Wood Frame Construction and Fire Resistance; Code Compliance?

Wood frame construction has long been a staple in building design, particularly in residential and mid-rise projects. Yet as buildings grow taller and more complex, ensuring adequate fire resistance becomes critically important. This article distills code compliance, focusing on fire resistance requirements for wood frame floors and walls. It draws from the Code-Conforming Wood Design (CCWD) document and the Design for Code Acceptance (DCA3) to provide practical, essential guidance.

The CCWD, a collaboration between the American Wood Council (AWC) and the International Code Council (ICC), serves as an index to the International Building Code (IBC) provisions specifically for wood construction. While it focuses on the IBC and not the International Residential Code (IRC), its most valuable feature is a series of tables that simplify building size and height calculations. These tables effectively replace many of the manual formula calculations typically required.

Understanding building construction type is fundamental to determining fire resistance needs. Construction type directly impacts allowable size, materials, and required fire ratings. For wood construction, Types III, IV, and V dominate. Type III construction requires exterior walls to be non-combustible or made of fire-retardant-treated (FRT) wood with a minimum two-hour fire resistance rating. Type IIIA also requires one-hour ratings for other building elements, while Type IIIB relaxes this requirement for interior components. Type V construction, often used in residential projects, permits most materials allowed by code but has less stringent exterior wall fire resistance requirements.

Height and area allowances for buildings are detailed in IBC Chapter 5 and supported by tables in the CCWD. Automatic sprinkler systems can significantly increase these allowances, providing up to a 300% increase in allowable floor area for one-story buildings and a 200% increase for multistory buildings. Sprinklers also enable a height increase of up to 20 feet or an additional story, depending on occupancy type.

One of the most practical tools provided by the CCWD is the tabular format that bypasses complicated calculations. These tables integrate occupancy classifications, construction types, sprinkler provisions, and open frontage considerations to guide users to maximum allowable height and area. While they do not replace the need for understanding the underlying formulas, they offer a fast and reliable reference for most projects.

Fire resistance requirements do not stop at surface materials; they extend to the structure’s design and assembly. Here, the DCA3 comes into play, offering tested and approved designs for wood frame assemblies. It covers both symmetrical (rated from both sides) and asymmetrical (rated from one side) assemblies, outlining configurations for floor-ceiling systems and wall intersections that meet specific fire resistance ratings.

A critical distinction in fire protection is between fire resistance and flame spread. FRT wood slows flame spread but does not inherently increase fire resistance as measured by ASTM E119 tests. To achieve a desired fire resistance rating, assemblies must be properly designed and tested or follow accepted guidelines like those in DCA3.

Particular attention must be given to floor-wall intersections in platform construction, where floors bear on walls below. DCA3 details configurations using rim boards, blocking, and gypsum wallboard to ensure two-hour fire resistance continuity at these junctions. Depending on the assembly, additional wood thickness, mineral wool insulation, or multiple layers of gypsum may be required.

The IBC provides multiple paths for establishing fire resistance: testing, approved design sources, prescriptive assemblies, and calculated methods. For example, the National Design Specification (NDS) for Wood Construction offers calculation methods, including char rate estimates, that help determine the fire resistance of exposed wood elements. According to NDS Chapter 16, solid wood typically chars at about 1.5 inches per hour. This means a nominal two-by (1.5 inches thick) wood member can provide roughly one hour of fire resistance when exposed on one side.

Exterior walls within 10 feet of a property line require symmetrical fire resistance, meaning they must resist fire from both the interior and exterior. Walls farther than 10 feet typically only need to resist interior fire exposure. DCA3 offers detailed case examples (Cases A, B, and C) showing how to configure rim boards, blocking, insulation, and gypsum to meet these requirements. Whether using a single rim board with additional blocking, double rim boards, or extra-thick rim boards, each solution is carefully calculated to meet the two-hour rating. Another critical area is horizontal assemblies, such as floor-ceiling systems. DCA3 includes one-hour and two-hour rated configurations, some using double layers of gypsum or specific insulation materials. For assemblies involving resilient channels, mineral wool insulation becomes a key component for achieving the required fire resistance.

Beyond assemblies, the IBC also defines fire-resistance-rated elements like fire walls, fire barriers, fire partitions, and horizontal assemblies, each with specific performance and continuity requirements. For example, fire walls must extend continuously from the foundation to or through the roof and are designed to divide a structure into separate buildings. Fire barriers serve to separate occupancies or provide shaft enclosures, while fire partitions typically divide dwelling units or tenant spaces. Each of these elements has its own rating and continuity rules.

Understanding these distinctions is essential for designers, builders, and code officials alike. Knowing which elements require rated assemblies, which can use FRT wood, and which demand noncombustible materials is vital to achieving both code compliance and safe building performance. While the detailed calculations and configurations may seem daunting, resources like the CCWD and DCA3 streamline much of the work. By carefully applying these guides, designers and reviewers can ensure that wood frame buildings not only meet code but also deliver robust fire performance, protecting both lives and property.

In summary, wood frame construction offers remarkable flexibility and efficiency, but it must be paired with thoughtful fire resistance design. By using the tested assemblies and detailed guidelines in the CCWD and DCA3, construction professionals can confidently meet the fire safety requirements of modern codes. With proper planning, coordination, and attention to detail, wood buildings can stand tall—and stand safe. For any further inquiries regarding this topic, as well as for code consulting and fire engineering design support related to your project, please don’t hesitate to contact us via email at contact@engineeringfireprotection.com.

Application of any information provided, for any use, is at the reader’s risk and without liability to Engineering Fire Protection (EFP). EFP does not warrant the accuracy of any information contained in this blog as applicable codes and standards change over time. The application, enforcement and interpretation of codes and standards may vary between Authorities Having Jurisdiction and for this reason, registered design professionals should be consulted to determine the appropriate application of codes and standards to a specific scope of work.