The Evolution of Breathable Roofing Felt: History, Benefits, and Modern Challenges

Breathable roofing felt revolutionized roofing by addressing condensation issues prevalent with earlier materials. Initially developed in Germany in the 1980s, breathable felt became common in British roofing only toward the end of the 20th century. Its robust design, made from non-woven polypropylene, allows moisture control and durability. However, proper installation and ventilation remain critical to prevent defects like overstretching or mold. This article explores the history, advantages, and challenges of breathable felt, alongside the evolution of roofing materials, including older sarking felts and their modern alternatives.

The Origins of Breathable Roofing Felt

The concept of ‘breathable’ roofing felt emerged in the early 1980s, originating in Germany as a response to stricter building standards. The first patents for breathable felt were granted in 1983, laying the foundation for its eventual adoption in British re-roofing projects toward the end of the 20th century. Before its widespread use, plastic-based felts like Monarfoil were common but caused severe condensation issues within roof voids.

What is Breathable Felt?

Breathable roofing felt is made from non-woven polypropylene, providing excellent durability and resistance to UV degradation. This material facilitates moisture escape while preventing water ingress, making it an ideal underlayment for modern roofs. However, the felt must be properly installed with additional ventilation sources like soffit vents or ridge vents to ensure effectiveness. The British Standard 5534 (2015) introduced best practices, including using eaves felt support trays to enhance performance at gutter levels.

Modern Challenges with Breathable Felt

Despite its advantages, improper installation can lead to issues with breathable felts. Common problems include:

1. Overstretching: Roof felts should drape loosely between rafters, with a sag of 15-25mm. This ensures wind-driven rain flows along the central sag, preventing contact with timber battens. Overstretching compromises this function, leading to defects like mold growth and reduced roof lifespan.
2. Ventilation Gaps: Many assume that breathable felt alone provides sufficient airflow, neglecting the need for additional vents. Poor ventilation can exacerbate condensation problems, affecting the longevity of the roof structure.

Evolution of Sarking Felt

Sarking felt, used before the advent of breathable membranes, was made from hessian coated in bitumen. While effective for its time, it had several limitations:

• Brittleness: Over time, exposure to UV rays and moisture caused sarking felt to become brittle and tear, leading to leaks.
• Condensation: Unlike modern breathable felts, traditional sarking felt trapped moisture, increasing the risk of rot and wood-boring insects.
• Eaves Issues: At eaves levels, deteriorated felt often allowed rainwater to collect on walls and timber wall plates.

Replacing aged sarking felt often involved significant costs. However, introducing eaves felt trays offered a cost-effective solution by reinforcing exposed sections and mitigating water damage.

Historical Perspectives on Roofing Materials

In the early 20th century, roof designs relied on high-quality slate or clay tiles laid in overlapping patterns, eliminating the need for underfelt. During the 1930s housing boom, “double-lapped” clay tiles provided excellent weather resistance. However, material shortages during subsequent decades led to the adoption of half-lapped tiles and bitumen-based sarking felts.

By the 1950s, materials like “SiselKraft”, a laminated bitumastic paper, gained popularity despite their short lifespan. Concrete interlocking tiles became standard by the 1960s, prompting the need for stronger underlays to prevent wind-driven rain from penetrating roof structures.

Modern Roofing Materials and Lifespan

Modern breathable felts resolve many issues faced by older materials, offering superior durability and moisture control. However, even newer roofs require regular inspections to identify defects like overstretched felts, mold growth, or improper ventilation.

Some roofs from the 1930s and earlier are still serviceable today, a testament to the durability of traditional materials like slate and clay tiles. However, these roofs often require thorough inspections to check for rot, rusting nails, and timber decay.

Addressing Condensation in Roof Voids

To combat condensation in modern roofs, several measures can be implemented:

1. Extractor Fans: Removing moisture-laden air at the source.
2. Vapour Barriers: Installing barriers at ceiling levels to minimize moisture transfer into roof voids.
3. Improved Ventilation: Adding vented soffits and ridge vents for enhanced airflow.
4. Re-Felting: Upgrading to breathable membranes to replace older, less effective materials like Monarfoil or sarking felt.

Conclusion

Breathable roofing felt has significantly improved roofing standards by addressing the challenges of moisture control and durability. However, the effectiveness of any roofing system depends on proper installation and adequate ventilation. Understanding the evolution of roofing materials and addressing common defects can help homeowners and professionals extend the lifespan of roof structures.

Sources

1. British Standard 5534: Code of Practice for Slating and Tiling (2015)
2. Historic England: “Traditional Roofing Materials”
3. Roofing Today Magazine: "The Evolution of Roofing Felts"

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