How to Install Synthetic Roofing Underlayment?

Synthetic roofing underlayment, also known as synthetic underlayment or roof underlayment, is a crucial component of a well-designed roofing system. It is a water-resistant or waterproof material that is installed directly on the roof deck, serving as an additional layer of protection against moisture intrusion and providing a secure base for the installation of the primary roofing material, such as shingles or metal roofing.

Unlike traditional felt underlayment, synthetic underlayment is typically made from polymeric materials, such as polypropylene or polyethylene, making it more durable, tear-resistant, and longer-lasting. This type of underlayment has gained popularity in recent years due to its ability to withstand harsh weather conditions and its superior performance compared to its felt counterparts.

Importance of Proper Underlayment Installation

Proper installation of synthetic roofing underlayment is essential for ensuring the longevity and effectiveness of your roofing system. A correctly installed underlayment can help prevent water damage, mold growth, and premature deterioration of the roof deck and other components. Additionally, it can provide enhanced protection during the construction process, allowing the roofing project to continue even in inclement weather conditions.

In this comprehensive guide, we’ll walk you through the step-by-step process of installing synthetic roofing underlayment, covering everything from preparation to best practices and tips for a successful installation.

Preparing for Installation

Gathering the Necessary Tools and Materials

Before starting the installation process, it’s essential to gather all the necessary tools and materials. Here’s a list of what you’ll need:

  • Synthetic roofing underlayment rolls
  • Utility knife or roofing knife
  • Hammer or roofing nailer
  • Roofing nails or cap nails (appropriate for your underlayment type)
  • Tape measure
  • Chalk line or string line
  • Safety equipment (hard hat, gloves, non-slip shoes, etc.)

Additionally, you may need other materials depending on your specific roof configuration, such as drip edges, flashing, and sealants.

Inspecting and Preparing the Roof Deck

Before laying the synthetic underlayment, it’s crucial to inspect the roof deck thoroughly. Look for any damaged or rotted areas and make necessary repairs or replacements. Ensure that the deck is clean, dry, and free from debris, as any irregularities or protrusions can potentially puncture or damage the underlayment.

If you’re working on an existing roof, remove the old roofing material and inspect the deck closely. Any protruding nails or fasteners should be either removed or hammered flush with the surface.

Safety Considerations

Working on a roof can be hazardous, so it’s essential to prioritize safety throughout the installation process. Wear appropriate safety gear, such as non-slip shoes, gloves, and a hard hat. If you’re working at significant heights, consider using a fall protection system or harness.

Additionally, be mindful of potential electrical hazards, such as overhead power lines, and take necessary precautions to avoid accidents or injuries.

Step-by-Step Installation Guide

Installing Drip Edges and Starter Courses

Before laying the synthetic underlayment, it’s essential to install drip edges along the eaves and rakes of the roof. Drip edges help prevent water from getting behind the roofing materials and causing damage to the roof deck or fascia boards.

Once the drip edges are in place, you can proceed to install the starter course or edge metal along the eaves. This will provide a secure base for the underlayment and ensure proper water shedding.

Laying the Underlayment

With the drip edges and starter courses in place, it’s time to lay the synthetic roofing underlayment. Begin at the eaves and work your way up the roof, ensuring that each course overlaps the previous one according to the manufacturer’s recommendations.

Overlapping and Fastening

Proper overlapping and fastening are crucial for a successful underlayment installation. Most synthetic underlayments have printed overlapping guidelines or lines that indicate the required overlap distance. Follow these guidelines carefully, ensuring that each course overlaps the previous one by the recommended amount.

Fasten the underlayment to the roof deck using the appropriate nails or cap nails for your specific product. Drive the nails through the underlayment and into the roof deck, spacing them according to the manufacturer’s instructions. Ensure that the nails are flush with the surface and do not create any punctures or tears in the underlayment.

Handling Valleys and Penetrations

When laying the underlayment, pay special attention to valleys, roof penetrations (such as vents or skylights), and other irregularities. These areas require additional care and attention to ensure proper water shedding and protection.

In valleys, it’s recommended to use a specialized valley underlayment or to create a water-tight valley by overlapping the underlayment courses in a specific pattern, as per the manufacturer’s instructions.

For roof penetrations, cut the underlayment carefully around the protrusion, leaving enough space for proper flashing installation. Seal any gaps or openings with appropriate sealants or flashing to prevent water intrusion.

Securing the Underlayment

Once the entire roof area is covered with synthetic underlayment, it’s essential to secure the edges and overlaps properly. Use cap nails, roofing nails, or other fasteners recommended by the manufacturer to secure the underlayment at the edges, overlaps, and any other areas where it may be susceptible to wind uplift or movement.

Additionally, apply sealants or adhesives as recommended by the manufacturer to ensure a watertight seal at overlaps, penetrations, and other critical areas.

Best Practices and Tips

Proper Ventilation and Moisture Management

Proper ventilation and moisture management are crucial for the longevity and performance of your roofing system. Ensure that your attic or roof cavity is well-ventilated to allow for air circulation and prevent moisture buildup.

When installing synthetic underlayment, follow the manufacturer’s recommendations for moisture management and vapor barrier installation. Some underlayment products may require the use of a separate vapor barrier or may have integrated moisture management features.

Handling Weather Conditions

Weather conditions can significantly impact the installation process of synthetic roofing underlayment. High winds, heavy rain, or extreme temperatures can make the installation more challenging and potentially compromise the integrity of the underlayment.

If possible, plan the installation during favorable weather conditions to ensure a smooth and successful process. If you encounter adverse weather during the installation, take appropriate precautions and follow the manufacturer’s recommendations for working in those conditions. For example, you may need to use temporary protective coverings or adjust your installation techniques to account for wind or moisture.

It’s also important to allow for proper drying and curing times if the underlayment gets wet during installation. Rushing the process or covering a wet underlayment can lead to moisture issues and potential damage down the line.

Manufacturer’s Guidelines and Warranties

Every synthetic roofing underlayment product comes with specific installation guidelines and warranty information provided by the manufacturer. It’s crucial to carefully read and follow these guidelines to ensure a proper installation and maintain the validity of any warranties.

Failure to follow the manufacturer’s instructions can potentially void the warranty and may lead to issues with the product’s performance or longevity. If you have any questions or concerns, don’t hesitate to reach out to the manufacturer’s technical support for clarification or assistance.

Benefits of Using Synthetic Roofing Underlayment

While the installation process for synthetic roofing underlayment may seem more involved than traditional felt underlayment, the benefits of using this material are numerous and can provide significant advantages for your roofing system.

Durability and Longevity

One of the primary benefits of synthetic roofing underlayment is its superior durability and longevity compared to felt underlayment. Synthetic underlayments are designed to withstand harsh weather conditions, UV exposure, and temperature extremes without deteriorating or losing their effectiveness.

With proper installation and maintenance, a high-quality synthetic underlayment can last for decades, potentially outlasting the lifespan of the primary roofing material itself.

Water Resistance and Moisture Management

Synthetic underlayments are highly water-resistant and can provide an effective barrier against moisture intrusion. Unlike felt underlayments, which can become saturated and lose its water-shedding capabilities, synthetic underlayments maintain their water-resistant properties even after prolonged exposure to moisture.

Additionally, many synthetic underlayment products are designed with moisture management features, such as breathable membranes or integrated vapor barriers, which can help prevent condensation buildup and promote a healthier, more energy-efficient roofing system.


While the initial cost of synthetic roofing underlayment may be higher than traditional felt underlayment, it can be a cost-effective choice in the long run. The extended lifespan and superior performance of synthetic underlayments can help reduce the need for frequent replacements or repairs, ultimately saving money over time.

Furthermore, the enhanced protection provided by synthetic underlayment can help prevent costly water damage or premature deterioration of the roof deck and other components, further contributing to its overall cost-effectiveness.

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Installing synthetic roofing underlayment is a crucial step in ensuring the longevity and performance of your roofing system. While the process may seem daunting at first, following the proper steps, best practices, and manufacturer’s guidelines can make it a straightforward and successful endeavor.

By investing in a high-quality synthetic underlayment and ensuring its proper installation, you’ll be providing your roof with an additional layer of protection against moisture intrusion, weather exposure, and potential damage. The benefits of durability, water resistance, and cost-effectiveness make synthetic roofing underlayments an excellent choice for homeowners and contractors alike.

Remember, a well-installed roofing system is an investment in the safety, comfort, and value of your property. Take the time to carefully plan and execute the installation of your synthetic roofing underlayment, and you’ll be rewarded with a reliable and long-lasting roofing solution.


Q1: Can I install synthetic roofing underlayment over an existing layer of felt underlayment?

It’s generally not recommended to install synthetic underlayment over an existing layer of felt underlayment. The felt may contain moisture or deteriorate over time, compromising the performance of the synthetic underlayment. It’s best to remove the old felt and start with a clean, dry roof deck.

Q2: How do I seal the seams and overlaps of the synthetic underlayment?

Most manufacturers provide specific recommendations for sealing seams and overlaps, such as using compatible sealants, tapes, or adhesives. Follow the manufacturer’s guidelines carefully to ensure a watertight installation.

Q3: Can I walk on the synthetic underlayment after installation?

It’s generally safe to walk on synthetic underlayment, but you should exercise caution and follow the manufacturer’s recommendations. Avoid excessive foot traffic or concentrated loads that could damage the underlayment.

Q4: How long does synthetic roofing underlayment last?

The lifespan of synthetic roofing underlayment can vary depending on the specific product, installation quality, and environmental conditions. However, most high-quality synthetic underlayments are designed to last for several decades, often outlasting the primary roofing material.

Q5: Do I need a separate vapor barrier when using synthetic underlayment?

The need for a separate vapor barrier depends on the specific underlayment product and the climate in your area. Some synthetic underlayments have integrated moisture management features, while others may require an additional vapor barrier layer. Consult the manufacturer’s guidelines and local building codes for specific recommendations.

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Spike Miller

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