Modified Bitumen Roofing and Telltale Signs – Part II of VII

Today’s discussion picks back up from earlier this week, in our discussion on modified bitumen single-ply roof systems.  In our last article we explained how modified bitumen systems are manufactured and the differences between smooth and granulated systems.  

Today, we will talk about standards for installation.  We refer to “Hard Requirements” as the actual rules that are enforced by law.  There aren’t many of these hard requirements, and there is even less practical enforcement,  unfortunately.  If there were more regulations and enforcement, the low quality contractors would be removed from the industry altogether.  

poorly installed modified bitumen roof system
The image above shows an example of a poorly installed smooth modified bitumen system.  This system requires the application of a protective coating but before hiring our company the building homeowner was not informed of the requirements by the original contractor.

The outline for the entirety of this seven-part series on modified bitumen flat roofing follows, with todays portion in bold:

  1. What is Modified Bitumen Roofing?
  2. Hard Requirements
    • Slope
    • Flashing:  
    • Crickets
    • Counterflashing
    • Galvanized Fasteners 
    • Priming
    • Limited Recovery
  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 continue, 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.

Hard Requirements

There’s a handful of items that are best practices but are not hard requirements. To start, it’s good to understand the meaning of a “hard requirement”.   Requirements that are specifically listed in the written building code are hard requirements.  Those requirements must be followed.  It sounds simple, but it isn’t always simple.  The building code, regarding flat roofing, is sparse and limited.  It’s as if the council decided to go light on that area because it was too technical, or not important enough, for them to elaborate.  It doesn’t seem to make sense.  The building code requires certain things, but these items are limited.  A short list of the main requirements follow:

Slope

The building code requires that a flat or low slope roof, even though called flat, is not technically flat at any location within the field of the roof. The minimum allowable slope in Washington DC is 2%.  Two percent translates to roughly 1/8 of an inch per linear foot so that means that for each linear foot of horizontal run of the roof system, the roof must have a positive grade or slope (or fall) of 1/8 of an inch downward away from the upper part of the roof.

Flashing: Coping

Copings are metals that span over top of parapets. They are a requirement of both residential and commercial construction, A parapet is a wall that continues from the building structure to above the roof line. These parapet walls are extremely common in the historic neighborhoods of Washington DC such as Capitol Hill, Dupont circle, in Georgetown. Almost all rowhomes in Washington DC have parapets. The roof membranes are required by code to terminate at a counterflashing or cap flashing (if they terminate or continue to area of coping). Coping is then required at the top of the parapet wall to cover the parapet wall.  An interesting side note is that coping is not required in all cases of other masonry walls where it is not part of a roof.

For example,  in a retaining wall, coping may not be required. However, with a roof system, a similar type of masonry wall is required to have a coping that spans the top of the masonry wall.

Flashing: At wall And roof intersection.  

Flashing is required where the roof meets an adjacent wall. This makes sense and in most cases where a roof meets an adjacent wall a counter flashing will be set into the wall to cover the base flashing. This achieves the code’s requirements for a base and cap flashing as the counterflashing serves as the cap flashing. In many cases with single-ply roof membranes the membrane can be run from the horizontal surface up over a cant strip onto the vertical surface of the wall and that membrane will qualify as the flashing itself. In this particular case of a continuous thermoset or polymer membrane, for example, it is better to have the continuous membrane run up the wall to under the counterflashing instead of having a separation in the horizontal area of the roof where there is a transition to a different material of base flashing.

Crickets

At chimneys over 30”, a roof cricket is a sloped structure installed on the high side of a chimney, diverting water away from the chimney’s base. It is a small ridge-like feature designed to prevent water accumulation and facilitate proper drainage on the roof. Building codes mandate the installation of a cricket for chimneys with widths exceeding 30 inches. The requirement serves to address potential water pooling issues associated with larger chimney structures. Without a cricket, rainwater or melting snow can accumulate behind the chimney, leading to water infiltration and increased risk of leaks. The cricket redirects water flow, preventing ponding and ensuring efficient runoff. This not only safeguards the chimney structure but also contributes to the overall integrity of the roof, helping to meet water management and waterproofing standards, against ponding water, reflected in building codes.  Here we are specifically referring to the building code requirements which do not necessarily require crickets art curbs for equipment or skylights, but later in this series when we address best practices, we will talk about crickets in those areas as well.

Counterflashing (or termination bars) 

The code, as written, often refers to “base” and “cap” flashing.  The building code requires cap flashing and in some limited cases requires counter flashing, but counter flashing should be installed at almost all areas of termination of modified bitumen. With alternative types of membranes such as thermoset or polymer membranes (such as TPO roofing), termination bars are also often an acceptable substitute to counter flashing.  In most cases, counter flashing is a strip of aluminum or  similar sheet metal which has been custom bent to fit the shape of the substrate and is inserted into adjacent masonry or attached to adjacent vertical services and then extended downward to cover over the top of the exposed edge of the roof membrane.

The picture below shows a parapet, not only not covered with a metal coping which is wrong but also unfortunately a very common low-quality shortcut in DC, but also with a loose lap edge which can allow water to enter under the roof membrane.

electrogalvanized fastner head
The list of problems with this installation is numerous, but there is also an exposed electrogalvanized fastener head shown in the photo above.  See #6 in our list here, below, for more info on the shortcomings of this type of galvanization.
aluminum coping
The image above shows a properly installed aluminum coping.
roof system without metal coping
This image shows another example of a roof system without metal coping.  Although this is a requirement of the Building Code, it is often omitted by low-quality contractors.

Galvanized fasteners

The building code’s allows for galvanized fasteners without specifying hot-dipped galvanized fasteners in roofing is often based on the assumption that both types meet the minimum requirements for corrosion resistance. However, the distinction between hot-dipped galvanized and electrogalvanized fasteners lies in their manufacturing processes and, consequently, their corrosion resistance properties.

Hot-Dipped Galvanized Fasteners

The hot-dipped galvanization process involves immersing steel fasteners in molten zinc, creating a comparatively thick and durable zinc coating that provides effective corrosion protection. Hot-dipped galvanized fasteners are generally considered higher quality because of the thicker zinc coating. This makes them more resistant to corrosion in harsh environments, including exposure to moisture, chemicals, and weathering.

Electrogalvanized Fasteners

Electrogalvanizing uses an electrolytic process to deposit a thinner layer of zinc onto the steel surface.  Electrogalvanized coatings are typically thinner than those achieved through hot-dipping. While they offer some corrosion resistance, electrogalvanized fasteners may be less durable in typical exterior conditions, making them potentially inferior for certain applications. 

The decision not to explicitly require hot-dipped galvanized fasteners in the building code may be based on the recognition that electrogalvanized fasteners look very similar to hot-dipped galvanized fasteners and not all supervisors, inspectors, or contractors are trained to recognize the difference.  (This is not a sufficient excuse though.) Specifically, in roofing applications where exposure to the elements is significant, and corrosion resistance is crucial for the long-term integrity of the structure, hot-dipped galvanized fasteners should be preferred. The thicker zinc coating provides enhanced protection against rust and corrosion, ensuring the longevity of the fasteners and the overall roofing system.

Priming

Priming of metal flashing to which bituminous materials are to be adhered.  Adhesion involves several mechanisms, including mechanical adhesion, physical adhesion, and chemical adhesion. Asphaltic primers are formulated to provide chemical adhesion, where the molecules in the primer create bonds with the molecules on the metal surface. This chemical interaction promotes a strong and durable bond.

Limited recovery 

You can only recover a roof one time. After the first recovery, that recovery can no longer be removed and have for the substrate to be recovered again, a successive  Application must be a full removal of both existing membranes if more than one exists.

modified bitumin roof system
The modified bitumen roof system shown here at a low angle is an uncoated smooth system.  A cant strip is missing

This coming week we are going to continue with this discussion and examine the difference between the roofing requirements we have described above and best practices. To many people’s surprise, best practices go way farther than the building code. Often other contractors do not even meet the minimum requirements of the building code but good contractors should consider the building code a minimum and our company has best practices that far exceed the requirements of the building code.

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.

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