Roofing felt, also known as underlayment, plays a critical role under flat roof membranes by offering an additional layer of protection against moisture, wind-driven rain, and temperature fluctuations. This guide explains what roofing felt is, the types most commonly used on flat roofs, installation considerations, maintenance practices, and how to choose the right product for long-term performance in American climates.
What Roofing Felt Is And Why It’s Used On Flat Roofs
Roofing felt is a layer of material laid beneath the primary roofing membrane to improve water resistance and provide a stable surface for the finished roof. On flat roofs, felt helps seal joints, improves wind uplift resistance, and can assist in temporary weatherproofing during construction. It also serves as a secondary barrier if the primary membrane is damaged. The material is typically asphalt-saturated felt made from organic or fiberglass bases, coated with asphalt to repel moisture.
Types Of Roofing Felt For Flat Roofs
There are several common types of roofing felt used on flat roofs, each with distinct characteristics and suitability for different systems:
- Organic Felt (pressphalt): Made from paper or felted organic materials saturated with asphalt. It is less common today due to lower moisture resistance in extreme climates but can be found in some older installations.
- Fiberglass-Saturated Felt: A lighter, more durable option with better moisture resistance and heat tolerance than organic felt. Often chosen as an underlayment for built-up roofs (BUR) and modified bitumen systems.
- Mineral-Surface Felt: Fiberglass or organic base with an embedded mineral surface to resist UV exposure. Commonly used as an underlayment in multi-ply BUR or APP modified bitumen roofs, providing a durable, weather-resistant layer.
- Self-Adhered (SA) Felt: Pre-primed and self-adhesive, activated by heat or pressure. Ideal for cold or complex roof details where torch or hot-mop methods are impractical.
Installation Considerations For Flat Roofs
Proper installation is essential to ensure roofing felt performs as intended. Key steps and considerations include:
- Substrate Preparation: Clean and dry the roof deck, repair any surface irregularities, and ensure a smooth, even base to prevent wrinkles and blisters in the felt.
- Underlayment Layout: Overlap seams according to manufacturer guidelines, typically 2–4 inches for shaft or horizontal seams, with staggered joints for added waterproofing.
- Adhesive And Fastening: Use compatible asphalt-based adhesives or mechanical fasteners as recommended. Self-adhered felt requires proper primer and sufficient heat or pressure to activate the adhesive.
- Ventilation And Drains: Maintain proper drainage and prevent ponding. Ensure there are no gaps around penetrations, flashings, or edges where water could back up under the membrane.
- Transition Details: Pay careful attention to edges, parapets, and penetrations (vents, vents, drains) with compatible flashings and sealants to avoid leaks.
Maintenance, Lifespan, And When To Replace
Roofing felt on flat roofs typically lasts 10–20 years depending on climate, exposure, and the type of felt. In harsher environments or with improper installation, it may degrade sooner. Regular inspections after significant weather events help identify signs of aging, such as cracking, curling edges, or exposed asphalt. If the felt shows extensive deterioration or if the primary roof membrane requires frequent repairs, replacement should be considered alongside roof system upgrades.
Common Problems And How To Address Them
Flat roofs using roofing felt can experience several issues. Awareness helps prevent costly damage:
- Ponding Water: Accumulated water can accelerate felt deterioration. Improve drainage and repair deck slopes if necessary.
- Blistering Or Wrinkling: Usually from moisture under the felt, poor adhesion, or substrate irregularities. Remove and re-install affected areas, ensuring a clean surface.
- Cracking At Seams: Result of temperature cycling and aging. Re-seal seams or replace the felt in affected zones.
- UV Degradation: Prolonged sun exposure can soften asphalt; use compatible membranes and avoid exposing felt longer than recommended during construction.
Choosing The Right Roofing Felt For A Flat Roof
Selection depends on the type of flat roof system, climate, and expected service life. Consider these factors:
- Roof System Compatibility: Ensure the felt type matches BUR, modified bitumen, or single-pply membranes used above it.
- Climate And UV Exposure: In hot, sunny regions, choose high-performance felt with strong heat resistance and UV stability.
- Fire And Regulatory Requirements: Some projects require specific underlayment fire ratings or code-compliant materials.
- Installation Method: Self-adhered products can simplify complex details but may require proper surface preparation and temperature control.
- Warranty And Availability: Purchase from reputable suppliers with documented warranties and clear installation guidelines.
Best Practices And Practical Tips
Adopting best practices helps maximize performance and minimize leaks:
- Follow Manufacturer Guidelines: Adhere to temperature, exposure, and overlapping requirements provided by the felt and membrane manufacturers.
- Quality Control: Inspect rolls for tears or defects before installation and verify that adhesive or fasteners are correctly applied.
- Professional Installation: For complex roofs, hire experienced crews familiar with flat-roof felt applications to ensure proper detailing and flashings.
- Documentation: Keep records of materials, installation dates, and inspection notes to support future maintenance or warranty claims.
Frequently Asked Questions
Is roofing felt necessary on a flat roof? While some modern systems rely solely on membranes, felt underlayment provides an extra layer of protection against moisture and aids in nail sealing and temporary weatherproofing during installation.
Can roofing felt be installed in cold weather? Some self-adhered felts are designed for colder conditions, but performance can be limited. Always follow manufacturer temperature guidelines.
How often should a flat roof with felt be inspected? Conduct a professional inspection at least once per year and after severe weather events to catch issues early.
Conclusion
Roofing felt remains a versatile and valuable underlayment option for flat roof installations, offering moisture management, improved sealant performance, and added durability when properly installed and maintained. By choosing the right felt type, aligning with the specific roof system, and adhering to best practices, homeowners and builders can extend the life of flat roof assemblies and minimize maintenance costs.