What is a Modified Bitumen Roof?

Modified bitumen, also known as ModBit, is more common on residential roofs but is also used on commercial roofs. Although it has a lot of advantages, it isn’t for everyone. Continue reading to learn what a modified bitumen roof is, how long it lasts, how much it costs, the benefits of a modified bitumen roof, and more.

Before we talk about modified bitumen, we should mention what bitumen is. Bitumen (pronounced ‘bit-oo-men’), also known as asphalt, is a thick, sticky, black liquid. It’s a form of petroleum.

What is modified bitumen?

Modified bitumen is bitumen that has rubbers or plastics added to it then adhered to fiberglass or polyester fabrics. It was developed in Europe in the mid-1960s and has been used as a flat roof option since 1975. Modified bitumen comes in two types: SBS (Styrene Butadiene Styrene. Most common.) and APP (Atactic Polypropylene). APP ModBit has plastics in it. SBS ModBit has rubber in it. Therefore, SBS is a little more flexible. We only offer SBS modified bitumen.

Modified bitumen is used on low slope or flat roofs, either commercial or residential, but more often on residential buildings such as homes, apartments, etc. You will also find modified bitumen on low-slope sections of generally steep sloped roofs.

modified bitumen on low slope on steep slope roof
modified bitumen on low slope of steep slope roof

Modified bitumen lasts 10-15 years, on average, depending upon all sorts of factors, such as if it was professionally installed. On average, modified bitumen costs around $4.25/sf. total, for materials and installation. This is just an average. Please contact us for a free estimate for your particular commercial roof.j

Advantages and Disadvantages of a Modified Bitumen Roof


Modified bitumen expands and contracts with the changing temperature, so it’s tear and crack resistant. It performs extremely well in areas where the temperature fluctuates greatly. It costs just a little more than asphalt shingles at $4.25/sf. Because of the added fiberglass or polyester, modified bitumen has a high tensile strength, which makes it resistant to tears and hail. It also holds-up extremely well in areas with high foot traffic. In the rare case modified bitumen does tear, it is quickly and easily patched with bitumen patches. Because SBS modified bitumen comes in rolls, it’s very easy to install.

Bitumen is essentially petroleum, or oil. Oil and water don’t mix. Modified bitumen has oil or asphalt in it. Therefore, it is extremely water resistant. Modified bitumen is a flat membrane that is mechanically attached to the roof deck. Therefore, it is extremely resistant to high winds.

Although modified bitumen naturally comes as black, a top reflective layer can be added to reflect sunlight, therefore, reducing your energy bill. Because modified bitumen is strong and weather resistant, it is naturally low maintenance. That said, it is a good idea to have your modified bitumen roof inspected regularly.

Modified bitumen comes in many different colors that can closely match shingles. For example, white, black, slate, tan, shake, green, etc.

Because SBS modified bitumen is mechanically attached, it provides excellent resistance to wind and wind uplift.


One disadvantage of modified bitumen, is its lifespan. At only lasting around 10 to 15 years, it doesn’t last as long as other commercial roofing, or residential roofing systems. It’s also not as impact resistant, such as from hail, as asphalt shingles are. If not properly installed, or if not allowed to drain adequately, pooling can occur, which can prematurely age your ModBit roof.

photo of water pooling on a flat roof