Base flashing and Counter Flashing – Part II

In our last article we took a look at the principles and requirements around the installation of base flashing and counterflashing. These types of assemblies generally occur where there are transitions in the layout or configuration of a roof structure.   Common examples include the penetration of a chimney through a roof or the perimeter termination roof to an adjacent wall.

Today we will look at section IV-V of the following outline:

    1. Why One Isn’t Enough
    2. Separation of Dissimilar Metals
    3. A Required Seam
    4.  Inherent Differences Between Residential and Commercial
    5. Pitch Pockets as an Alternative to Counter Flashing

Inherent differences between residential and commercial

In the past article on this topic, we looked at the diagram below. You can click the image to see a close up view of the details of how the roof membrane attaches to a chimney in a low slope or flat roof configuration.

base flashing 1

In the past article we also took a look at the picture of the chimney below. Instead of installing a step flash and, generally the best method of installing counter flashing at a chimney, in this case, the contractor cut right through the edge of the brick to set the counter flashing top edge into a diagonal reglet or raggle created with a kerf in the face of brick.

Cutting a kerf through the face of a brick is problematic because it becomes a point of fracture or weak link in the structural surface of the brick.


Our company was hired by the building owner to check various details of the prior contractor’s installation. We found several problems with the installation, but we did find that in this case the counter flashing had been at least set into a cut raggle or reglet. Although this is not the best practice, to cut through the face of the brick, it’s better than installing counter flashing with no recess or raggle or reglet at all. Although it’s not right, many contractors install counter flashing by just gluing it to the side of a chimney, which is the worst of all methodologies and prohibited by the Code.

cutaway area
The cutaway area of the sealant at the top of the base flashing shows a turn of the aluminum flashing metal into the masonry.


In the next picture below, we show the small area of the cut out, approximately ⅜ inch by ⅜ inch where the sealant was removed and then later resealed with an elastomeric polyurethane sealant as a sign for future inspections that this area has been checked in the past.

step flashing
The continuously run flashing is lower quality than step flashing, yet better than counter flashing not set into the masonry, with the exception of termination bar or metal flashings with an intended top reservoir.

In most facets, residential roofing  Is quicker and cheaper than commercial roofing. In this case we refer to flat roofing as commercial roofing and slope roofing with shingles as residential roofing. By-and-large, that disambiguation or bifurcation of the industry holds up, except for the case of Washington DC. Here in Washington DC there are many commercial type low slow roof membranes installed on residential rowhomes. In most other parts of the country, it’s uncommon,from an empirical perspective, to see low-slope roof systems on residential buildings.

Pipe penetrations are often treated, in low slope or flat roof systems, with a concentric wrap of base and counter flashing, as well. In this particular case though the flashing membrane must be adhered to the pipe outer walls. Unlike with the rough contour of masonry, single-ply membranes can often seal well to the face of metal or PVC piping.

In the picture below though, the pipe is not properly wrapped, instead and asphaltic mastic has been liberally applied between the low-slope roof membrane and the sides of the PVC plumbing vent pipe. As the materials react to temperature swings at different rates with differential movement, the PVC has separated from the asphaltic mastic.  Asphaltic mastic, over time is susceptible to deterioration from exposure to ultraviolet rays. In this case the mastic has hardened almost like a rock and where the PVC responds quickly to temperature differentials the asphalt application has remained more rigid and separated from the pipe.

poor installation
Cheap attempts at upkeep and maintenance are poor solutions to insufficient installation methodologies.


At the same poorly applied roof system, another PVC pipe penetration has been treated by liberally applying additional asphaltic mastic. Often a handylady or handyman or DIY’er will make such applications as a cheap or quick and dirty method to stop a leak, but this type of application is not long-lasting.

signs of deterioration
The overall roof surface shows significant signs of deterioration from exposure to ultraviolet rays. Currently there is a coating of aluminum paint on the roof surface but the substrate below is already in rough condition.

Pitch Pockets as an Alternative to Counter Flashing

Pitch pockets are a common alternative to counter flashing, but they’re most often used at other types of penetrations and configurations. Generally, pitch pockets are used where a collection or grouping of pipes and wires need to be sealed together ganged or grouped as one. They work in a very similar way though.

The picture below shows a area of complicated penetrations of pipes, wires, and refrigerant lines

seprated penetration
The individual separated penetration of a PVC pipe is sealed with a pre-molded closed boot pipe seal, different from a split seal.

Unlike the pipe shown in the picture above, the 3 pipes in the picture below are very similar in material type and nature, overall, but they are grouped so close together that there is insufficient space to install a pre-molded closed type seal. Instead, in this case, a pitch pocket is the better type of installation, for this specific context.

pitch pockets
Some types of pitch pockets are adjustable in the field so they can be set to different dimensions to customize to the specific grouping of elements which need to be sealed.

The next picture shows a pitch pocket at a steel guardrail post. The guard rail post itself is most likely connected to the framing structure below, but the penetration itself is sealed by the pitch pocket. The pitch pocket in this case is a bit better than a combination of base flashing and counterflashing type installation. Even where the modified bitumen membrane were to be welded directly to the steel, the pitch pocket has better tolerances for movement and flexibility.

walls of pitch pocket
The walls of the pitch pocket are mechanically secured to the base membrane, but the pourable sealant and applied sealants create the waterproofing.

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 fill out the webform below and drop us a line.  We will be in touch if we can help.

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