Choosing the Best Underlayment for Asphalt Shingle Roofs

Installing a high-quality underlayment beneath asphalt shingles is a critical step when replacing a roof. The right underlayment provides a secondary barrier against leaks, reinforces the roof deck, and improves overall durability. With the wide variety of underlayment materials available today, selecting the optimal product for your roof requires careful consideration of several key factors.

Why Underlayment Matters Under Asphalt Shingles

Asphalt shingles are composed of a fiberglass or organic felt mat, saturated with asphalt and topped with protective mineral granules. Despite their resilient construction, over time shingles can incur damage from weathering, foot traffic, falling branches, or simple aging. When shingles become compromised, water could leak through to the vulnerable roof deck below. This is where a high-quality underlayment serves a vital role in preventing leakage and moisture infiltration into the attic or living spaces of the home.

Underlayments provide a water-shedding secondary layer to deflect water away from vulnerable areas in the event of damaged or missing shingles. Underlayments also reinforce the roof, help smooth out imperfections, and improve overall durability and longevity when properly installed.

Main Types of Asphalt Shingle Underlayments

There are three primary categories of underlayment used under asphalt shingles:

• Roofing Felt - Asphalt-saturated felts made from paper, fiberglass, or organic materials. Often called tar paper. Typically the most affordable option.
• Synthetic Underlayment - Woven or sheet product made with synthetic materials like polypropylene coated with weatherproof resins and minerals.
• Self-Adhering Underlayment - Flexible membranes like rubberized asphalt that stick to the roof without adhesives.

Key Factors When Selecting an Asphalt Shingle Underlayment

With an overview of the primary underlayment categories, several specifics about your roof and climate should guide you toward the ideal product for your needs:

Roof Pitch and Slope

The steepness of your roof, measured by its vertical rise over the horizontal span, significantly impacts the underlayment selection. On lower sloped or nearly flat roofs, self-adhering rubberized asphalt underlayments are strongly recommended, as they form a reliable, waterproof bond to the roof decking without risk of slipping. On steep sloped roofs, synthetic underlayments are lighter and easier to install while still shedding water effectively.

Local Climate Conditions

In regions with substantial annual snowfall, the reinforced durability that underlayments provide against the tremendous weight of snow accumulation makes their use critical. In hot and arid climates, increased UV and heat exposure shortens the lifespan of roofing components, so selection of a rugged, weather-resistant underlayment can prolong roof performance. In damp climates and coastal regions prone to wind-driven rain, mold resistant underlayments paired with proper roof ventilation helps guard against moisture damage and decay.

Potential Exposure to Foot Traffic and Falling Debris

Roofs located under trees or overhanging branches may be more prone to punctures and tears from fallen debris. Rooftops exposed to regular foot traffic for maintenance access similarly demand underlayments with exceptional tear strength and puncture resistance. Synthetic options typically offer greater durability and longevity compared to the tear susceptibility of some felts.

Areas Prone to Leaks - Openings, Projections and Valleys

Vulnerable roof zones around skylights, chimneys, valleys and vents call for the dimensional stability and 100% waterproof security that self-adhering rubberized asphalt underlayments provide through their ability to seal around fasteners and roofing interruptions. Nailing felt or synthetic underlayment securely around such irregular roof aspects can prove challenging, increasing leak risks.

Weighing Pros and Cons of Different Underlayment Materials

With a grasp of key selection criteria, comparing the inherent strengths and weaknesses of different underlayment materials is essential:

Roofing Felt Underlayment

Roofing felt remains a cost-effective underlayment choice, available in grades #15 to #30, indicating weight per 100 sq ft. Heavier #30 fights rips and is recommended for underlayment use. However, durability challenges make roofing felt a poor performer when directly exposed to weather long-term at seams, laps or tears. Felt best serves as an affordable underlayment layered entirely beneath shingles, rather than a temporary exposed covering awaiting shingle installation.

Synthetic Underlayment

Offering improved tear-resistance and longevity compared to felt, polypropylene-based synthetic underlayments represent an upgraded option. Although costlier than felt initially, the extended service life of some synthetics paying dividends over multiple shingle lifecycles makes them a sound value. Tricky adhesive-based attachment on low slopes remains a drawback for synthetic sheets however.

Rubberized Asphalt Underlayment

Self-adhesive rubberized asphalt membranes provide dependable, waterproof performance at a moderate cost. The absence of mechanical fasteners through the flexible membrane avoids potential leak sources. Limitations come from difficulty adhering self-stick products during cold weather installation. Stiffness in extreme cold also requires prompt coverage with shingles to prevent uplift.

Best Asphalt Shingle Underlayment Recommendations

Given the capabilities of different underlayment products, recommendations can be made for optimal selections based on roofing configurations and climate conditions:

By Roof Pitch and Slope

On low slope roofs up to 2:12 pitch, self-adhering rubberized asphalt underlayment prevents slippage and reliably bonds to roof decks. On steep pitches from 4:12 and greater, lighter-duty synthetic underlayments sufficiently shed water at a lower cost and easier installation than adhered membranes.

By Climate and Weather Conditions

In wet climates with wind-driven rain, rubberized asphalt seals out moisture through fully waterproof, self-sealing membrane installation. Where heavy snow loads accumulate for months, the durability boost underlayments provide against ice dams and water ingress makes their use a necessity. In warm, high-UV regions, UV-resistant underlayments prevent premature brittleness and degradation.

For High Traffic/Debris Exposure Risks

On roofs exposed to overhead trees or maintenance foot traffic, synthetic underlayments like polypropylene provide exceptional tear strength and puncture resistance compared to felt alternatives prone to ripping when snagged or pierced.

For Vulnerable Areas Needing Waterproofing

The absence of mechanical fasteners piercing through rubberized asphalt membranes makes them ideal for reliable waterproofing around roof openings for skylights, plumbing vents and chimneys. They seal tightly and conform around flashing and fasteners via self-adhesion.

Proper Underlayment Installation for Optimal Function

While material selection marks a significant portion of underlayment effectiveness, proper installation remains equally vital to prevent leaks and realize the product's intended benefits:

• Attach underlayment securely to roof deck using mechanical fasteners like cap nails, or adhesive bonding centered over rafters.
• Overlap seams correctly, whether lapped shingle-style or taped/sealed depending on product specifications.
• Extend underlayment sufficiently past roof edges and overlaps with flashings to allow water drainage over fascia boards.
• Ventilate attic spaces to avoid interior moisture accumulation if condensation forms under the membranes.

When underlayment application follows manufacturer instructions, it establishes a critical second line of defense against water infiltration past damaged or aging asphalt shingles.

Signs Your Asphalt Shingle Underlayment Needs Replacement

While asphalt shingles visibly weather and crack over 15-25 years, underlayments concealed beneath them may still adequately protect from water despite similar aging. However certain symptoms indicate underlayment failure requiring replacement:

• Water stains on interior ceilings and walls indicating moisture seeping through from inadequate underlayment water protection.
• Curled, stiffened, or slipping underlayment visible through missing shingles signaling loss of self-adhesive bond to the roof deck.
• Sudden increase of ice dam formation and icicle buildups in winter reveal insulation saturation from chronic underlayment leaks.

Replacing worn underlayment immediately helps prevent irreversible moisture damage from escalating into mold, rotting rafters, and roof collapse over time.

Budgeting Factors and Expected Lifespans

When estimating roofing project costs, underlayment expenses represent approximately 10-30% of total new roof materials expense. On average, proper underlayment installation leads to a service life spanning two to three traditional asphalt shingle replacements before renewal is recommended. Factoring in durability and maintenance benefits, enhanced underlayment can provide excellent return on investment.

While protective underlayment remains a vital component underlying asphalt shingles, lack of visibility makes it easily overlooked by homeowners. Consultation with professional roofing contractors offers invaluable guidance as to prudent selections for your home's specific roof configuration and regional weather patterns. They will ensure the optimal underlayment keeps your largest defense against the elements soundly weatherproofed for decades to come.