Felt Roofing: A Comprehensive Guide

Felt roofing, often referred to as Built-Up Roofing (BUR) or simply 'felt', is a prevalent and time-tested method for waterproofing flat or low-sloped roofs. Its ubiquity in both residential and commercial structures is a testament to its durability and cost-effectiveness. Unlike the more complex interlocking tiles or shingles found on pitched roofs, felt roofing systems rely on multiple layers of bitumen-saturated felt, meticulously applied to create a seamless, waterproof barrier. This guide will delve into the intricacies of felt roofing, exploring its composition, installation, common problems, and essential maintenance practices to ensure its longevity.

What Exactly is Felt Roofing?

At its core, felt roofing is a multi-ply system. The primary material is a robust felt, typically made from organic fibres or fibreglass, which serves as a strong base. This felt is then saturated and coated with bitumen, a thick, black, viscous liquid derived from petroleum. Bitumen is a highly effective waterproofing agent, providing excellent resistance to water penetration. The system is built up in layers, with each layer of felt and bitumen overlapping the previous one. This layering technique is crucial for creating a robust and reliable waterproof membrane.

The Layers of a Felt Roof

A typical felt roofing system will consist of several key components, laid down in a specific order:

• Vapour Control Layer (VCL): This initial layer prevents moisture from the building's interior from migrating into the roof structure and causing condensation.
• Insulation: Often a rigid insulation board is laid over the VCL to improve the thermal efficiency of the building.
• Underlay Felt: The first layer of bitumen-coated felt is applied over the insulation or directly onto the roof deck.
• Interfelt Layers: Subsequent layers of felt and bitumen are applied, with each layer overlapping the one below. The number of layers can vary depending on the desired performance and expected lifespan of the roof.
• Cap Sheet (Surfacing Felt): The final layer is often a surfaced felt, typically embedded with mineral granules. These granules protect the bitumen from UV radiation, which can degrade the material over time, and also provide a more aesthetically pleasing finish.

The Advantages of Felt Roofing

Felt roofing has maintained its popularity for several compelling reasons:

• Cost-Effectiveness: Compared to many other flat roofing systems, felt roofing is often the more budget-friendly option, both in terms of initial material cost and installation labour.
• Durability and Longevity: When installed correctly and maintained properly, a felt roof can last for 20 to 30 years, sometimes even longer. Its multi-layered structure provides excellent resistance to wear and tear.
• Waterproofing Capabilities: The bitumen-based system creates a highly effective, continuous waterproof membrane, ideal for flat surfaces where water might otherwise pool.
• Repairability: While damage can occur, felt roofs are generally quite straightforward to repair. Minor splits or blisters can often be sealed with specialist repair compounds, making it easier to address small issues before they become major problems.
• Versatility: Felt roofing is suitable for a wide range of flat roof applications, from garages and extensions to larger commercial buildings.

Common Issues with Felt Roofs

Despite its strengths, felt roofing is not immune to problems. Understanding these common issues can help in early detection and prevention:

Blisters

Blisters are raised areas that form in the felt. They are typically caused by trapped moisture or air within the layers, which expands when heated by the sun. While not immediately critical, significant blistering can compromise the integrity of the waterproof membrane.

Splits and Cracks

Over time, the bitumen can become brittle, especially if exposed to extreme temperature fluctuations or UV radiation without adequate protection. This can lead to splits and cracks, creating pathways for water ingress.

Pooling Water

Flat roofs require adequate drainage. If the roof deck sags or the drainage outlets become blocked, water can pool on the surface. Prolonged standing water can put excessive stress on the felt and exacerbate any existing weaknesses.

Damage to Edges (Eaves and Verges)

The edges of a roof, known as eaves and verges, are particularly vulnerable. These areas are where guttering is typically attached to channel water away. Damage to the felt or flashing at these points can lead to leaks and water damage to the underlying structure.

UV Degradation

While surfacing felts offer protection, older or poorly maintained roofs may experience UV degradation of the bitumen, leading to a shortened lifespan.

Repairing a Felt Roof

Addressing issues with a felt roof promptly is key to preventing more extensive damage. For minor damage, such as small splits or blisters, a DIY approach might be feasible:

DIY Repairs: You can purchase specific sealant products designed for felt roof repairs. These are typically applied to clean, dry surfaces over the damaged area. For blisters, the blister may need to be cut open, dried out thoroughly, and then sealed with the repair compound. Always follow the manufacturer’s instructions carefully.

Professional Repairs: For more significant damage, such as widespread blistering, large cracks, or issues with the roof edges and drainage, it is highly recommended to call in a qualified roofing professional. They have the expertise and tools to assess the problem accurately and carry out more robust repairs, which might involve patching sections of felt or even replacing larger areas.

The Importance of Eaves and Verges

As mentioned, the eaves and verges are critical points on any roof, especially a flat one. They are the first line of defence against water ingress at the roof edges. The correct installation and maintenance of the flashing and guttering systems here are paramount. Blocked gutters, for instance, can cause water to back up under the felt, leading to significant damage. Regular clearing of gutters and inspection of the felt at these vulnerable points is a crucial aspect of felt roof maintenance.

Note: Lifespans and costs are approximate and can vary based on installation quality, climate, and maintenance.

Maintaining Your Felt Roof

Regular maintenance is the secret to maximising the lifespan of your felt roof. Here are some key maintenance tips:

• Regular Inspections: Conduct visual inspections of your roof at least twice a year, ideally in the spring and autumn. Look for any signs of blistering, cracking, splitting, or areas where the surfacing granules may have worn away.
• Clear Debris: Remove any leaves, twigs, or other debris that accumulates on the roof, especially around drainage outlets. This prevents blockages and standing water.
• Clean Gutters: Ensure your gutters and downpipes are clear of obstructions to facilitate proper water drainage away from the building.
• Address Minor Damage Promptly: Don't ignore small issues. A quick repair to a minor split can prevent a major leak and costly damage later on.
• Professional Check-ups: Consider having a professional roofer inspect your felt roof every few years to identify any potential problems you might have missed.

Frequently Asked Questions

Q1: How long does a felt roof typically last?

A: A well-installed and maintained felt roof can typically last between 20 and 30 years, though some can last even longer.

Q2: Can I walk on a felt roof?

A: While felt roofs are durable, it's generally advisable to avoid unnecessary foot traffic. If you must walk on it, do so carefully and wear soft-soled shoes. Avoid sharp objects or anything that could puncture the surface.

Q3: What is the best way to repair a small hole in felt roofing?

A: For small holes, a specialist felt roof repair compound or bitumen-based sealant can be used. Clean the area thoroughly, ensure it's dry, and apply the sealant generously over and around the hole, extending slightly onto the surrounding felt.

Q4: Is felt roofing suitable for very hot climates?

A: While bitumen can soften in extreme heat, modern felt roofing systems, especially those with mineral granule surfacing, are designed to withstand a wide range of temperatures. Proper installation and adequate surfacing are key.

Q5: When should I consider replacing my felt roof instead of repairing it?

A: If your felt roof shows widespread signs of damage, such as numerous blisters, extensive cracking, delamination of layers, or if leaks are persistent despite repairs, it is likely time for a full replacement.

In conclusion, felt roofing remains a practical and economical choice for flat roof applications. By understanding its construction, potential issues, and committing to regular maintenance, homeowners and building managers can ensure their felt roof continues to provide reliable protection for many years to come. Always remember that for significant repairs or concerns, consulting a seasoned roofing professional is the wisest course of action.