At the Roof Edge: Water, Air, Thermal, and Vapor Control

"Out on the edge you see all the kinds of things you can't see from the center." - Kurt Vonnegut
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Sponsored by GAF
By Benjamin Meyer, AIA, LEED AP

Learning Objectives:

  1. Discuss the requirements to managemoisture, air, vapor and thermal continuity.
  2. Review code requirements and how toachieve compliance.
  3. Outline design and specificationrequirements to set achievableperformance.
  4. Develop critical details blending form andfunction.


1 GBCI CE Hour

The parapet is so much more than the intersection of roof and wall. It’s also the junction where building aesthetics meets structural performance, air and moisture management, energy efficiency, construction trade sequencing and operational maintenance. Each of these perspectives is critical for the long-term performance of the building, but they are often at odds with one another. At such a critical interface, proper parapet detailing, installation coordination, and execution are paramount. Continuity of water, air, thermal and vapor control layers are necessary for long-term performance.

All images courtesy of GAF, except as noted

Types of Parapets

Parapets can be assembled in many configurations and each requires project-specific detailing. The 2018 International Building Code (IBC) defines a parapet as “the part of any wall entirely above the roofline.” To simplify the discussion a bit, this article will look at a baseline flush edge condition and two primary parapet types—platform framed and balloon framed—defined by how the roof and wall structure are connected.

Flush edge roof-to-wall and two parapet configurations.

Parapets can generally be composed of structural materials such as wood framing, light gauge metal framing, pre-engineered steel, concrete or masonry. In this context, the terms “platform framed” and “balloon framed” are referring to the configuration of the wall and roof structure to form a parapet. These terms are applied to parapets throughout this article based on the parapet configuration and are inclusive of all materials comprising the assembly.

The flush edge roof-to-wall connection is the simplest approach, with the roof structure placed above the wall system. Compared to the platform or balloon framed parapets, the flush edge configuration provides the least wind uplift protection for the system at the roof edge and the most limited aesthetic options.

Platform framed parapets are similar to the flush edged construction, with the roof structure sitting directly on the wall system, but include a parapet wall assembly on top of the roof structure. In this configuration, the roof structure acts as a platform for the parapet wall above. Depending on the attachment method, height and materials of the parapet wall, additional lateral and/or wind bracing strategies may be needed for this type of parapet.

Balloon framed parapets are formed when the wall system bypasses the roof system to form a wall that extends above the roofline. In this configuration, the roof structure is commonly hung from the wall structure or supported by a separate superstructure inboard of the wall system.

Control Layer Continuity

To better understand common parapet challenges, it is important to review continuity across the roof and wall systems, specifically the key four control layers: water, air, thermal and vapor.

These four key control layers should generally be continuous across all six sides of the building enclosure. ASTM E2947 defines the term “building enclosure” to “refer collectively to materials, components, systems and assemblies intended to provide shelter and environmental separation between interior and exterior, or between two or more environmentally distinct interior spaces in a building or structure.” It is difficult—but not impossible—to achieve effective control layer continuity across the building systems, especially at significant transitions like a parapet or flush edge, where the roof system meets the wall system.

When beginning to think about designing the enclosure, it’s helpful to start with an ideal scenario. The configuration of the ideal wall system can be considered as follows: the cladding on the outside, continuous insulation keeping the rest of the control layers tempered in the middle, and structure to the inside. This “ideal” configuration can also be applied horizontally to the roof assembly. For the transition example, the roof and wall meet as an “ideal” flush edge with very simple transitions. As a system moves away from the ideal configurations, like with the inclusion of a parapet, wood blocking or actual project constraints, the transition details become more challenging and trade-offs have to be made.

An “ideal” roof and wall transition.

For more complex scenarios, like parapets, there are simple design tools to connect the control layers as they transition from the wall to the roof. The “pen test”—tracing each of the control layers across the building enclosure—is a helpful tool to design and communicate to the field the intent of the critical components and functions of the building enclosure.

Example of air control “pen test” continuity across the building enclosure.

Water Control

Water control diagram for flush roof edge (left), platform framed parapet (center), and balloon framed parapet (right).

Goal: Keeping water out of buildings is a function of both roofs and walls, so it’s reasonable to assume parapets and flush edges should do the same.

Principles: Construction-related moisture, installation deficiencies and damage in the use-phase can introduce moisture into the roof and wall systems. Construction acceptance testing, scheduled inspections and regular maintenance play an important role in ensuring the systems are able to meet their intended performance over time.

Water control elements highlighted in blue, an example of parapet continuity.

The figure above shows the individual components to be considered. In a parapet condition, it starts with managing the flow of water on the parapet coping cap which is sloped back to the roof system; this also helps prevent staining on the exterior wall. Where the roof membrane meets the parapet wall, the membrane should be installed to allow for the possibility of differential movement, and terminated with flashing/counterflashing under an appropriate transition membrane under the coping cap. Wall systems commonly include a secondary water management layer behind the exterior cladding. For instance, it is important to protect the top of the wall assembly with a membrane below the parapet cap, sealing fastener penetrations for coping cap cleats, and lapping over the wall’s secondary water management layer in shingle fashion.

A flush edge condition shares many similar considerations for bulk water management. Properly layering the edge components provides water-control redundancy and the metal edge provides a level of protection for the main water control layer. Wood blocking provides the support and fastening location for the edge of the field sheet and L-shaped metal edge. Concealed and fastened cleats or clips are installed so that they overlap and extend past the top edge of the exterior wall. The vertical face of the edge metal is secured by engaging the drip edge to the cleats. Splice plates, interlocked joints, or concealed sealant in the seams of the metal edge provide bulk water management where metal edge joints are visible. A membrane stripping ply is installed on top of the roof-side metal edge flange for bulk water control. A raised ‘gravel-stop’ metal edge keeps rain water on the roof side to drain properly, not down the exterior wall. An L-shaped metal edge allows bulk water to flow over it, ideally into a water management system (e.g., gutters and downspouts).


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Originally published in May 2021