Occupied Roof Decks - Code Changes in 2018 and 2021 IBC

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 roof at the Facebook headquarters in Menlo Park, CA (image courtesy of Facebook).

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:

Where a structure is proposed for a purpose that is not specifically listed in this section, such structure shall be classified in the occupancy it most nearly resembles based on the fire safety and relative hazard. Occupied roofs shall be classified in the group that the occupancy most nearly resembles, according to the fire safety and relative hazard, and shall comply with Section 503.1.4. (Excerpt from 2018 IBC 302.1)

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:

A roof level or portion thereof shall be permitted to be used as an occupied roof provided the occupancy of the roof is an occupancy that is permitted by Table 504.4 for the story immediately below the roof. The area of the occupied roofs shall not be included in the building area as regulated by Section 506.

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:

1. If your building if fully sprinkler-protected (NFPA or 13R) and you have fire alarm notification on the occupied roof, you are permitted to have any occupancy on the occupied roof.
2. If your building is Type I or II construction, you are permitted to have assembly occupancies on the roof of parking garages.

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.

Sketch of a 4 story building with occupied roof. Assuming Type IIIA construction, fully sprinkler-protected with fire alarm notification throughout, a Group A-3 occupancy is now explicitly permitted 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.

The 2021 IBC clarifies that spaces used to access an elevator or stairway providing access to the roof are permitted to be considered a penthouse, and therefore not a story (Image courtesy of Dana Schulz).