Exterior Wall Fire Ratings Demystified

Multifamily project under construction in Seattle, Washington. Wood framed exterior walls are sheathed with gypsum sheathing. Photo by the author.

Figuring out the required fire rating of an exterior wall can be a bit tricky. A number of sections of the International Building Code¹ (IBC) govern fire resistance requirements for exterior walls, and the appropriate fire rating for an exterior wall is often based on a multitude of factors such as building construction type, fire separation distance, occupancy group, and whether or not the wall itself is load bearing.

So where do we start?

Well, when it comes to fire resistance ratings of wall assemblies, Chapter 7 is always a good place to jump in, and particularly relevant in our case is Section 705 which pertains to exterior walls. Section 705.5 addresses fire resistance ratings and largely redirects to Tables 601 and 602. Importantly, 705.5 also includes some stipulations about whether an exterior wall must be rated from the inside only or from both sides. We’ll come back to that in just a moment. Our first step is to determine the wall’s fire rating by referencing the two tables in Chapter 6.

Table 601 applies to both interior and exterior walls while Table 602 applies to exterior walls only. Since both Tables 601 and 602 specify fire resistance ratings for exterior walls, both must be consulted and the higher fire rating of the two will govern (as directed by Table 601 Footnote e and Table 602 Footnote a).

Since the required ratings in both Tables 601 and 602 are based (in part) on the type of construction, I’ll break my analysis below down along those lines. I’ll focus specifically on a few types of construction that are common to multifamily residential architecture (Type IIIA, IA, and VA), but the approach discussed below can be applied to exterior walls for a building of any construction type.

Type IIIA Construction

Type IIIA construction is commonly used in multifamily residential buildings, especially for the upper levels of a “podium” building. Exterior walls in Type IIIA are likely going to be platform framed using Fire Retardant Treated (FRT) wood, which means that as we search for the appropriate fire rating(s), we’ll assume that the exterior walls are generally bearing walls.² Per Table 601 exterior bearing walls in Type IIIA construction must be rated for 2 hours.