Introduction to Torch-on Felt Roofing
What Is Torch-on Felt Roofing?
Torch-on felt roofing is a method used to waterproof flat or low-slope roofs, utilising sheets of modified bitumen that are heated and bonded to the underlying surface. The system is built up in multiple layers, with each layer offering a specific function within the roof’s structure. The primary component, modified bitumen, is an asphalt-based product that is enhanced with either plastic (such as Atactic Polypropylene) or rubber (like Styrene Butadiene Styrene). This enhancement gives the material flexibility, allowing it to cope with seasonal temperature changes without cracking or melting.
Basic Components
The typical torch-on felt roofing system comprises:
- Modified bitumen sheets, forming the bulk of the waterproofing.
- A reinforcement mat, often made from fibreglass or polyester, which strengthens the membrane.
- A top mineral layer, sometimes embedded with granules for added UV protection and fire resistance.
How Installation Works
Installation starts by preparing the roof deck, ensuring it is clean and dry. Rolls of modified bitumen are then heated on the underside using a propane torch. As the bitumen softens, each sheet is rolled into place and firmly bonded. Overlaps between sheets are also heated, creating a continuous, watertight surface. Each layer builds on the previous one, with all seams sealed using the torch to avoid penetration by water.
Why It’s Popular
Torch-on felt roofing is especially favoured for flat and low-slope roofs due to its proven water resistance and durability against harsh weather. Unlike some materials, torch-on felt can expand and contract to accommodate both heat and cold, making it suitable for changing climates. Its tough, layered structure helps prevent leaks and withstands impact from debris or foot traffic.
Understanding the 3-Layer System
The Structure of Torch-on Felt Roofing
A high-performing torch-on felt roof is built using a three-layer system. Each layer plays a distinct and essential role:
- Venting Base Layer: This is the lowest layer against the roof deck. Its main function is to help moisture escape from beneath the roofing. It often includes perforations and a thermofusible film, which melts during installation and allows bitumen to bond through, creating a partial bond. This venting reduces the risk of trapped moisture and condensation issues, protecting the roof structure over time.
- Waterproofing Underlay: Installed above the base, the underlay is the primary barrier against water ingress. It provides reinforced protection, adding to the thermal insulation and boosting the roof’s ability to keep water out. Properly fitted, this layer is crucial for the longevity and durability of the roof.
- Mineral Cap Sheet: This is the final, exposed layer. It features a granular mineral finish that defends against harsh sunlight, mechanical impact, and the weather. The cap sheet also adds an extra waterproof barrier and preserves the roof’s appearance over time.
Layer Contributions and Comparison
Each of these layers is interdependent. The venting base manages moisture, the underlay delivers the core waterproofing, and the mineral cap ensures long-term resistance to UV and weather.
When comparing two-layer and three-layer systems, the three-layer build provides stronger protection and a longer lifespan. While a two-layer system uses just a base and cap sheet, it lacks the added granulated layer found in three-layer constructions. The extra cap sheet found in three-layer roofs increases energy efficiency, fire resistance, and general durability, with reduced maintenance needs and fewer problems over time.
Materials Used in Torch-on Felt Roofing
Modified Bitumen: The Core Waterproofing Material
Torch-on felt roofing relies on modified bitumen as its main waterproofing component. Modified bitumen combines traditional asphalt (bitumen) with modern polymers. Two main types of polymers are used: Atactic Polypropylene (APP) and Styrene Butadiene Styrene (SBS). APP is a form of plastic that adds flexibility and resilience to the felt, helping it withstand temperature changes. SBS is a type of rubber compound that allows the material to stretch and contract without cracking. This polymer modification makes torch-on felt roofs highly flexible and ensures they perform well in both hot and cold climates.
Reinforcement Layers: Polyester and Fibreglass
To improve the strength and lifespan of torch-on felt, the bitumen sheets include a reinforcement core. This core is usually made of polyester, fibreglass, or a mixture of both. Polyester reinforcement makes the felt resistant to tearing and is known for its flexibility, which helps accommodate slight roof movements or settling. Fibreglass adds dimensional stability and reduces stretching over time. These materials act as a tough backbone for the bitumen layers, helping the whole system resist physical damage and weathering.
Vapour Control Sheets and Mineral Finishes
The base layer of a torch-on roof often includes a vapour control sheet. This layer prevents condensation and moisture from beneath the roof structure from seeping into the system. Vapour control sheets both restrict excess humidity and allow necessary ventilation where required.
The top or cap sheet is finished with mineral granules. These granules provide several benefits: they protect the surface from UV rays, add colour, make the roof more resistant to fire, and reduce physical wear. The mineral finish forms a durable barrier against weather and sunlight, improving the roof’s life expectancy and performance.
Taken together, these specialised materials create a multi-layered roofing system that maximises weatherproofing, strength, and longevity. This careful material selection underpins the long-term reliability of torch-on felt installations.
The Installation Process
Preparation and Surface Assessment
Installing torch-on felt roofing begins with careful preparation. The surface must be dry, clean, and free from debris. Any existing damaged material is removed to ensure a stable base. If re-roofing, blisters or cracks in the old felt should be cut out, and the substrate must be primed for better adhesion of new layers. If the structure requires insulation, it is laid first, followed by a moisture barrier to help protect against trapped dampness and condensation.
Layer Application: Step-by-Step
The system is constructed in distinct layers.
- A venting base layer or vapour control sheet is applied first. This helps manage moisture and serves as the foundation.
- Next, a waterproofing underlay is rolled out, and precise alignment is checked.
- The mineral-finished cap sheet forms the top layer, giving durability and resistance to UV and weather.
Each sheet is measured, cut, and overlapped with the previous layer to avoid weak points. Proper overlaps are essential, commonly at least 75–100mm at seams.
The Role of the Propane Torch
A key feature of this method is the use of an open-flame propane torch. The bottom of the bitumen sheet is heated until it becomes semi-liquid and sticky. The installer steadily unrolls the membrane while applying the torch, bonding the sheet to the roof and sealing the overlaps at the seams. This process melts the surfaces of adjacent layers together, creating a watertight and flexible seal that protects against pooling water and harsh weather.
Importance of Professional Installation and Safety
Working with an open flame presents notable fire risks. Only trained professionals should carry out this task. Certified roofers understand how much heat to apply and how to create strong bonds without burning through materials. Safety measures are vital: protective gear, fire extinguishers on-site, and dry weather conditions are essential standards. Failure to follow safety practices may lead to property damage or injury.
Advantages of Torch-on Felt Roofing
Durability and Longevity
Torch-on felt roofing is valued for its impressive durability. When installed and maintained correctly, it regularly outlasts other flat roofing felt products. A typical torch-on felt roof will provide robust protection for at least 10 years. With routine care, it is common for these roofs to last 15 to 30 years, making them one of the most long-lasting flat roofing choices available. This superior performance is largely due to the structure and materials involved—particularly the reinforced bitumen and multiple protective layers—which resist tears, punctures, and everyday wear far better than traditional roofing felts.
Waterproofing and Weather Resistance
A key benefit of torch-on felt roofing is its exceptional resistance to water infiltration. The three-layer system, when properly torched and bonded, creates a seamless and watertight barrier. This waterproofing ability ensures the roof does not suffer from leaks, even in challenging weather. The layers manage both standing water and driving rain, which is especially important on flat and low-pitched roofs where water can pool. Additionally, the system withstands wind, rain, snow, and UV radiation. The cap layer provides an extra defence against sunlight, preventing degradation from heat and UV exposure.
Flexibility Across Climates
Torch-on felt roofs are highly adaptable to a broad range of temperatures and environmental conditions. The modified bitumen used in these systems allows them to expand during heat and contract in cold weather without cracking or melting. This flexibility is crucial in climates with large temperature swings, heavy snowfall, or intense sunlight. The roofing remains stable and effective whether exposed to freezing conditions or prolonged heat, maintaining its waterproof qualities and structural integrity throughout.
Lifespan and Maintenance
Expected Lifespan
Torch-on felt roofing is recognised for its reliability and ability to last. Most torch-on felt roofs can be expected to serve effectively for 10 to 20 years. The actual lifespan depends on factors such as the quality of materials chosen, the skill of installation, and local weather conditions. A professionally installed roof with premium components may last closer to the upper end of this range.
Maintenance Requirements
Routine care is important to maximise the longevity of a torch-on felt roof. Maintenance tasks are straightforward but crucial for early detection of any issues:
- Inspect the roof at least twice a year, ideally in spring and autumn.
- Remove leaves, branches, and other debris to prevent water pooling, which can degrade the felt and shorten its life.
- Check that gutters and downpipes are clear, as blockages can cause water to back up onto the roof.
- Examine overlaps, seams, and edges, as these can be points of potential weakness.
- Monitor flashings and seals, especially around vents and upstands.
Small repairs, such as patching minor blisters or splits, can be performed using compatible bitumen repair compounds.
Extending Roof Life
Addressing minor defects promptly is one of the best ways to protect against larger issues. Applying a reflective coating can also slow down the deterioration of the mineral cap sheet from UV exposure. Regular professional inspections allow for early detection of deterioration or damage that might not be obvious from a visual check at ground level.
Identifying and Addressing Issues
Common problems include blisters caused by trapped moisture, cracks from movement or extreme weather, and lifting at the edges. If these are caught early, repair is usually straightforward and can prevent more expensive issues developing. Persistent problems, such as recurring leaks or significant splitting, may indicate deeper problems requiring a full roof assessment.
Torch-on Felt vs Other Roofing Systems
Comparing Torch-on Felt and Hot-Mopped (Built-Up Roofing) Systems
Torch-on felt and hot-mopped roofing both serve flat and low-slope roofs, but there are crucial differences in material, installation, and performance. Torch-on felt uses sheets of modified bitumen, which are bonded to the roof with a propane torch. In contrast, a hot-mopped system involves layers of bituminous felt built up with hot liquid asphalt, spread with a mop to form a continuous waterproof layer.
Lifespan is similar for both systems, with torch-on felt typically lasting 15 to 30 years and hot-mopped roofs generally expected to last around 15 to 20 years. However, torch-on felt provides better resistance to punctures and UV damage, enhancing its long-term performance, particularly in harsh weather conditions. Hot-mopped roofs may be less expensive to install upfront, but they can be more vulnerable to tears and sun damage over time. Hot-mopped installation also produces more fumes, requiring extra precautions for worker safety and building occupants.
Torch-on Felt vs EPDM Rubber Roofing
EPDM (Ethylene Propylene Diene Monomer) rubber roofing is another common flat roof covering. It is a synthetic rubber material, usually supplied in large rolls for fewer seams. When comparing with torch-on felt:
- Durability: EPDM tends to outlast felt in many cases, with some membranes lasting over 30 years. EPDM’s flexibility and resistance to weathering make it less prone to splitting or cracking.
- Environmental Impact: EPDM is non-toxic and does not release oils, making it a more environmentally friendly choice, especially for green roofs.
- Seams and Installation: Torch-on felt is installed in smaller rolls, leading to more seams, which can be potential points of failure if not properly sealed. EPDM’s larger sheets reduce the number of seams and simplify installation.
- Cost: While EPDM may cost more to buy, torch-on felt can have higher installation costs due to labour and safety requirements.
Each roofing option has specific strengths, and the best choice depends on the building’s needs, environmental goals, and budget.
Conclusion: Is Torch-on Felt Right for Your Roof?
Key Takeaways and Main Benefits
Torch-on felt roofing stands out as a flat roof covering due to its remarkable durability, waterproof nature, and cost-effectiveness. Using a three-layer system of modified bitumen, reinforced mat, and a mineral finish, it creates a strong, weather-resistant surface that effectively seals out water and withstands UV exposure. This system can reliably last from 15 up to 30 years with proper maintenance, making it one of the most robust felt options.
Flexibility is another major benefit. Torch-on felt adapts well to complex roof shapes and features, allowing installation on both new builds and existing roof structures, including those with curves, slopes, or awkward sections. Its strong bond resists wind uplift, and routine maintenance can keep it performing for decades.
Important Considerations Before Choosing
Before opting for torch-on felt, consider the following:
- Safety: Installation involves an open flame, which must be handled by trained professionals to mitigate fire risks. This is not a DIY roofing solution.
- Installation Quality: The lifespan and watertight performance of torch-on felt depends heavily on correct installation. Poor workmanship can lead to early failure.
- Maintenance Commitment: While generally low-maintenance, regular inspections and quick attention to any issues will extend roof life and prevent costly water damage.
- Roof Suitability: Torch-on felt is ideal for flat and low-slope roofs but may not be the best solution for steeply pitched designs.
The Value of Professional Assessment
A professional roofer can evaluate your building’s specific needs, check the underlying structure, and recommend whether torch-on felt fits your circumstances. Their knowledge ensures that the right layering system is chosen, safety protocols are observed, and long-term performance is maximised.
Choosing the right roofing system means weighing up your property’s demands, budget, and your willingness to invest in periodic upkeep. Careful assessment and expert installation are essential to enjoying the full lifespan and benefits of a torch-on felt roof.
