Modified bitumen is an asphalt-based roofing system developed in Europe in the 1960s and widely used in North America since the 1970s. It serves as one of the most common flat and low-slope roofing systems for smaller commercial buildings, residential additions, and garages.
How Does Modified Bitumen Roofing Work?
Manufacturers build a modified bitumen flat roof as a two-layer system containing a base sheet and a cap sheet. Both layers are made from asphalt modified with specific polymers — typically APP or SBS.
- APP modifier: Makes the asphalt harder and more UV resistant. Best for hot climates.
- SBS modifier: Makes the asphalt rubberized and highly flexible. Best for colder regions like Upstate NY.
SBS-modified bitumen is generally preferred for its cold-weather flexibility in Upstate NY winters.
Installation Methods for Modified Bitumen
Torch-Applied (Torch Down)
A roofer uses a propane torch to melt the underside of the membrane, bonding it directly to the substrate. The melted asphalt creates a strong, watertight seal. Torch down roofing is the most popular installation method — so much so that "torch down" has become the nickname for the entire system.
Torch-applied mod bit requires an experienced installer — improper torching creates a severe fire hazard.
Cold-Applied (Self-Adhered)
Uses a peel-and-stick membrane that requires no open flame — safer and faster. However, this method requires a perfectly clean and dry substrate and creates a slightly less durable bond in extremely cold temperatures.
Hot-Mopped
The traditional approach using hot liquid asphalt mopped directly onto the substrate. Less commonly used today as torch-applied and cold-applied methods offer greater efficiency.
Lifespan and Cost
A properly installed system offers a lifespan of 20 to 30 years. The installed cost ranges from $4 to $8 per square foot — a mid-range cost compared to competing systems. It costs slightly more than EPDM but prices out similarly to TPO.
Modified bitumen is one of the most repairable flat roofing systems — contractors can patch small damaged areas easily with matching material.
Modified Bitumen vs. TPO and EPDM
- Modified bitumen: Asphalt-based, excellent foot traffic resistance, heavier weight, proven technology
- TPO: Thermoplastic, heat-welded seams, better energy efficiency through a reflective surface, lighter weight
- EPDM: Rubber membrane, excellent cold-weather flexibility, lowest upfront cost
Modified bitumen handles foot traffic much better than either TPO or EPDM. Choose it if your roof requires regular access for HVAC or equipment maintenance.
Frequently Asked Questions
Modified bitumen is an asphalt-based multi-layer system, while TPO is a single-ply thermoplastic membrane. Modified bitumen handles foot traffic better and allows for easier repairs. TPO is lighter, more energy-efficient due to its white surface, and features heat-welded seams. The right choice depends on your specific building application.
Yes. Torch down roofing is the common industry nickname for torch-applied modified bitumen. Because the torch-applied installation method remains the most popular, the nickname stuck and is used interchangeably.
A properly installed system lasts 20 to 30 years. Regular maintenance and prompt repair of any damage can extend the service life toward the upper end of that range. Annual inspections are highly recommended.
Yes, specifically SBS-modified bitumen. The rubberized SBS polymer keeps the membrane flexible at low temperatures, significantly reducing cracking. For Upstate New York, specify SBS-modified bitumen rather than APP-modified when requesting estimates.
In some cases, yes. Local building codes may permit a single overlay. However, most municipal codes limit flat roofs to a total of two layers. Contractors often prefer a full tear-off because it allows them to inspect and repair the underlying roof deck before installing the new membrane.
