Torch down roofing is a modified bitumen roofing system installed by heating the underside of rolled roofing membranes with a propane torch. The heat softens the asphalt, allowing the membrane to bond to the roof deck, insulation board, or underlying roofing layer.
Torch down roofing is also called torch-applied roofing, torch-applied modified bitumen, or torch-on roofing. It is most common on flat and low-slope roofs, including commercial buildings, garages, porches, extensions, and some residential structures.
Properly installed and maintained modified bitumen roofing can last 20 years or more, according to Polyglass. Its service life depends on the membrane, installation, drainage, climate, maintenance, foot traffic, and the rest of the roof assembly.
Torch Down Roofing at a Glance
| Feature | Description |
|---|---|
| Main material | Polymer-modified asphalt, also called modified bitumen |
| Installation | A propane torch heat-welds roofing membranes together |
| Typical roof type | Flat and low-slope roofs |
| Common polymers | APP and SBS |
| Construction | Usually a base sheet plus a granulated cap sheet |
| Main advantage | Strong water resistance and durable seams |
| Main risk | Open-flame installation can cause fires if performed improperly |
| DIY suitability | Not recommended |
How Does Torch Down Roofing Work?
Torch down roofing is installed in several layers. A typical installation includes the following steps:
- Roof preparation: The contractor checks the roof deck, drainage, flashing, and existing roofing.
- Base sheet installation: A smooth modified bitumen sheet is attached with approved fasteners, adhesive, or heat welding.
- Cap sheet installation: A mineral-granulated membrane is rolled into position.
- Heat welding: The contractor heats the underside of the membrane until the modified asphalt softens and bonds to the layer below.
- Seam and flashing work: Overlapping edges, roof penetrations, parapets, and wall transitions are sealed to limit water intrusion.
The finished system creates a continuous water-resistant layer. It is not a single sheet with no joints. It still has seams and flashing details, but properly heat-welded laps can form strong, watertight connections.
What Materials Are Used in Torch Down Roofing?
Torch down roofing uses modified bitumen, which combines asphalt with polymers and reinforcement such as polyester or fiberglass.
APP Modified Bitumen
APP, or atactic polypropylene, is a plastic-modified asphalt membrane commonly installed with a torch. APP membranes generally provide strong UV and weather resistance and are often used for cap sheets.
SBS Modified Bitumen
SBS, or styrene-butadiene-styrene, is a rubber-modified asphalt membrane designed for greater flexibility and elongation. Depending on the product and roof assembly, SBS membranes may be torch-applied, cold-applied, hot-mopped, or self-adhered.
The right membrane depends on the roof design, climate, manufacturer requirements, fire classification, and installation method. APP and SBS describe the membrane chemistry. "Torch down" describes how the membrane is installed.
What Are the Benefits of Torch Down Roofing?
Torch down roofing can provide strong water resistance and a durable membrane system for suitable flat and low-slope roofs.
Strong Water Resistance
Heat-welded seams can create durable bonds between membrane layers. Torch-applied modified bitumen is therefore used on low-slope roofs that need protection from rain and occasional ponding water.
Reinforced Construction
Modified bitumen membranes usually contain reinforcement that helps resist splitting and tearing. Multi-layer systems also add redundancy because more than one membrane layer contributes to water resistance.
Better Performance Through Temperature Changes
Polymer modification helps the asphalt membrane handle expansion and contraction better than conventional unmodified asphalt. SBS generally provides more rubber-like flexibility, while APP is commonly selected for UV and weather resistance.
Use in Repairs and Reroofing
Torch-applied membranes can be used for new roofs, reroofing projects, flashing details, and repairs to some existing modified bitumen or built-up roofing systems. The contractor must confirm product compatibility before installation.
Potentially Long Service Life
Polyglass states that properly installed and maintained modified bitumen roofing can have an average life expectancy of 20 years or more. The result depends on membrane quality, installation, drainage, climate, foot traffic, maintenance, and the complete roof assembly.
What Are the Disadvantages?
Torch down roofing has one major limitation: installation uses an open flame. The roof deck, insulation, framing, and surrounding materials must be suitable for that method.
Open-Flame Fire Risk
Torch down roofing uses a propane torch. OSHA warns that improper torch use can transfer enough heat through the roofing system to ignite combustible decking, wood framing, or concealed materials. OSHA guidance also recommends fire-watch inspections for at least two hours after the last torch is extinguished.
Installation should be handled by a qualified roofing contractor using hot-work procedures, fire extinguishers, and fire-watch practices.
Not Suitable for Every Roof Deck
The contractor must evaluate the roof deck and the materials beneath the membrane before using an open flame. Torch application may be unsuitable near combustible construction, concealed voids, insulation systems, vents, air intakes, or other sensitive areas.
Installation Quality Matters
A seam that is not heated enough may fail to bond. Too much heat can damage the membrane or the materials below it. Poor flashing, misplaced laps, and weak details around penetrations can also lead to leaks.
Odors and Disruption
Torching asphalt can create smoke and odors during installation. This may affect occupied homes, offices, hospitals, schools, and neighboring properties.
Repairs and Inspections Are Still Required
A torch down roof is not maintenance-free. Debris, blocked drains, standing water, punctures, failed flashing, and damaged seams can shorten its service life. Roof drains and vulnerable details should be inspected regularly.
Is Torch Down Roofing the Same as Built-Up Roofing?
No. Torch down roofing and built-up roofing are both asphalt-based low-slope roofing systems, but they use different materials and installation methods.
| Roofing system | How it is installed | Main distinction |
|---|---|---|
| Torch-applied modified bitumen | The membrane underside is heated with a torch | Uses factory-made modified bitumen rolls |
| Built-up roofing, or BUR | Multiple asphalt-saturated felts are layered with hot asphalt | Uses several plies and traditionally hot-mopped asphalt |
| TPO or PVC | A single-ply plastic membrane is heat-welded | Does not use asphalt or an open flame for normal seam welding |
| EPDM | A rubber membrane is usually adhered or mechanically fastened | Uses synthetic rubber rather than modified asphalt |
| Self-adhered modified bitumen | Factory adhesive bonds the membrane | Avoids torch installation in many assemblies |
GAF identifies torch-applied, hot-mopped, cold-applied, and self-adhered systems as different installation methods for modified bitumen roofing.
Is Torch Down Roofing Good for a House?
Torch down roofing can be a good choice for a house with a flat or low-slope roof, but it is not normally appropriate for a conventional steep-slope roof.
It may suit:
- Flat-roof additions
- Garages
- Porches
- Dormers
- Commercial-style residential roofs
- Low-slope roof sections where shingles cannot provide adequate drainage
The contractor should confirm that the roof has adequate slope and drainage. Torch down roofing does not correct poor roof design, blocked drains, structural movement, or chronic ponding water.
How Much Does Torch Down Roofing Cost?
Torch down roofing prices vary based on roof size, access, tear-off requirements, insulation, deck repairs, flashing, location, membrane type, and warranty requirements.
The installation method can increase labor and insurance costs because it requires specialized equipment, hot-work procedures, and fire protection. A reliable estimate should identify:
- Whether the old roof will be removed
- The number of membrane layers
- Base sheet and cap sheet products
- Insulation or cover board
- Deck repairs
- Flashing and edge-metal work
- Drainage improvements
- Manufacturer and contractor warranties
- Fire-watch and safety procedures
Do not compare estimates by price per square foot alone. A cheaper two-layer assembly may not provide the same durability, warranty coverage, or fire classification as a specified three-layer system.
What Should You Ask a Roofing Contractor?
Before approving a torch down roofing project, ask:
- Which membrane will be installed, APP or SBS?
- How many layers will the system contain?
- What roof slope and drainage improvements are required?
- Is the roof deck combustible or otherwise unsuitable for torch application?
- How will the contractor protect walls, vents, insulation, and concealed spaces from heat?
- What fire-watch procedure will be used after torching stops?
- Which manufacturer installation instructions apply?
- What are the workmanship and manufacturer warranty terms?
- Will the contractor provide proof of licensing and insurance?
- Are self-adhered or mechanically attached alternatives safer for this building?
Bottom Line
Torch down roofing is a modified bitumen roofing system that uses a propane torch to heat-weld membrane sheets onto a flat or low-slope roof. It offers strong water resistance, reinforced construction, and flexibility, but the open flame creates a serious fire risk.
It is worth considering when a qualified contractor can install the specified membrane assembly safely and the roof has sound decking, adequate drainage, and well-designed flashing. A self-adhered or mechanically attached modified bitumen system may be a better option where open-flame work is unsafe or impractical.