Do IRC Limitations Provide Sales Advantages for CMs?

Originally published by the following source: SBC MagazineMay 20, 2019
by Jess Lohse with contributions from SBCA Professional Engineers


Editor’s Note: The following article is part of SBCA’s Scope of Work Initiative, which is exploring how the construction industry currently function and the role component manufacturers play within that structure. Your feedback is encouraged to help us complete this project.

The International Residential Code (IRC) is largely a prescriptive code that lays out direction for materials and installation methods as outlined in this article. While many materials and methods are specifically referenced in the IRC, there are items where the code carves out product provisions and limits the applicability of those provisions to a certain size building. These items include:

  • Cold formed steel floor framing
  • Steel framed walls
  • Exterior concrete walls
  • Structural Insulated Panels
  • Trussed roofs
  • Steel roof framing

General Product Provisions and Limits

The IRC imposes product limits and provisions generally by structure size and environmental factors. These generally include the length of the building perpendicular to the joist/rafter/truss span and the width of the building parallel to the joist/rafter/truss span. Additionally, the number of stories above grade is considered when determining potential application of IRC code. Environmental constraints focus on ultimate design wind speed, exposure categories and grown snow loads (GSL).  ​​

Application 2018 IRC Reference Length Width Stories Ultimate Design Wind Speed Exposure Category Ground Snow Load
Cold-Formed Steel Floor Framing R505.1.1 60' 40' N/A <140 MPH B or C <=70 PSF
Cold-Formed Steel Wall Framing R603.1.1 60' 40' <=3 <140 MPH B or C <=70 PSF
Exterior Concrete Wall Construction R608.2 60' 40' <=2 <160 MPH B or C N/A
Structural Insulated Panel Wall Construction R610.2 60' 40' <=2 Varies Varies <=70 PSF
Wood Roof Framing: Wood Trusses R802.10.2.1 60' 40' <=3 <140 MPH B or C <=70 PSF
Cold-Formed Steel Roof Framing R804.1.1 60' 40' <=3 <140 MPH B or C <=70 PSF

CFS Floor Framing

Section R505.1.1 of the IRC limits building size of cold formed steel (CFS) floor framing to buildings not greater than 60 feet in length perpendicular to the joist span, not greater than 40 feet in width parallel to the joist span and less than or equal to three stories above grade. In other words, if a building utilizes CFS floor framing greater than 40 feet in span, or for a run of 60 feet or greater, that portion of the building must be designed by an engineer and is not prescriptively covered by the IRC. Additional limits include ultimate design wind speed less than 140 miles per hour (MPH), Exposure Category B or C and GSL less than 70 pounds per square feet (PSF). 

CFS Wall Framing

Section R603.1.1 relates the applicability limits CFS wall framing.  Similar to CFS floor framing, CFS wall framing is limited to buildings not greater than 60 feet in length and not greater than 40 feet in width, limited to 3 stories above grade or less.  CFS wall framing is limited to ultimate design wind speed less than 140 MPH and Exposure Categories B or C with GSL 70 PSF or less. The applicability limits in this section apply to exterior and load bearing interior walls.

Exterior Concrete Wall Construction

Exterior Concrete Walls are covered by R608.2 and are also limited to 60 feet in length.  However, separate from the CFS materials, the IRC limits exterior concrete wall construction to floors with clear spans not greater than 32 feet and roofs with clear spans not greater than 40 feet. Building height is also limited to a mean roof height of 35’ or two stories above grade. Floor/ceiling dead loads cannot exceed 10 PSF, roof/ceiling dead loads cannot exceed 15 PSF and attic live loads cannot exceed 20 PSF atop exterior concrete walls. Roof overhangs are also limited to 2 feet and a dead load of 8PSF.  Environmental factors are limited to a maximum design wind speed of 160 MPH for Exposure B structures, 136 MPH for Exposure C structures and 125 MPH for Exposure D structures. 

Structural Insulated Panel Wall Construction

Structural Insulated Panels (SIPs) under IRC R610.2 are also limited in structure size to 60 feet in length and 40 feet in width and 2 stories above grade. Wall heights for SIPs under the IRC are limited to 10 feet. Ultimate design wind speed limits are limited to 155 MPH for Exposure B and 140 MPH for Exposure C structures. GSL is limited to 70 PSF for SIPs. 

Wood Roof Framing: Wood Trusses

IRC R802.10.2.1 covers applicability limits for wood roof trusses. Similar to other product provisions, structure length and width limits are listed. Structures are limited to 60 feet in length perpendicular to truss span and limited to 36 feet in truss span. Truss use is also limited to structures 3 stories in height or less and must have a roof pitch greater than or equal to 3/12 and less than or equal to 12/12. IRC limits wind speed applications to 140 MPH or less, with Exposure Categories of B or C and a maximum GSL of 70 PSF. 

CFS Roof Framing

Under R804.1.1, the IRC limits CFS roof framing exactly the same was as it limits wood roof trusses with one exception. Structure length is limited to 60’ however, structure width is expanded to 40’ similar to other materials listed here outside of wood roof trusses. Similar to wood roof trusses, structures are limited to 3 stores in height, pitches between 3/12 and 12/12, ultimate design wind speeds of 140 MPH, Exposure Categories of B or C and a maximum GSL of 70 PSF.

Beyond the IRC

Once a project exceeds the provisions of a specific product within the IRC, an engineering solution can be used to address that specific project’s needs with a deeper understanding of a product’s characteristics and limitations. This can be done through engaging a Professional Engineer (PE) or a firm that provides product design services and solutions, such as a component manufacturer, to specify particular products and installation methods, which are then sealed and signed by a PE, where required by law. The IRC building code leaves room for certain items that fall outside of the prescriptive scope to be used and approved under a section of the code entitled “alternative materials, design and methods of construction”.  This allows engineered applications to be used to address situations where the IRC is otherwise limited. A specific product, with proper documentation, is submitted to building officials for review under the relevant sections of the IBC. While requiring additional engineering for a specific material or its installation, the use of engineering does not disqualify the remainder of the structure from the IRC code. 

For additional information on alternative materials and methods, view the following two articles: