Roofing projects rely on multiple layers to protect a home from weather, and the choice of underlayment is crucial. Roofing Felt and Ice And Water Shield are two common options, each serving different roles in a roofing system. This article explains what each product is, how they work, and how to decide which is best for a given climate, roof type, and budget. Readers will gain practical guidance on installation considerations, costs, and durability to inform smarter roof investments.
What Is Roofing Felt
Roofing felt, also known as tar paper or asphalt-saturated felt, is a traditional underlayment placed over the roof deck to provide a moisture-resistant layer beneath shingles. It comes in two main weights: 15-pound and 30-pound, with synthetic felt now common in many installations. The felt’s primary purpose is to protect the roof deck from incidental moisture during construction and to offer a secondary barrier against minor leaks after shingles are in place. It is not designed to be a stand-alone, long-term waterproof barrier, especially against wind-driven rain or heavy snow.
Traditional felt is installed with nails or staples and overlaps to create a continuous surface. While older homes often used organic (wood fiber) felt, modern installations favor asphalt-saturated felts or synthetic felts that resist tearing and provide better performance in moisture-prone climates. In warm, sunny environments, felt can deteriorate more quickly, whereas synthetic felts tend to resist UV exposure and temperature fluctuations better. Because felt is relatively cost-effective, it remains a common choice for many re-roofing projects when paired with a reliable roof covering.
What Is Ice And Water Shield
Ice and Water Shield is a self-adhesive, rubberized asphalt membrane designed to provide a waterproof layer where leaks are most likely to occur. It is typically applied at roof edges, valleys, around chimneys, and at other penetrations. The membrane forms a seamless barrier that can resist wind-driven rain, melting snow, and ice damming. With its self-adhesive backing and flexible composition, Ice And Water Shield adheres tightly to the roof deck and can accommodate expansion and contraction with temperature changes, improving durability in harsh climates.
This underlayment is designed for critical vulnerable areas where ice dams form in cold climates or where heavy rainfall occurs. In many regions, building codes or manufacturer guidelines require or strongly recommend Ice And Water Shield in eaves and along overhangs, valleys, and penetrations. Depending on the product, shield membranes range in thickness from roughly 30 to 40 mils and may be sold as 1- or 2-ply systems. Some installations cover entire roof sections in high-wind areas or for metal roofing, while others limit shield use to the lower edges of the roof to control cost while still providing essential protection.
Key Differences
- Function: Roofing Felt mainly provides a secondary moisture barrier and construction protection; Ice And Water Shield provides a true waterproof membrane in high-risk areas.
- Location: Felt is installed over the deck under the shingles; Ice And Water Shield is placed at eaves, valleys, and around openings, sometimes covering large portions of the roof in certain regions.
- Durability: Felt can degrade under UV exposure and heat; Ice And Water Shield resists moisture and remains tacky and flexible in cold temperatures, reducing leak potential at critical points.
- Costs: Felt is generally less expensive per square foot; Ice And Water Shield adds higher upfront cost but may extend roof life and reduce leak-related repair costs.
- Code and Warranty: Local codes and shingle warranties may influence whether shield membranes are required or recommended, especially in regions prone to ice damming.
Where They Are Used In Roof Systems
The combination of underlayers is common for modern roofs. A typical approach is to install roofing felt over the entire roof deck as a base underlayment, followed by the Ice And Water Shield in critical areas such as eaves, hips, valleys, and around penetrations. In harsh climates or high-wire wind zones, some contractors extend shield coverage beyond the edges, sometimes covering the entire roof surface for maximum protection. The exact configuration depends on climate, roof pitch, roof covering, and local building codes.
Installation Considerations
Installing Roofing Felt involves laying the material with consistent overlaps—typically 2 inches for standard felt and 4 to 6 inches for more weather-sensitive synthetic felts—then securing with roofing nails or staples. Seams are often sealed with adhesive tape or cement to reduce moisture infiltration. Proper ventilation and deck preparation are essential to prevent moisture entrapment beneath the felt.
Ice And Water Shield installation centers on surface adhesion and securement. The self-adhesive membrane adheres to the deck after removing the release film, with certain products requiring a primer on rough or porous substrates. Overlaps are generally 6 to 12 inches depending on the product and the roof slope. In steep pitches or complex roof geometries, contractors may use additional fasteners along the edges to prevent wind uplift. Proper ventilation and roof-edge detailing also influence the shield’s long-term performance.
Key installation notes include ensuring clean, dry decking before applying either underlayment, avoiding gaps at penetrations, and following manufacturer guidelines for adhesion, overlaps, and wind exposure. For DIY installers, following local building codes and manufacturer instructions is essential to avoid voiding warranties or compromising roof performance.
Cost And Longevity
Costs vary by product quality, roof size, and regional labor rates. Roofing Felt generally costs less per square foot than Ice And Water Shield. Typical ranges might be $0.25 to $0.60 per sq ft for 15-pound felt, $0.40 to $0.90 per sq ft for 30-pound felt, and $0.70 to $1.50 per sq ft for Ice And Water Shield. When including installation, total project costs reflect additional labor for sealing, overlaps, and edge detailing. Longevity depends on climate and exposure: felt may last 15 to 30 years under typical conditions but can fail sooner in extreme heat or moisture; Ice And Water Shield often endures the life of the roof under normal installed conditions, providing decades of protected edge and penetration areas, though the shield itself does not replace the need for proper roof covering.
In terms of performance, felt is susceptible to tearing and moisture infiltration if shingles are damaged or if water sits on the deck. Ice And Water Shield remains flexible in cold weather and resists water intrusion effectively at critical points, potentially reducing ice damming risk and interior leaks. The combination of a base felt layer with shield protection can provide a balanced solution that addresses both cost and performance, especially in climates with freeze-thaw cycles and heavy snowfall.
Practical Scenarios And Recommendations
| Scenario | Recommended Underlayment Approach | Rationale |
|---|---|---|
| Moderate climate, asphalt shingles, low to moderate snowfall | Roofing Felt as base; Ice And Water Shield at eaves | |
| Cold climate with frequent ice dams, high snowfall | Roofing Felt base plus Ice And Water Shield on entire roof or at minimum across eaves, valleys, and penetrations | |
| High-wind zone or complex roof geometry | Ice And Water Shield in critical areas; consider additional edge fasteners and wider overlaps | |
| Budget-conscious reroof | 35–45 mil ice/water shield in key locations; standard felt base for the rest |
Maintenance And Replacement Considerations
Underlayment maintenance focuses on preserving the barrier before and during a roof replacement. Inspect after severe weather for lifted edges, tears, or exposed deck material. If shingles are removed or damaged, check the underlayment for moisture infiltration and replace the affected sections as needed. In regions with rapid temperature swings or persistent moisture, synthetic felt may offer improved longevity over traditional organic felt. When reroofing, evaluate whether to upgrade to a full Ice And Water Shield installation in high-risk areas to maximize protection and align with current codes or manufacturer recommendations.
In summary, Roofing Felt and Ice And Water Shield serve complementary roles within a roof system. Felt provides a cost-effective base moisture barrier and construction protection, while Ice And Water Shield delivers essential waterproofing at edges, valleys, and penetrations. The optimal choice depends on climate, roof design, and budget, with many installations benefiting from a layered approach that combines both products to balance protection, durability, and cost for American homes.