Flat Roof Counterflashing – PART II of II

In our last article, we looked at several pictures of a chimney where a modified bitumen roof membrane had been installed as a base flashing, up to the sides of the chimney, but a proper counterflashing was missing and had possibly never been installed.

Another picture of a hole in the parge coat, at the area of delamination, follows below. You can notice a pronounced shadow where the surface of that cementitious material is missing.  There is a pronounced void and deep shadow at the vertical surface above the base flashing.  

hole in a parge coat

On the other side of the same chimney, there is deamination of the modified bitumen base flashing. A counterflashing allows a base flashing to potentially delaminate in the future after installation, but still protects the area by providing a redundant cover which limits or prohibits the entry of water from the elements such as rain, sleet, and snow.   This issue of missing counterflashing coincident with delaminated base flashing is seriously problematic, it’s a weak link in the overall flat roof system of a building like this, as shown in this example.  And while it’s a serious problem, it’s also a very common problem when proper counterflashings or termination bars are not installed.

deamination of the modified bitumen base flashing

The next pictures below show a different chimney with similar problems. The roof membrane runs right up to and around the base of the chimney and the modified bitumen membrane there turns upwards to be installed as a base flashing.  

voids in the mortar

The next picture below shows the deep voids in the mortar of this chimney.  Historic properties, built from historic brick, need to be repointed. This particular chimney should have been repointed at the time of installation of the previous roof membrane. In this case, as a repair, a termination bar was installed to secure the loose and delaminating modified bitumen base flashing.

A picture of the aluminum termination bar follows below. This aluminum termination bar has both flexibility to roughly follow relatively close to the mishapen contour of the historic chimney. It also has enough rigidity to hold its shape in place, only secured at interspersed screws spaced at approximately 8 inches apart, on-centers.

aluminum termination bar

The fasteners used to secure the termination bar in place have had each respective fastener head covered in a liberal generous application of elastomeric sealant.  The elastomeric sealant does not last nearly as long as the single-ply field membrane itself, but still does a pretty good job to allow for the differential movements inherent in applied metals in both termination bars and roof flashings.  These sealants are highly cohesive and have significant resiliency. Yet, there are no gun applied sealants yet available, based on the current limitations of technology, which can last as long as single ply roof membranes like TPO and even the highest quality modified bitumen roof systems.

elastomeric sealant

The next picture below shows the area of deteriorated mortar, at a joint between the bricks at a corner of this chimney. The lower brick, at the outer edge, is in the header position at this side of the chimney. The brick above is in the stretcher position. By understanding which brick facing is in which position, you can infer the depth of omission or deterioration of the brick mortar in the joint between the header and stretcher bricks. The void area at the outer border reaches halfway into the depth of the brick in the header position, meaning that the mortar is deteriorated up to or more than 1.75 inches deep. That amount of deterioration is significant, but at rooftop chimneys, significant deterioration of mortar joints is very common because those particular historic masonry elements do not have the protection, by the nature of the layout of adjacent walls, from the elements, found at the main walls of a historic brick building.

deteriorated mortar

The final picture below shows the other side of the brick joint, from a different facing of that chimney. It’s important to understand that the condition of masonry has little to do with the quality of installation of a single ply roof membrane, yet these elements are interconnected. Roofing materials generally require substrate or accessory components of the roof to be in relatively good condition. In many cases these accessory elements are outside of the scope of work of many roofing contractors, yet these elements are still important. Our company works both with historic masonry and with many types of flat roof systems and elements. We recommend working with a company like ourselves because it’s important to have a roof contractor who is at least knowledgeable about alternative adjacent rooftop system elements.

brick joint

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.

On Key

Related Posts

what damages metal roof paint

Terne Metal Roof Paint Deterioration

Terne-coated metal roofs are an element of historic architecture, somewhat common in Capitol Hill and other historic neighborhoods of Washington, DC. However, the terne coating