Image courtesy GAF I Siplast

While some attribute success to being at the right place at the right time, roofing success is more about thoughtful design and installation rather than happenstance. Roofing plays an extremely important role in protecting the building from the elements, impacting energy efficiency, and contributing to the resilience goals of a building. Selection of every roofing assembly component, from the insulation to the membrane, including the attachment method can impact both the service life and the performance of the roof assembly. A successful roof design, which incorporates the building’s design parameters, will withstand the test of time.

Setting the Right Requirements—The Code

Codes provide the minimum requirements for construction. Code represents a baseline for design and has few regulatory provisions that focus on resilience. Designers can, and often should, design beyond the minimum code requirements to improve the durability and longevity of their buildings. The code requirements come from the International Building Code® (IBC) and the International Energy Conservation Code® (IECC), and are either performance or prescriptive requirements. The code also references specific key standards from documents issued by organizations including ASTM International (ASTM), ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers), and The American Society of Civil Engineers (ASCE).

Building Code Prescriptive Requirements

Chapter 15 of the IBC provides minimum requirements for the design and construction of roof assemblies and rooftop structures. The chapter addresses Roof Drainage (1502), Weather Protection (1503); Requirements for Roof Coverings (1507); Flashing (1503.2); Coping (1503.3); Wind Resistance of Roofs (1504.1, ASCE 7); and Edge Securement, Low-slope Roofs (1504.5). Such sections include requirements for emergency overflow drainage, gutter securement, and the ability of roofs to withstand wind events, foot traffic, and fires. While these topics may seem obvious to include, roof coverings must be designed and installed in accordance with this code and the manufacturer’s approved instructions. First, code considerations must be taken into account during roofing materials selection and installation. Then, they can be enhanced for each unique roof design.

Energy Code—Commercial Requirements

The IECC recognizes the roof’s role in energy performance. Energy codes apply to roofs because the R-value established in the roof assembly enables containment of conditioned air inside the building. The roof's contribution towards maintaining the enclosure and keeping the interior environment separate from the exterior means that heating, ventilation, and air conditioning (HVAC) systems do not have to work as hard, resulting in reduced energy consumption. 

Under IBC Chapter 13, Energy Efficiency, “Buildings shall be designed and constructed in accordance with the International Energy Conservation Code - 1301.1.1.” Designs can comply with either the requirements of ANSI/ASHRAE/IESNA 90.1 or the requirements of the IECC Commercial Provisions, which apply to all buildings except for residential buildings 3 stories or less in height.

The version of the Code is adopted at the state level; the most recent version is dated 2021, where significant updates were made in regards to the air barrier requirements. Since 2010, the IECC states that a continuous air barrier must be provided throughout the building thermal envelope (2021 IECC §C402.5.1. Air Barriers). The air barrier is permitted to be located on the inside or outside of the building envelope, located within the assemblies composing the envelope, or any combination thereof. The air barrier selected needs to comply with Sections C402.5.1.1 for materials and C402.5.1.2 for assemblies. This section is important as the roof air barrier, whether a dedicated air barrier or the roofing membrane as the air barrier, is required to be continuous across the roof as well as continuously transitioned to the exterior wall air barrier. This requirement is critical to understand when detailing the roof at both roof penetrations (discontinuities) and at the roof to exterior wall interface.

The energy code additionally sets requirements for thermal performance, including a minimum R-value for the roof. This R-value is based on climate zone and geographic location (IECC C301.1 and 90.1 Annex 1). It should be noted that the R-values in the Code are denoted as minimum R-values. The unique building type should be taken into account when designing the R-value for the roof. Owners selecting to achieve higher energy savings, or specialty buildings such as cold storage, frequently require more insulation than required by the Code.

Guidelines for installation, as provided by the codes, are also critical to achieving predicted energy efficiency. Starting in 2018, the IECC Section C402.2.1 requires two layers of insulation at the roof and provides that this installation should be staggered and offset during installation, to prevent airflow between the joints. 

Wind Design

Determining the wind loads affecting a roof system is based on requirements contained in the IBC and the American Society of Civil Engineers (ASCE) 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures.

Wind mapping tools, such as the ASCE 7 Hazard Tool, helps select a location-based wind speed (in miles per hour, mph) that is used to determine the loads that will be acting on the roof system.

Recent building codes have become more stringent in determining design wind loads for roof systems. The requirements are based on historical data that has been collected over decades. Variables, such as the local building code and building dimensions, building use, occupancy, exposure, and type, are also used in various equations to determine loads acting on a roofing system. The wind speed, in miles per hour, is then converted into a wind uplift pressure, in pounds per square foot (psf). It is these wind uplift pressures that the roof assemblies must be able to withstand.

The main objective is to have the capacity of the roof greater than the projected wind loads. Wind uplift pressures will vary for each unique building and therefore, calculating the wind pressures for each roof is imperative.

Please see below for further resources and information on Wind Design:

Wind Design and (the new!) ASCE 7-16

Prevailing Winds and Prevailing Codes, A Summary of Roof Related ASCE 7-22 Changes

Establishing the Right Design Parameters

Design parameters provide the starting point to determine which roofing assembly to install on a property. It is these requirements that shape the building design, including building envelope and structural considerations. Owner requirements, including building type and use will dictate some design parameters for the roof, with the code, including regional and local climate factors setting the remainder of the requirements. Selection of the roofing assembly will influence service life and energy savings over the life of the roof. Selecting the right roof ultimately means mitigating risk and ensuring that the roof will perform as intended. 

Building Use

The building use, including rooftop use, may have the most impact on a building design. The interior spaces will influence the building shape, size, and interior conditions, all of which influence the roof design. The roof will need to incorporate desired energy considerations, which influences the amount of insulation. Airflow, vapor drive, and moisture potential, will each influence the roof assembly as consideration of a dedicated air or vapor retarder may be required based on interior conditions.

While these are not all roof design considerations, these are some to consider or some questions to be asking:

Are there requirements for impact resistance?

• Consider an appropriate coverboard based on the type of impact resistance required. Foot traffic may require a coverboard with less compressive strength whereas impact requirements for hail, such as very severe hail, will require a coverboard with more compressive strength. A thicker and more robust membrane will also provide impact resistance.

Is there a particular level of fire resistance required?

• Consideration may be given to a fire rated assembly.

Will there be exercise rooms or shower facilities that may increase the moisture level in the building?

• Consider a dedicated vapor retarder at the roof deck level to prevent moisture from entering the roof assembly.

Will enhanced chemical, or grease and oil resistance be required, such as from food processing or cooking operations?

• Consider a membrane that will be resistant to the chemicals, greases, or oils that may be expelled onto the roof. Membrane types will differ on their resistance to each type of chemical.

Is the required interior temperature lower than a typical building temperature, such as for a cold storage facility or a hockey rink? 

• Consider additional insulation to maintain interior temperatures as well as appropriate air sealing to prevent condensation within the roof assembly.

Is there a high humidity interior environment such as for a natatorium or laundry facility?

• Consider the use of a vapor retarder at the roof deck level to prevent condensation and uncontrolled air movement.

Will the roof need to integrate a steep to low slope design? 

• Consider the transition between steep slope and low slope materials including compatibility and warranty.

Are there any use restrictions for the building, such as a laboratory or hospital which requires the building to be fully pressurized?

• Consider a dedicated vapor retarder or air barrier at the roof deck level with appropriate air sealing detailing to ensure no air loss occurs into the roof system.

Will there be overburden on the roof, including rooftop solar or an amenity deck?

• Consider a coverboard and insulation with a high compressive strength. A thicker and more robust membrane will also provide resistance to added foot traffic and weight of the overburden system.

Building Location and Climate

The climate zone where the building is sited also impacts assembly design and specification—whether the structure is in warm and humid Florida, in the dry and cold mountains of Colorado, in temperate and windy Indiana, or within a major metropolitan heat island, like New York City or Los Angeles. The geographic location of a building will affect the weather that the building needs to endure including the amount of rain, snow or sun. More robust membranes and roofing assemblies should be considered where there are extremes of temperature or weather conditions, including high winds and hail. Weather and temperature constraints can also vary greatly depending on exposure and the structure’s position within an urban or rural environment. A more robust system will allow for the roof assembly to be resilient to weather events.

Consideration to wind resistance also varies widely, as coastal locations are impacted by higher wind speeds. Additionally, the wind speed that a particular building will need to resist can vary widely even from one part of the state to the next, depending on its immediate surroundings, whether the building is on a hill or isolated location, or more sheltered from wind impacts by taller surrounding buildings or trees. 

Local city and county regulations may have specific requirements for reflectivity and volatile organic compounds (VOCs). Reflectivity requirements impact the membrane color selection, which require lighter colored membranes. The limitation of the VOCs will impact the roof attachment method as certain adhesives may not meet the regulations. 

Kristin Westover is part of the Building and Roofing Science Team where she works with designers on all types of low-slope roofing projects to review project design considerations so designers can make informed roof assembly decisions.

Building Enclosure  |  BuildingEnclosureOnline.com  |  Winter 2024

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