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What information is available to help designers detail floor-to-exterior wall conditions in Type III wood-frame structures?

Under the International Building Code (IBC), exterior walls in Type III construction are required to be framed with fire retardant-treated wood (FRTW) or non-combustible materials (IBC 602.3), while the floor and roof framing can be standard, untreated wood framing. This raises the question of how to detail the intersection where the floor and wall assembly meet, particularly with the common platform-frame style of construction where floor framing rests on top of the exterior wall framing below.

Type III construction is typically used for five-story wood-frame residential projects, with or without a podium, and may also be used for six-story wood-frame office buildings. When designing these buildings, code requirements for wall and floor detailing come from many parts of the IBC. For multi-family construction, a common combination includes a 2-hour fire-resistance rating for exterior bearing walls (IBC 2015 Table 601) and a 30-minute or 1-hour rating for the floor construction (IBC Table 601, Sections 420.3 and 711.2.4.3).

Questions that arise which are not clearly answered in the IBC include:

  • What fire-resistance continuity requirements exist for the 2-hour rated exterior wall through the depth of the floor (which is not usually rated for 2 hours)? Unlike provisions listed for fire-resistance continuity of fire walls, fire barriers and fire partitions, IBC does not specify continuity requirements for exterior walls.
  • What FRTW material continuity requirements exist where an untreated, wood-framed floor (framing and sheathing) intersects a FRTW-framed wall?

A challenge for building design and construction professionals is that there has been a significant variety in what is considered acceptable from one building jurisdiction to another.

To help address this, a committee of experts from the American Wood Council (AWC), WoodWorks and partner organizations investigated code requirements and testing completed for the floor-to-exterior wall configuration. Detailing strategies were developed which are both “framer friendly” and meet IBC requirements. The strategies and rationale behind them are included in the 2018 update of the AWC’s DCA 3 – Fire-Resistance-Rated Wood Floor and Wall Assemblies. See Figures 1A, 1B, 1C and 2.

One example, Figure 1A, shows the detail and rationale of how a 1-hour fire-rated floor assembly with ceiling membrane of Type X or Type C gypsum wallboard can be platform-framed on a 2-hour fire-rated wall. The required 2 hours of protection from within the room through the exterior wall is provided by a combination of the ceiling membrane, sacrificial wood blocking (in the depth of the floor) and a rim board at the end of the floor framing in the plane of the exterior wall. Additionally, DCA 3 notes that neither the sacrificial blocking in the wall, floor framing, or floor sheathing that penetrates the exterior walls needs to be FRTW as these are simply extensions of the floor assembly, not members of the exterior wall. IBC only requires that exterior wall framing and sheathing be FRTW.

A more general discussion of allowable building sizes and detailing of mid-rise construction can be found in this article.