Base flashing and Counter Flashing – Part I

Today, we start a two-part series discussing the elements and components associated with base and counter flashing. Penetrations or connections at joints at the perimeter of roof systems generally require a combination of both base and counterflashing, especially where there are dissimilar materials between the roof system and for substrate adjacent wall or chimney or piping systems.

The outline for this two-part series follows below:

  • Why One Isn’t Enough
  • Separation of Dissimilar Metals
  • A Required Seam
  • Inherent Differences Between Residential and Commercial
  • Pitch Pockets as an Alternative to Counter Flashing

Today, in this article, we will talk about topics I-III.

Why One Isn’t Enough

The picture below shows a modified bitumen flat roof, at the base of a chimney. At the particular installation, a metal counterflashing was not installed to cover the top of the base flashing. As a cheap shortcut some unprofessional roofer, maybe a handyman, just smeared asphalt mastic on top of the base flashing, many years in the past. At this point in time the base flashing has delaminated from the substrate chimney brick and allows a wide open pocket for water to enter the building.

base flashing
Base flashing without counter flashing is a prolific yet costly omission.  The resulting wide open pocket allows water to run right into the building, worse than just separation, more like a funnel.

The combination of both base flashing and counter flashing in roofing systems is needed, in most applications due to the configuration associated with the diverse geometric planes of walls and roofs, particularly in configurations involving walls or chimneys. Installation of counter flashing that seamlessly or continuously wraps the contours of both the roof and the adjacent vertical structure is often impractical, like gift wrapping a box with more than six rectilinear planes.  Given the distinct geometries of wall and roof configurations, the feasibility and facility of the installation are optimized by employing two separate elements. 

The concentric overlay of these materials not only facilitates the shedding of water but also allows the individual areas of the flashing to move independently with the inherent or natural movements of the building over time. In essence, the combination of base and counter flashing provides a weather-resistant application to the challenge of differing substrate materials at the intersection of roofs and walls in various structural, mechanical, and architectural configurations.

The diagram below you can see that the counterflashing, in this context, is set directly into a mortar joint of the chimney. There are acceptable alternatives to setting the counter flashing into a reglet or raggle at the chimney, but the standard traditional method is by removing the mortar at a selected mortar joint to encase the top of the counter flashing with an applied sealant.   Accepted alternatives, in limited cases, may include termination bar and/or specific bent flashing with a reservoir at the top of the flashing to receive a gun applied sealant.   Although these methodologies are required by the building code, they are often skipped or omitted by unprofessional roofing installers.

chimney and roof interface
The diagram above shows a section view of the chimney and roof interface.

Counter flashing serves as a protective element installed over base flashing.  The counter flashing shields the upper edge of the base flashing, preventing exposure to the elements, including rain, wind, and debris.  Counter flashing should be adaptable, conformable, or compatible with the base flashing material, but they can actually be totally different types of materials as long as they work together, (dissimilar metals would be one example of materials that do not work well together) to ensure a paired integration and effective weatherproofing.  

Separation of Dissimilar Metals

When dissimilar metals are set in a building assembly, in contact, they undergo galvanic corrosion, a phenomenon where a more noble metal extracts electrons from a less noble, active metal. This interaction leads to the formation of iron oxide, with ferrous metals, commonly known as rust, and compromises the structural integrity of the metals involved. The consequences of corrosion extend beyond aesthetic issues, potentially causing damage to the weather tightness of the roof system.

A Required Seam 

Inherently, there will be a seam between the base flashing and the counter flashing, that seam is not a problem but they should be installed in a near seamless way so that the general flow of materials through gravity alone does not allowed water to enter up the base flashing and behind the counter flashing or through the assembly at any point.

Counter flashing materials often mirror those of the base flashing, including metals like aluminum, copper, or galvanized steel, or other weather-resistant materials.

In the second article in this series we will also talk about the stark contrast between the use of base flashing and counterflashing in the context of residential vs. commercial construction. A lot of the principles of overall fundamentals of roofing are very similar between commercial and residential roofing but in the case and counter flashing, there are significant differences between low slope and moderate or high slopr roof systeVS.

As a preview into the upcoming article in this series, the picture below shows a typical chimney at a asphalt shingle roof assembly.

improper counter flashing
The picture above shows an improper assembly of counter flashing.

Smart and proactive replacement, construction, upkeep and maintenance of low slope roof and mansard roof systems requires an enthusiastic interest and understanding of historical methodologies, waterproofing principles, and building science.   Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building. 

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the roof and related systems of your building in Washington DC, contact us or fill out the webform below and drop us a line.  We will be in touch if we can help.

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