Modified Bitumen Roofing and Telltale Signs – Part I of VII

We recently had a new client call with a concern about a relatively new roof on an old historic building, here in Washington DC. The roof wasn’t leaking, but they had just bought a recently renovated row home in DC.  Like many victims of dirty flips, the new homeowners were concerned that there might be issues with the quality of the recent renovations.

The outline of the seven-part series on modified bitumen flat roofing follows, today we will start with section #1:

  1. What is Modified Bitumen Roofing?
  2. Hard Requirements 
  3. Best Practices
  4. Telltale Sign Of Experience
  5. Core Sampling 
  6. Rounded Corners and Why They are Better
  7. Bleed-out as a Sign for Proper Application 

We will start by explaining the basic fundamentals of what a modified bitumen roof membrane is and how it is made and how it is applied.  Later in this series we will look at telltale signs of quality and problems hidden just beneath the surface that are observable in modified bitumen low-slope roof systems.

What is Modified Bitumen Roofing?

Modified bitumen roof membranes are one of the many types of modern single-ply roofing materials used on low slope and flat roofs.  Modified bitumen is considered a modern single-ply material, but it’s older than most of the other alternative types. In fact, modified bitumen, while not a historic type of material, has been used for decades.

modified bitumen roof materials
Modified bitumen roof materials are manufactured in either a smooth or rough texture.

The photo shows a rough texture ceramic granule covered membrane.  The granules are available in a variety of colors. The smooth variety of membrane is lighter weight, but requires more maintenance, as it must be recoated and reposted throughout the life of the roof.  

The most common type of bitumen, around us everyday, is the asphalt type of material used on roadways. We see that almost everywhere we go.  Modified bitumen, by comparison to asphaltic bitumen, is very different because modified bitumen, when manufactured, has a high velocity air applied to the surface which changes the material on a molecular level.  This distinction in the manufacturing process sets it apart from conventional asphaltic bitumen. During production, modified bitumen experiences this application of high-velocity air to its surface, inducing molecular changes. This process, known as hot air leveling, involves blowing heated air onto the bitumen surface. The high-velocity air serves multiple purposes, primarily aiding in the dispersion and alignment of polymer modifiers within the bitumen matrix. As the air interacts with the bitumen, it facilitates the creation of a more uniform and stable molecular structure. This molecular alteration enhances the material’s overall properties, imparting increased elasticity, flexibility, and resistance to temperature fluctuations. The modified bitumen’s molecular transformation contributes to its superior performance in roofing applications, providing a durable and weather-resistant solution. This nuanced manufacturing approach distinguishes modified bitumen from traditional asphaltic bitumen, creating its engineered molecular composition tailored for enhanced functionality in roofing systems.

Modified bitumen single-ply flat roof membranes are composed of a blend of asphalt and synthesized rubber-like modifiers, Modified bitumen roll membranes are often reinforced with multiple internal layers of polyester or fiberglass for added strength and tear resistance. The application, onto a roof deck or polyisocyanurate substrate on the roof, involves torch / flame activation (melting) of the bottom layer’s emulsion which allows the membrane to adhere to the roof deck or recovery board substrate The torch-welded seams create a continuous, near watertight surface. 

low-slope front porch roof
This image shows a low-slope front porch roof with a modified bitumen smooth roof membrane.   The original installer did not apply a coating to this roof and that coating is now needed.

Alternatively, self-adhesive versions use a peel-and-stick method for installation, yet actually do not simplify the process.  In fact, self-adhesive type applications are known to be fraught with problems such as later detachment or delamination.

Somewhere around half of the flat roofs in Washington DC are built with modified bitumen materials, but it’s arguably not the best material available today. As an alternative to modified bitumen, synthetic polymer or thermoset materials like TPO are available and for a slightly increased price, TPO offers many advantages over modified bitumen roofing.  Nonetheless, because it’s so prolific, our company, as a trusted roofing contractor, must be familiar with the modified bitumen materials and system requirements so we can both troubleshoot and repair roofs where we are called out to for these needs.

Upkeep, Care and Maintenance of Washington DC Roofs

The upkeep, maintenance, and general care of flat and mansard roof systems should be driven by an understanding and passion for historical methodologies, waterproofing principles, engineering and building science. Washington DC, a city built with both vintage charm and contemporary modernities, residential and commercial buildings of substantial value. 

The roofs of these buildings are their defensive shield from the harsh elements of nature. To our clients, as well as all readers of this article and our blog, we emphasize the importance of quality construction and active building maintenance.  Our website includes informative resources you can use to understand and learn about best practices on preservation of your building.  If you are in need of further guidance on the roof and its associated systems for your Washington DC property, we are here to assist, where we can. Simply contact us or complete the webform below and drop us a line and we’ll respond if we can help.

delaminated seam
The picture shows a delaminated seam at a rear termination drip edge, discovered at a routine maintenance check up, a defect from tbe original installer.
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