Openings in Exterior Walls by Jonathon Clark
As California continues to wrestle with a housing crisis, many of the projects that I work on are getting more dense. I’m sure there are many reasons that contribute to the densification of residential projects, among them are high land costs, desirable location, project financial feasibility, etc. As projects get closer to other properties and even the different parts of a project get close to each other, we as architects need to be aware of the building requirements of these dense projects.
There are a couple of major parts of the code that are fundamental to understand as we begin talking about opening in exterior walls. The first is construction type found in chapter 6 of the CBC, specifically Table 601. I hope everyone is familiar with if not an expert with what is contained in this table, but if you are not I will briefly explain. There are five different types of construction identified in the code. Type I through V. Generally, Type I is the most fire protective and Type V is the least. There is a subclass of fire protection indicated with an ‘A’ or ‘B’. ‘A’ being more fire protective and ‘B’ less. You can see that Type IV has some other items that are just most specific cases that the code allows. (Also, the next code cycle will update this table significantly to allow for cross-laminated timber construction.) Much of the work that I do falls in Type V-B, V-A, and we are starting to do some building that needs Type III-A/B. Type of construction is important because it is the basis for how large a building can be with respect to area and height. It is also the first major part of the code that defines how much fire protection a building will need. It is also used in Table 602.
Table 602 is where the code describes the second level of fire protection. This table links together type of construction, occupancy group, and fire separation distance, to determine a second level of fire protection. Again, I would hope most are very familiar with this table. This table addresses the idea that the closer a building is to something else (whether that is another building, property line, or public way), the more fire protection it needs. From this table we can say that if buildings have 30 feet or more separation, Table 602 doesn’t require any fire-resistance rating. Between 10 feet and 30 feet, there is a mix of requirements depending on occupancy and construction type. And less than 10 feet, there is a fire-resistance requirement.
So when we are designing buildings that require a certain level of fire-resistance (Table 601) and/or locating these buildings so that they have a short fire separation distance (Table 602) what happens if we need to put openings (doors or windows) in those same fire-resistant walls?
In Chapter 7 of the CBC, we are provided with fire and smoke requirements. CBC Section 705 specifically addresses the requirements of exterior walls and Section 705.8 addresses the requirements of openings in exterior walls. I think most of us understand openings to mean windows and doors. But I have had the term “openings” interpreted broadly to include almost anything that penetrates the exterior wall.
Again, we are provided with a table, Table 705.8, that correlates fire separation distance with a level of opening protection to provide an allowable area or openings.
This table takes a more detailed approach when it comes to fire separation distance by segmenting out distances from zero to 30 in five foot increments. It also distinguishes between protected and unprotected openings and whether or not unprotected openings occur in sprinklered or non-sprinklered buildings. So generally, the more fire separation distance, then the more openings you can have.
It is important to note that a sprinklered building is a building that is equipped with an automatic fire sprinkler system that complies with the NFPA 13 requirements. And, a non-sprinklered building is on that doesn’t have an NFPA 13 fire sprinkler system. So there can be a situation where a building has fire sprinklers but would still be considered a non-sprinklered building when applying this table. One example would be a building with a NFPA 13R sprinkler system (lower standard for design) or a partially sprinklered building.
Another important item to note is that the allowable area is defined as, “…the percentage of the area of the exterior wall, per story.” (see footnote a) This is important because of how the code defines a story, which is, “That portion of a building included between the upper surface of a floor and the upper surface of the floor or roof next above. A story is measured as the vertical distance from top to top of two successive tiers of beams or finished floor surfaces and, for the topmost story, from the top of the floor finish to the top of the ceiling joists or, where there is not a ceiling, to the top of the roof rafters.” This is important because I have often seen where calculations were done that either don’t include enough wall area (not going to the upper floor level) or include too much wall area (including parapets as part of the wall area).
It is important to keep these code sections in mind as projects become more dense. I have had a couple of projects over the years go through the lengthy California planning process and be approved only to get into construction documents to find out that there is too much window percentage for a particular location. Design revisions have to occur and sometimes planning approval has to be revisited. I have had other more complex challenges with the application of this code section that I will share next time.
Architect | Principal
Additional Comments from Kerwin Lee: Remember that all numeric values in the code are arbitrary and are selected by the code writers as a number to design to. The basic intent for separation and the protection of openings are established to prevent the spread of a fire from one building/structure to another, whether they are on the same property or not. There is an ongoing debate whether the code requirements for separation are to prevent fires from spreading to adjacent properties or to protect your property from a neighboring fire. This basic intent goes back in history to the origin of building codes. One such event is the Great Chicago Fire of 1871, which burned over 17,000 structures and left more than 100,000 people homeless. This event prompted fire regulations to prevent such conflagration from happening again. With wildfires, the intent or code approach is changing or can the building code address this type of event?