What is Modified Butimen for Flat Roofs

First developed in Europe in the 1960s, modified bitumen roofing systems were designed to incorporate the benefits of built-up roofing systems with revolutionary polymer chemistry. The result is a sturdy, versatile roofing system that easily outperforms other conventional low slope roof systems.

What Is Modified Bitumen Roofing Made Of?

Modified bitumen is a type of built-up roofing system consisting of asphalt bitumen layers that have been modified or enhanced with synthetic polymers to improve the material’s longevity, strength and flexibility compared to traditional asphalt roofs.

The two most common polymers added to bitumen are:


  • APP polymer: Atactic polypropylene (APP) membranes give the asphalt a more plastic-like quality.
  • SBS polymer: Styrene-butadiene-styrene (SBS) adds a rubber-like quality.


Modified bitumen roofs can be installed in a number of ways: hot-applied, cold-applied, torch-applied and self-adhered. The type of polymer used dictates the installation method: APP materials are usually torch-applied (welded method), while SBS can be installed using the hot-applied (mop method), cold-applied or self-adhered methods.

What Projects Is Modified Bitumen Best For?

Commercial roofers could recommend modified bitumen roofs for a number of clients and climates. Like other forms of built-up roofing systems, modified bitumen roofing is best for use on buildings that have a low slope or flat roof. This makes it an ideal option for most commercial building projects.

One of the benefits of a modified bitumen roofing system is that it is applied in two or three layers, which makes it less likely to leak. They are also strong enough to withstand heavy foot traffic, so if employees frequently access a commercial building’s roof, modified bitumen is a good option. And it’s easy to repair: If it does get torn or damaged, modified bitumen can be easily and economically patched.

This material is also ideal for hot, sunny climates. It’s available in solar reflective or thermal emission cool coatings, and is designed to resist UV-light damage, blistering, cracking and oxidation. It also performs well in cold-weather conditions — because the material is so flexible, it will not crack or become brittle, even when temperatures drop below freezing.

While it is generally considered as a commercial roofing system, modified bitumen’s durability and flexibility mean it would also work well as a residential flat roofing option.

Considerations When Using Modified Bitumen

There are a few safety and installation concerns contractors should keep in mind while working with modified bitumen. Because they are made with polymers, the consistency of modified bitumen products can vary. Product quality depends on the compatibility of the ingredients and “the recipe used during the blending process,” notes the National Institute of Building Sciences. Manufacturers have to choose and monitor their raw asphalt carefully. Do your research and ask your distributor for help choosing a quality modified bitumen.

Cool temperatures and precipitation will affect the application of hot-applied modified bitumen systems. Installing in wet conditions increases the chances of blistering. If it’s too cold, the products become less flexible. Sheets need to be rolled out to relax, so installation in air temperatures below 45 degrees Fahrenheit is not recommended.

For the hot-applied method, an asphalt temperature of at least 425 degrees Fahrenheit is critical for the adhesion of modified bitumen products to create a bond between all layers of the roof. For the torch-applied application method, working with an open flame is inherently dangerous to roofers and a potential fire hazard to the building structure. Only experienced roofing experts should undertake the torch method of application. When dealing with any heated chemical products, it is important to avoid inhaling fumes. Ensure that all roofers are using face masks at all times during the installation process.

While modified bitumen roofing isn’t the only roofing option available for commercial buildings, its viability and durability make this roofing system the ideal choice for many commercial projects.

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