance with local building codes.
- Seal around roof penetrations (vents, chimneys, skylights) with appropriate sealant or flashing as directed by the underlayment manufacturer and the shingle brand.
- Install outer roof covering (shingles) according to the manufacturer’s instructions, ensuring proper alignment and nailing patterns to prevent water intrusion and shingle blow-off.
Following code requirements and manufacturer guidelines is essential. Some regions require a specific underlayment type or additional barriers, so verify local codes before starting work.
Maintenance And Inspection
Regular inspection helps preserve the integrity of the roof system beneath shingles. After severe weather, inspect for lifted shingles, loose edges, or obvious damage to the underlayment. Look for signs of moisture intrusion in the attic, such as staining or musty odors. If any underlayment shows signs of wear, consider replacement or reinforcement to prevent future leaks. Schedule professional inspections every few years, especially if the roof is nearing the end of its expected lifespan or if the climate features frequent freeze-thaw cycles. Early detection of underlayment issues saves repair costs and protects interior spaces from water damage.
Frequently Asked Questions
Do all roofs require underlayment? Yes. Underlayment provides a secondary moisture barrier and protection during installation. The type and thickness depend on climate, roof type, and local codes.
Can I install shingles directly over old underlayment? Generally not. Most systems require replacing underlayment during reroofing to ensure optimal protection and warranty compliance.
Which underlayment lasts longer? Synthetic underlayment typically offers longer life and better performance in extreme temperatures compared with traditional felt.