When comparing commercial roofing systems, one of the most common questions property owners ask is: What is the main difference between Modified Bitumen and Built-Up Roofing (BUR)? While both systems are designed for low-slope and flat commercial roofs and have decades of proven performance, they differ significantly in their construction, installation methods, flexibility, maintenance needs, and long-term performance.
Understanding these differences helps building owners, facility managers, and commercial property investors choose the roofing system that best fits their building, budget, and climate.
The Primary Difference
The biggest difference between Modified Bitumen (Mod Bit) and Built-Up Roofing (BUR) is how the roofing membrane is constructed.
- Modified Bitumen consists of factory-manufactured asphalt sheets reinforced with fiberglass or polyester and modified with polymers such as SBS (Styrene-Butadiene-Styrene) or APP (Atactic Polypropylene). These sheets are installed in one or two layers.
- Built-Up Roofing (BUR) is created on-site by alternating multiple layers of asphalt (or bitumen) with reinforcing fabrics or felts, then finished with gravel or a mineral cap sheet.
In simple terms, Modified Bitumen uses pre-manufactured membranes, while BUR is built layer by layer during installation.
Construction and Installation
Modified Bitumen systems are designed for faster and more consistent installation. Contractors can install them using torch application, cold adhesives, hot asphalt, or self-adhered membranes depending on the product and project requirements.
BUR systems require several alternating layers of hot asphalt and reinforcing felts, making installation more labor-intensive. Once the layers are complete, a protective gravel surface or cap sheet is applied.
Because BUR involves multiple built-up layers, installation generally takes longer and requires experienced roofing professionals.
Flexibility and Weather Performance
One of Modified Bitumen’s biggest advantages is flexibility.
SBS-modified membranes remain flexible during temperature changes, allowing them to expand and contract without cracking. This makes them particularly effective on buildings that experience structural movement or varying weather conditions.
BUR systems are durable but generally less flexible due to their layered construction. While they provide excellent waterproofing, they may become more susceptible to cracking as they age if not properly maintained.
Shieldline Roofing’s Expert Opinion
The main difference is how the membrane is constructed: BUR is built on the roof from alternating layers of bitumen and reinforcing plies, while modified bitumen uses factory-made reinforced sheets containing polymer-modified asphalt. For Florida low-slope roofs, we evaluate the building, exposure, and installation requirements before recommending either system.
Our Key Insights
The National Roofing Contractors Association describes modified bitumen as a prefabricated reinforced waterproof sheet made with asphalt, polymer modifiers, and reinforcing fabrics, while BUR is assembled from multiple plies and bitumen on-site. This factory-controlled construction gives modified bitumen more flexibility in installation methods, including self-adhered, cold-applied, and heat-welded systems.
Durability and Waterproofing
Both roofing systems offer excellent waterproof protection when professionally installed.
BUR has earned its reputation through redundancy. Multiple layers create several barriers against water penetration, meaning minor damage to one layer does not necessarily result in an immediate leak.
Modified Bitumen typically relies on fewer layers but compensates with advanced polymer technology that enhances puncture resistance, waterproofing, and flexibility. Learn More
