Control Layer Summary

Discussing control layers as they apply to a roof or wall alone is fairly manageable. But the process gets much more complicated when the roof meets the wall at the parapet condition. The “pen test” is relatively easy in theory, but it can get complicated as we zoom in and consider the control layers at each condition, penetration and transition.

In summary, the following are key points to maintain continuity of the control layers:

• Water Control is managed by the roof membrane and the cladding. A secondary water control layer is often found against the structure, behind or below the exterior insulation.
• Air Control can be managed at the deck level of the roof, which more readily can be married into the wall air barrier. The roof membrane can also be used as an air barrier as long as the detailing and transitions are done carefully.
• Thermal Control continuity is maintained by connecting the roof and wall insulation, which can be challenging. It’s important to be mindful of cavity insulation and the potential design risks of condensation at the thermal bridges.
• Vapor Control can also be in the same plane as the air control layer, based on location needs, construction methodologies and occupant use of the building.

Code References for Parapets

This section will make reference to the following national model codes and standards: 2018 International Building Code (IBC), the 2018 International Energy Efficiency Code (IECC), and the ANSI/ASHRAE/IES Standard 90.1-2016 (ASHRAE 90.1). It is worth noting that the summary provided below is not an exhaustive list of requirements for exterior wall and roof systems in the referenced national model building and energy codes. Different versions of the referenced codes have additional and/or different requirements; these requirements may also vary by adoption and modification by the local authority having jurisdiction. It is important to refer to local codes for the applicable requirements.

The requirements for parapets generally come from the building code (IBC) and the energy code (IECC and ASHRAE 90.1). The requirements within the building and energy codes can be mandated prescriptively, as a performance threshold, or by reference through specific key standards. The performance standards are important because they don’t attempt to regulate by providing exhaustive lists and itemized component requirements, like a prescriptive method. These performance requirements establish the design benchmark and then provide a methodology to demonstrate compliance with the benchmark.

The building codes and standards do not always address parapets exclusively, but many refer to “Exterior Walls” separately from “Roof Assemblies.”

Exterior Walls in the Building Code

The exterior wall requirements for parapets are covered in Chapter 14 of the IBC which addresses “exterior walls, wall coverings and components.” For parapets, the requirements for weather protection, water-resistive barriers (WRBs), managing vapor and flashing apply as they do for the rest of the exterior building walls.

Exterior Wall Flashing: Flashing is very important and is generally repeated in both the wall (IBC 1404.4) and roof provisions of the code. The IBC includes the principle that “flashing shall be installed… to prevent moisture from entering the wall or to redirect that moisture to the exterior.” This is an important starting point for parapet design where the sequencing can be a challenge among numerous wall- and roof-system contractors.

While not an exhaustive list, IBC 1404.4 includes a minimum list of areas requiring exterior wall flashing. These are summarized below:

• Penetrations and terminations
• Intersections with roofs, chimneys, porches, decks, balconies and similar projections
• Built-in gutters and similar locations where moisture could enter the wall
• Flashing with projecting flanges, installed on both sides and the ends of copings

At all of the prescriptive flashing locations listed in the IBC, the purpose is two-fold. The first is for the flashing to be installed in a way that prevents water from entering the wall system. This concept is known as “shingle fashion,” or installing components of the roof, exterior wall and parapet “such that upper layers of material are placed overlapping lower layers of material to provide for drainage via gravity and moisture control” (IBC 202). Logistically, this is best accomplished on-site by applying materials from the bottom of the building to the top, so the next progressive layer or system is then lapped correctly.

The second, and more challenging flashing requirement, is to also be installed in a manner that permits water to exit the wall system if it enters incidentally. This requires the parapet to be designed with a method and pathway for water to drain from the flashing, even from behind the cladding (think weep holes at masonry shelf angles). In addition to providing a means for drainage, the IBC also includes a drainage scenario to avoid exterior wall pockets (1404.4.1). Wall pockets or crevices are locations within a wall assembly “in which moisture can accumulate.” These scenarios can be common in parapets where the exterior wall, roof and parapet wall above might not always be in alignment. In parapets, these wall pockets should be avoided or protected with appropriate flashing for the application.

Exterior Wall Weather Protection: The weather protection section (IBC 1402.2) requires that the exterior wall “shall be designed and constructed in such a manner as to prevent the accumulation of water within the wall assembly.” One of the methods prescribed in this section is to include a secondary water management layer, or “water-resistive barrier” (WRB), behind the exterior cladding in the exterior wall portion of a parapet. Beyond including the WRB layer, “a means for draining water that enters the assembly to the exterior” must also be provided in the parapet wall design. There are exceptions to the secondary WRB and drainage requirements provided in the IBC for concrete and specifically tested systems, but the benefits for designed water control is applicable for all construction types.

Exterior Wall Vapor Retarders: In exterior parapet walls, protection against condensation is also required to be compliant with the vapor retarders portion (IBC 1404.3). Vapor retarder materials are separated into three classes by ASTM E96 testing (Procedure A, desiccant method):

• Class I: 0.1 perm or less;
• Class II: 0.1 < perm ≤ 1.0 perm;
• Class III: 1.0 < perm ≤ 10 perm

The vapor retarder classes are referenced in the IBC to identify by climate zone if a material is permitted in the assembly in a prescriptive manner (IBC 1404.3.1 and 1404.3.2). It is important to note that all materials have vapor retarding properties to some degree and may limit vapor transmission without the addition of a dedicated vapor control layer. This is also why the IBC includes the alternate performance compliance of providing a “design using accepted engineering practice for hygrothermal analysis” as described in the initial language of 1404.3.

In most cases, if a vapor control layer is needed, it is a good idea to select a vapor retarder that will allow some amount of drying from diffusion. High-humidity interior environments such as natatoriums, manufacturing facilities and grow houses may require a vapor barrier for long-term performance. However, the decision of whether or not to add a vapor control layer to a roof assembly is normally based on risk and is best made with a building enclosure consultant. The weather protection and vapor retarding sections of the IBC apply to exterior walls, but parapets may have very different design and performance requirements than the wall assembly below the roof. That is why it is important to maintain continuity of the four control layers at this interface. 

Roof Assemblies in the Building Code

The roofing portion for parapets is covered in Chapter 15 of the IBC which addresses roof assemblies, specifically the “design, materials, construction and quality” of roofs. Regarding parapets, the roof system requirements impact the wall where terminations and transitions occur. The requirements include weather protection, flashing, coping, wind resistance design, edge securement and specific requirements for various types of roof coverings.

Roof Assembly Weather Protection: The requirements for weather protection (IBC 1503) are fairly broad, requiring roof decks covered with approved roof coverings. Much more detail is covered in the additional IBC sections regarding roofing and parapets. In the roofing provisions, it is important to note that compliance with “the manufacturer’s approved instructions” doesn’t just affect a project’s eligibility for warranty, but is also required for building code compliance.

Roof Assembly Flashing: The requirements for flashing (IBC 1503.2) are repeated in part across the wall and roof portions of the code. This repetition highlights the importance of managing water control at the transitions. The code requirements for both roofs and walls support the water control layer principles in the pen test discussed previously. The roofing chapter in the code also directly mentions the parapet walls as a critical location for both roof system transition flashing and requirements for copings. While not an exhaustive list, IBC 1503.2 includes a minimum list of areas requiring roof flashing. These are summarized below:

• Flashing joints in copings
• At moisture-permeable materials
• At intersections with parapet walls
• At other penetrations through the roof plane

Roof Assembly Coping: The roof requirements for parapet wall copings are spread across many categories. One section specific to copings (IBC 1503.3) has a limited scope, requiring materials to be limited to “noncombustible, weatherproof materials” and be installed with a “width not less than the thickness of the parapet wall.” Many other requirements in the code also apply to copings in the code, such as flashing, wind design loads and edge securement performance. More will be discussed about copings in those sections.

Roof Wind Resistance: The wind resistance for low-slope commercial roof decks and roof coverings (IBC 1504.1) is required to be designed in accordance with IBC 1609.5, which ultimately leads to utilizing ASCE 7 for determining design wind loads. There are numerous updates to ASCE 7—2005, 2010 or 2016—and each has its own nuance as to how it impacts roof design loads.³ Because ASCE 7 is a performance standard, it is possible to use a version with higher performance requirements because designs do not need to be the minimum allowance. Parapets are a combination of wall and roof pressures. The exact height of the parapet is not factored into the roof wind uplift calculations, but if the parapet is 3 feet or higher, the perimeter values can be used at the corners, lowering the uplift requirements for that portion of the roof area.

Roof Edge Securement: Securing the edges on low-slope roofs (IBC 1504.5) has a significant impact on preventing failure and allowing the roof system to resist loads as it was designed. In addition to designing the wind resistance performance for the entire building (i.e., walls, roofs and parapets) per ASCE 7, metal roof edges are required to be tested for resistance in accordance with Test Methods RE-1, RE-2 and RE-3 of ANSI/SPRI ES-1. The referenced standard ANSI/SPRI ES-1 is a performance requirement that is specific to the strength of metal roof edges.⁴ ES-1 covers the “baseline” flush roof edge as well as parapet coping caps. When designing, it is important to specify compliance with ES-1 in the construction documents.

Roof Coverings: The IBC provides minimum installation criteria (IBC 1507) for various roof systems, based specifically on the attributes of that roof covering. In addition to the prescriptive criteria listed within, the IBC also mandates that “roof coverings shall be applied in accordance with the… manufacturer’s installation instructions.” Generally, the content of these roof covering sections address minimum substrate requirements, minimum roof slope, ballast requirements, and relative ASTM references to material standards, such as D6878 Standard Specification for Thermoplastic Polyolefin (TPO) Based Sheet Roofing.