Proper Target Patch Installation Technique

Because there are very few barriers to entry in the construction industry, it’s extremely common to find roofs installed by contractors or people who do not have proper training. (Hire Dupont Roofing and this will never be a problem. ) If most of the industry in Washington DC would simply read our website and blog, there’s so much information there that it could significantly improve their quality of installations. However, particularly here in Washington DC we find that many installers will skip proper steps to install a roof correctly. One of the reoccurring issues that we find done incorrected by other installers is improper termination and flashing at target patches, particularly at pipe penetrations through single-ply roof membrane systems. Single-ply roof membranes are low slope roof systems found at the majority of the typical buildings in Washington DC like residential row homes in Capitol Hill and most of the historic parts of the city.

The picture below shows a typical plumbing vent pipe penetration through a flat roof. This roof is a granulated modified bitumen membrane.

patch installation technique

(In this particular installation, the base flashing is not properly installed and the first of the two required parts of concentric or overlaid flashing is missing and omitted.)

The process of installing a target patch at a pipe penetration on a flat roof involves a series of relatively simple steps to create a proper installation. The first step is surface preparation, which entails ensuring the area around the pipe penetration is clean, dry, and free from debris, rust, or any other contaminants that could impede proper adhesion of the roofing materials.  The cleaning part of the process becomes particularly important in re-roofing. Brand new buildings generally do not have the same level of cleaning needs. Particularly where past modified bitumen, or built up roofing was installed tar and melted Emulsion may be found on or at the sidewall of the pipes. This step and most other steps apply equally to ductwork and most other types of penetrations through flat roof systems.

A base layer of roofing membrane or flashing material, typically made of the same or similarly compatible material as the base membrane, depending on the makeup of the system and field membrane, is installed around both the horizontal roof membrane, at the penetration and the vertical base of the pipe. This base layer should extend at least 6 inches (or as recommended by the manufacturer) from the base of the pipe in all directions, forming a near uniform layer. This base layer serves as the foundation for the subsequent layers in TPO membranes, but in modified bitumen membranes the sequence of layers is reversed.

proper patch installation technique

The proper installation technique required varies based on the the main types of flat single ply roof membranes, in modern times, are: modified bitumen, TPO, and EPDM. There are several different alternative types of roof membranes and even within the three named here, there are many varieties. Within those categories though, there are sets of general requirements that hold true at least within each respective category.  In modified bitumen single plan membranes, the upper layer which wraps around the base of the pipe is applied first and the lower target patch type application is applied last.

The secondary layer, in modified bitumen, a larger square or circular piece of roofing membrane or flashing material with a hole in the center to fit over the pipe and the previously installed vertical collar, is then applied. This primary layer should extend at least 6 to 9 inches (or as required based on the specific system) from the base of the pipe in all directions. In polymer or TPO systems, a compatible sealant or adhesive may be applied to the base layer and the underside of the primary layer, allowing a redundant  seal where the membrane is stretched around the piping in a horizontal position.

types of patch installation technique
The pictures above and below, you can look closely at target patch at a modified bitumen granulated flat roof.  This particular flashing setup is installed in the concentric methods explained above. The first application in this methodology is the upper portion of the flashing which wraps around the base of the pipe. That portion of the flashing also extends on to the horizontal rooftop below the secondarily applied target patch. The primary application, visible at the base of the pipe has been cut into slender slivers sometimes referred to as fingers which spread out as the upper portion wrapped around the pipe. Those lower fingers of the upper portion of the flashing are adhered around the base of the pipe at the horizontal rooftop surface.

proper target patch installation technique
Specifically for, in TPO and PVC roofing systems, a termination bar, worm drive clamp, caulking, and / or sealant is often needed to terminate the edges of the top layer and create a redundant seal around the top of the flashing installation. Gaps, fishmouths, or areas where water could potentially penetrate are mechanically and chemically sealed. Additional sealant or adhesive is applied as needed to create redundant extra seal.

closed view of patch installation technique
Modified bitumen membranes can be either torch-applied (heat-welded) or self-adhered (peel-and-stick). The installation method for the base layer and subsequent layers will vary depend on the type of modified bitumen system being used.  Across the board though, the industry should understand that the peel and stick types of modified bitumen should be avoided almost all costs for residential or commercial buildings in Washington DC,  the materials are barely any cheaper but have very stringent requirements for environmental conditions during installation and are generally substandard or of lesser quality than properly installed alternative types of single-ply roof systems.

The picture below looks closely at the seam of a granulated modified bitumen system. In this case the seam, with bleed-out exposed at the edge of the concentric overlapping layer, has been set with loose granule to offer additional UV protection.

material used in patch installation technique
The vertical collar around the pipe is usually formed using a separate piece of modified bitumen flashing material, which is either torch-applied, depending on the system. It’s obviously important to ensure a tight fit and complete coverage around the pipe to prevent water infiltration.  The membrane can often be heated to a low temperature above ambient temperatures and cold / soft formed by easing the material by hand in a dry Form, and then torched into place permanently.

Termination and flashing details vary, depending on the shape and materials sealed or flashed in modified bitumen roofs. Depending on the system, worm-drive clamps, termination bars, lead jacket flashing, or other flashing materials may be also used to secure the upper edges of the target patch and create a watertight seal around the pipe penetration, in specific cases.

target patch installation technique
We recommend every building owner in DC who values the longevity of their roof (and their investments) and building use a contractor who values the simple and important principles of proper roof construction like Dupont Roofing DC. Learn more about our company and the proper techniques of working with roofing on historic buildings in Washington DC here on our blog at, and you can call us at (202) 840-8698 and email us at

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