Flat Roof Counterflashing – PART I of II

In the past, on the Dupont Roofing website, we’ve talked a lot about flashing and the importance of continuity between differing rooftop and associated roof element type materials. Flashings provide a bridge to span between transitions and or different types of materials.

It’s also commonly found that flat roof chimneys have counterflashing missing at the base of the chimney or found that those required flashings are improperly installed. Today we take a more thorough look at those details on typical flat roofs, like the modified bitumen roof systems so common in Washington DC. Other contemporary flat roof systems common in Washington DC such as TPO systems, EPDM, or other single-ply materials have similar principles related to rooftop chimney flashings and counter flashings.

The first picture below shows a modified bitumen SBS smooth membrane on a rooftop with a low strength and poor functional connection between the flat roof membrane and the brick chimney. This chimney currently has no counter flashing. In a shingle roof, at a typical medium to high slope free standing building, the shingles would have individual pieces of step flashings intersperse between each of the asphalt shingles where it was positioned next to the chimney.

modified bitumen sbs smooth membrane

Looking at a closer angle in the picture below, you can clearly see the base flashing and an asphaltic trowel applied sealant above.   That gray color, originally black but now faded, asphalt mastic was smeared on top of a parge coat of cementitious materials on top of the brick chimney. It would would have probably been best to repoint the brick in this chimney before applying the asphaltic mastic and or avoiding the need for the asphalt mastic altogether. Instead, this lower quality approach was done by low-skilled contractors in order to seal up the area above the flashing. There is no counterflashing installed at this brick chimney.

flat roof counterflashing

Asphaltic mastic can be effective in emergency type repairs, in some cases maybe materials like this can even be applied in wet conditions, where there’s a leak, for example, in the middle of a rainstorm. Asphaltic mastic, though, does not have an intrinsic resistance to ultraviolet rays. In fact, an asphaltic mastic like this is highly susceptible to degradation and deterioration from exposure to sunlight and the ultraviolet rays found in sunlight.  By comparison, many better, far superior performance materials are available instead of using an asphaltic mastic.  The correct way to install base flashing is to apply an overlay of counter flashing. An acceptable alternative to counterflashing, in many cases, is the installation of a termination bar. Here, neither a termination bar nor counterflashing has been installed. 

In the next picture below, you can see that some of the cementitious parging coat has delaminated and the asphaltic mastic, used as a type of membrane in this case, has also partially delaminated in a round chunk in the center, approximately 2.5 inches in diameter.

cementitious parging coat

Here in the next photo below, you can see that area more closely, the relatively thick cementitious parging coat, on top of the chimney, is delaminated and now missing.  This is problematic because years ago when the parge coat of cementitious material was applied on the surface of the exterior of the brick chimney, the mortar in the brick area was already deteriorated.  Now that the parge coat is partially missing, water can enter the area of deteriorated brick mortar and go down behind the roof membrane at the base flashing.

delaminated chimney

In the next article in this two-part series, we will look at this chimney again, and we will also look at other chimneys with missing base flashing and alternative types of installations, which can repair or fix some of the problems created by installing roof membranes without counterflashing.

Smart 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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