As more and more states move to adopt the 2018 IBC, it's important to know about a few code updates that impact the design of occupied roof spaces. And if you're jurisdiction is on the ball and already adopted the 2021 IBC, there are a few additional items that apply to you.
Occupied Roofs Under the 2018 IBC
In the 2015 IBC and prior editions, the code remained silent in regards to occupied roof decks and requirements for allowable height and area. Under the 2018 IBC, there are now specific provisions addressing this issue.
First, IBC 302.1 has been updated with specific requirements for classifying an occupied roof space:
This added language forces designers to classify an occupied roof with an occupancy, whereas some jurisdictions has not previously required it.
Next, IBC 503.1.4 brings in a major new requirement:
This section limits the occupancy on an occupied roof to those allowed on the story immediately below the roof. So if your building construction type allows an assembly occupancy on the top story, you are also permitted an assembly occupancy on the roof. The code does offer two major exceptions:
Take the image below for an example, a 4 story building consisting of Type IIIA construction. If the building is fully sprinkler protected in accordance with NFPA 13, a Group A-3 occupancy would per permitted on Level 4. As long as occupant notification is provided on the roof, the 2018 IBC now explicitly allows a Group A-3 occupancy on the roof.
The provision of Section 503.1.4.1 do limit elements on the roof to no more than 48" above the roof surface, with exceptions for penthouses, tower, domes, spires and cupolas. If you have elements above this height, the roof would have to be classified as a story. As an example, the overhang in the image above of the Facebook building would likely trigger the roof being classified as a story.
Another common question is whether an occupied roof can trigger classification as a high-rise building. The IBC itself does not address this issue, but this staff opinion from the ICC is very helpful in clarifying the intent. The opinion clearly states, "Just because a roof is an occupied roof does not make it a floor with respect to the definition of a high-rise building. "
Occupied Roofs Under the 2021 IBC
The 2021 IBC updates Section 503.1.4 to explicitly clarify that an occupied roof should not be included when determining building height or number of stories per IBC 504. Language was also added to clarify that this only applies when penthouse and other rooftop structures, when present, comply with IBC 1511. This further supports the notion described above that an occupied roof should not by itself trigger a classification as a high-rise building.
Additionally, the 2021 IBC clarifies that the Exception 1 to 503.1.4 for occupant notification would only require a voice fire alarm system on the roof if the system is required elsewhere in the building. In other words, if you aren't required to have a voice fire alarm in the building, you don't have to provide it on the roof in order to use Exception 1.
Finally, the 2021 IBC updates Section 1511.2.2, Use Limitations for Penthouses, to specifically allow "ancillary spaces used to access elevators and stairways" to be considered part of a penthouse. This means that a stair or elevator tower to the roof will not force an occupied roof to be classified as a story, even though those elements are taller than 48" from the roof surface.
In the past few months, I've had several clients ask me questions regarding grade plane calculations - most often related to determining whether a particular story counts as a story above grade plane or as a basement. Here is quick rundown of how to do the calculation and the implications.
Average Grade Plane
Chapter 2 of the IBC defines Grade Plane (often referred to synonymously as Average Grade Plane) as:
A reference plane representing the average of finished ground level adjoining the building at exterior walls.
So if you have a completely flat site, the elevation where the finished ground level adjoins the building exterior walls is your grade plane. If your site is sloped, you need to take several elevations where the finished ground level adjoins the building and average them to determine the grade plane.
The most common question I receive on this: how many elevation points do you need to account for this in average?
First, the IBC does not give any specific requirements or instructions to answer this question. Your local jurisdiction could potentially have some guidance on how they want to see the calculation performed, but in my experience, most do not.
Typically, I advise clients to base the number of elevation points on the slope of the site. If you have a relatively flat site, a small number of elevation points will give you an accurate grade plane elevation. You may only need one elevation point per face of the building. On the other hand, if you have an irregular, highly-sloped site, you will need many elevation points. I once worked on a building that was built into the side of a steep hill - there we measured the elevation every 10 feet in some areas. If in doubt, the greater number of elevation points in your calculation will result in a more accurate measurement.
In the simple example above, the grade plane measurement is just the average of the elevations at the end of each wall segment (e.g. for the east wall, (280+270)/2 = 275'). However, with a more severe grade, particularly where the slope is irregular, you will need to take additional measurements, as shown in the example below.
Another common question: What if my site slopes perpendicular to the exterior walls?
The Implication - Determining a Story Above Grade Plane
Why does the grade plane calculation matter? On my projects, this always comes up when a client is trying to determine whether a particular story is actually a story above grade plane or a basement.
Chapter 2 of the IBC defines "Story Above Grade Plane" as:
Any story having its finished floor surface entirely above grade plane, or in which the finished surface of the floor next above is:
This definition has caused some confusion in the past, so here is a step by step breakdown:
There are numerous code requirements that differ between a story and a basement, but the most common one is complying with the allowable number of stories based on occupancy and construction type. I had a project this year where the floor surface of the level above was 5 feet, 10 inches above the grade plane elevation, just barely passing as a basement. 2+ inches more and the building would haven been one story taller, resulting in requirements for a more robust construction type, higher shaft ratings and substantial additional cost.
If your situation is that close, I suggest including a plan in the permit package that clearly shows the grade elevations around the building and shows your grade plane calculation. This makes it easy for plan reviewers to follow your logic and hopefully agree with your approach (remember, there are no specific IBC instructions or requirements for how to do the calculation).
What do you Think?
Have you had a project that required a detailed grade plane calculation? Have you every had a plan reviewer or code official disagree with your calculation? Let me know in the comment box!
October 2020 Update: I am moving all calculators to the Tools menu at the top right of the screen. I have also added in a slider bar to allow for varying frontage amounts (see the new tools page for this function).
After several weeks of working on this, I'm excited to release a beta version of the 2015 IBC Allowable Height, Area and Number of Stories Calculator. Take a look and let me know what you think!
A few notes:
In the AEC world, the terms “mixed use” and “mixed use building” are commonplace. Developers refer to new buildings that contain both office and retail spaces as mixed use projects. Architects and Engineers refer to a floor containing multiple occupancy types as mixed use. You’ll even hear AHJs drop the term mixed use in plan review meetings on occasion.
Colloquially, “mixed use” is meant to refer to a building or space that contains multiple occupancy types. But did you know that other than parking garages, the building code has essentially no requirements pertaining to a mixed use building? In fact, if you search for “mixed use” in the International Building Code (IBC), you won’t find any requirements related to building height, building area, construction type, fire-rated construction or means of egress. *
So why is “mixed use” a common term in the AEC world but not addressed in the codes? Long story short: there is a difference between use and occupancy in the IBC and people frequently confuse the two. We’ll do a full analysis of the differences between use and occupancy in a later article, but here is a simple way to understand the difference.
Mixed Use Vs. Mixed Occupancy
The use of a space is a description of how the space will actually be used. This could be a broad, general description such as “office” or “conference room”, or it could be more specific, such as “visiting team locker room and shower.” The use of a space is helpful in determining (1) the occupancy classification and (2) the appropriate occupant load for a space.
On the other hand, the occupancy classification of a space is 1 of 10 categories (plus sub categories) in Chapter 3 of the IBC. These occupancies are:
For example, most offices and places of business have conference rooms, where multiple people come together for meetings, presentations or collaboration. The use of such spaces might lead you to think that the conference room is an assembly occupancy. But per IBC 303.1, if your office has a conference room with 49 people, the conference room is considered a business occupancy. There are many similar examples where the use of a space does not necessarily align with the occupancy, so always refer to the definitions in Chapter 3 when determining the occupancy classification.
Now getting back to the original discussion, “mixed use buildings” do not have specific requirements because nearly every building contains multiple uses. Does your office suite have a storage area? You have at least two uses there. Does your retail store have an office in the back? Multiple uses. Does your apartment building contain an amenity space? You get the point.
So when people use the term “mixed-use,” it doesn’t mean much from a code standpoint, and practically, almost all buildings contain multiple uses. But what most folks intend to convey by this term is that the building has at least two distinct occupancy types, which the IBC would consider “mixed occupancy.” And unlike “mixed-use,” there are numerous requirements for “mixed occupancy buildings” in the IBC. In fact, IBC Chapter 5 has sections for allowable height, allowable area, allowable number of stories and required separation of occupancy, all of which are impacted when a building is mixed occupancy.
Conversationally, “mixed use building” is generally understood to mean a building containing multiple occupancy types. Architects, engineers, contractors and code officials all use this term and it typically does not create a misunderstanding. But as far as the code language goes, mixed use doesn’t mean anything for the requirements for a building. Mixed occupancy, on the other hand, carries many code requirements that must be understood for a code-compliant building design.
*While IBC 508 is entitled "Mixed Use and Occupancy,' it does not actually have specific requirements for mixed use buildings, only mixed occupancy buildings.