Roof Felt

What is Roof Felt?

Roof felt is a sheet material designed to waterproof the roof structure and interior of a building from water ingress in the event that the outer covering is compromised - whether by wind-driven rain, snow, ice dams, or physical damage. Historically made from organic fibres saturated in tar, modern felts predominantly use a base of fibreglass or polyester, impregnated and coated with bitumen (asphalt), sometimes modified with polymers for enhanced performance.

Why Is It Important? 

• Waterproofing: Provides protection against water ingress should the primary roof covering (tiles, slates, shingles) fail or allow wind-driven rain/snow underneath.
• Weather Barrier: Protects the roof deck from rain and snow exposure during construction or if the primary covering is damaged.
• Control of Wind Uplift: Sufficiently fixed underlay contributes to securing the roof covering against wind forces.
• Temporary Weatherproofing: Allows a building to be temporarily weatherproofed during construction phases.
• Component of Fire Safety: Can contribute to the overall fire rating of a roof system when specified correctly (e.g., Class A).
• Prevents Chemical Interaction: Acts as a separation layer between potentially incompatible materials (e.g., certain wood treatments and bitumen).

Understanding Roof Felt Materials: From Traditional to Modified

The performance and lifespan of roof felt are heavily influenced by its composition:

• Base Material:
  • Organic Felts: Historically used materials like wood cellulose, jute, or hessian. Largely obsolete now due to poor durability and moisture absorption leading to rot.
  • Fibreglass Felts: Offer good dimensional stability and fire resistance but can be more brittle than polyester.
  • Polyester Felts: Provide excellent tensile strength, flexibility, and puncture resistance, making them very durable.
• Bitumen & Modifiers: Standard bitumen provides waterproofing but can become brittle in cold and soft in heat. Modifiers enhance its properties: 
  • SBS (Styrene-Butadiene-Styrene): An elastomeric (rubber-like) modifier. Provides excellent flexibility, especially at low temperatures, making it resistant to cracking from thermal movement or structural stress. It has good recovery properties. Installation requires careful torching as it becomes sticky rather than liquid. Preferred in climates with wide temperature fluctuations like the UK. Lifespans often 15-30 years.
  • APP (Atactic Polypropylene): A plastomeric (plastic-like) modifier. Offers a higher softening point, making it very resistant to heat and UV radiation. It melts to a more liquid state when torched, considered easier for less experienced installers but can be over-torched. Less flexible than SBS, especially in the cold, making it potentially more prone to cracking over time due to movement. Often chosen for warmer climates or where ease of install is key.

Key Types of Roof Felt Systems

Different applications require different types and systems:

• Torch-On Systems (Mainly Flat Roofs): Multi-layer systems (typically 2-3 layers) where SBS or APP modified bitumen sheets are bonded together and to the substrate using a propane torch. Layers often include a vapour control layer (VCL), reinforced underlay(s), and a mineral-surfaced cap sheet for UV protection. Highly durable and reliable when installed correctly by trained professionals. 
  • Example: IKO TGX Torch-On range.
• Pour and Roll (Built-Up Roofing - BUR): A traditional flat roofing method involving layers of felt bonded with hot poured bitumen. Increasingly less common due to safety and the rise of torch-on/single-ply systems.
• Self-Adhesive Felts: Sheets with a peel-off backing exposing an adhesive layer. Safer (no flame) and easier for DIY on sheds, porches, or small garages. Performance depends on quality and surface preparation. Can include underlays and cap sheets. Lifespans often 15-20 years.
• Shed Felt: Economical bitumen felt, often with a mineral surface, designed specifically for non-habitable outbuildings. Easy to install with nails and sometimes adhesive. Offers basic weatherproofing for 5-15 years depending on grade (standard vs. 'super' or polyester reinforced). 
  • Example: IKO Shed Felt & Super Shed Felt.

Relevant UK Standards and Regulations

Compliance with standards ensures minimum performance levels:

• BS 6229:2018: Code of practice for flat roofs with continuously supported coverings.
• BS 8217:2005: Code of practice for reinforced bitumen membranes (RBMs).
• BS 5534:2014+A2:2018: Code of practice for slating and tiling (pitched roofs), including underlay requirements and wind uplift fixing specifications.
• BS EN 13707: Flexible sheets for waterproofing - Reinforced bitumen sheets for roof waterproofing.
• BS EN 13859-1: Flexible sheets for waterproofing - Underlays for discontinuous roofing (covers breathable membranes).
• BS 5250: Code of practice for control of condensation in buildings (guides ventilation requirements for cold roofs).
• Building Regulations Part L: Conservation of Fuel and Power - dictates thermal insulation requirements, impacting roof design (Warm vs. Cold).
• BBA Certification: Third-party agrément certificates assure fitness-for-purpose for specific products and systems.

Warm Roof vs. Cold Roof Construction

The position of insulation defines the roof type, significantly impacting felt choice and ventilation needs:

• Cold Roof: Insulation is placed between the roof joists/rafters (at ceiling level for flat roofs, or rafter level for pitched). The roof deck and structure above the insulation remain 'cold'. Requires ventilation above the insulation (typically a 50mm gap) to disperse moisture vapour, as per BS 5250. Traditional bitumen felt (non-breathable) is often used here, relying on ventilation. Can be prone to thermal bridging through joists.
• Warm Roof: Insulation is placed above the roof deck/rafters. The structure remains 'warm', on the inside of the insulation layer. A Vapour Control Layer (VCL) is essential on the warm side of the insulation (directly over the deck) to prevent moisture vapour from entering the construction. No ventilation is required within the roof structure itself. This is the preferred modern method for flat roofs and insulated pitched roofs due to better thermal efficiency and reduced condensation risk. Requires compatible felts/membranes applied over the insulation or supporting board.

Installation Best Practices & Safety

Correct installation is paramount for longevity. Key considerations include:

• Surface Preparation: Deck must be clean, dry, stable, and compatible with the chosen system.
• Laying Direction: Typically start at the lowest point, laying sheets perpendicular to the slope.
• Laps: Ensure correct minimum side and end laps (e.g., 100mm-150mm for underlays, specific requirements for multi-layer systems). Seal laps as required (heat welding, cold adhesive).
• Fixings: Use appropriate fixings (galvanised clout nails for underlays/shed felt, specific fasteners for mechanical fixing). Avoid puncturing VCLs unnecessarily.
• Detailing: This is where many roofs fail. Pay meticulous attention to: 
  • Eaves: Underlay must be supported (e.g., eaves tray) and dressed into the gutter.
  • Verges: Secure fixing and appropriate finishing (e.g., undercloak, dry verge system).
  • Abutments (Walls/Chimneys): Turn felt up minimum 100-150mm against abutment, supported. Integrate correctly with flashings (lead, GRP, etc.) and cavity trays.
  • Penetrations (Pipes, Vents): Cut felt neatly, turn up around penetration. Use flexible (homogenous/glass tissue) felt for collars/aprons. Seal effectively using methods like clip & mastic, proprietary weathering collars, or membrane sleeves. Ensure adequate support and isolation from membrane movement if needed.
• Torch-On Safety (Critical): Requires trained, competent professionals. Key safety points include: 
  • Risk Assessment & Permit: Hot works permit system often required.
  • Equipment Checks: Regularly inspect hoses (orange LPG hose, check date), regulators (correct thread, safety cut-off), torches (clean, stand available, electronic ignition preferred).
  • Fire Safety: Appropriate fire extinguishers readily available (CO2, powder, water mist depending on risk). Continuous 'Fire Watch' during work, breaks, and for at least 60 minutes after completion.
  • PPE: Flame-retardant overalls, heat-resistant gloves, eye protection, safety boots, hard hat.
  • Gas Handling: Keep cylinders upright, secure on trolleys, handle correctly.
  • Never torch directly onto timber.

Common Causes of Felt Roof Failure

Understanding why felt roofs fail helps prevent issues:

• Poor Installation: Incorrect laps, inadequate adhesion/fixing, poor detailing, wrinkles/bubbles trapping air/moisture.
• Water Pooling (Ponding): Inadequate roof falls/drainage allows water to stand, accelerating degradation and stressing the felt.
• UV Degradation: Sunlight breaks down bitumen over time, causing brittleness and cracking, especially if mineral protection is lost.
• Thermal Movement: Expansion and contraction cycles stress the felt, leading to splits, particularly with less flexible types or inadequate provision for movement.
• Mechanical Damage: Foot traffic, falling objects, hail, or subsequent work on the roof can puncture or tear the felt.
• Blistering: Trapped moisture or air expanding when heated by the sun, creating bubbles that can eventually rupture.
• Ageing: All materials degrade over time; felt eventually reaches the end of its service life.
• Lack of Maintenance: Debris blocking drains causes ponding; minor damage left unrepaired escalates.