Open Vs. Enclosed Parking Garages

For architects and engineers designing parking garages, two questions commonly arise when determining whether the garage should be an open or enclosed garage. These questions are:

• When can a parking garage be considered open?
• What are the differences in code requirements for an open garage vs. an enclosed garage?

In this post, we’ll provide a concise overview of the requirements related to these frequently asked questions. I use this writeup as a quick reference guide instead of opening up to the code every time. Hopefully it's helpful for you too! All code references are to the 2015 International Building Code (IBC). 

A naturally ventilated, open parking garage (image courtesy of Steve Morgan).

When Can a Parking Garage Be Considered Open?

Parking garages must meet all of the requirements of 406.5 to be considered open. In most cases, the limiting factor for an open vs. enclosed garage is meeting the opening requirements for natural ventilation.
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To achieve natural ventilation, an open parking garage must (IBC 406.5):

• Have uniformly-distributed openings on two or more sides
• Have openings not less than 20% of the perimeter wall area of each tier
• Have openings not less than 40% of the perimeter length of each tier
  • This 40% length requirement does not apply if the openings are uniformly distributed on two opposing sides of the garage

​One additional point is that these opening calculations are for the free area of the openings. If a screen or other form of cover is applied to the opening, the free area ratio of the screen must be multiplied by the area of the opening to determine the free area.

What are the differences in code requirements for an open garage vs. an enclosed garage?

Key differences in the code requirements are:

Ventilation
Where open garages are naturally ventilated, closed parking garages require mechanical ventilation complying with the International Mechanical Code (IMC). Mechanical ventilation alone can add hundreds of thousands or even millions of dollars to a project.

Sprinkler Protection
Open parking garages are not required to be sprinkler protected. Enclosed parking garages require sprinkler protection in accordance with NFPA 13. If the garage is not maintained above 40 degrees Fahrenheit, a dry-pipe sprinkler system is required by NFPA 13. Both open and enclosed garages typically require standpipe systems.

Enclosure of Vertical Openings
Stairs serving an enclosed parking garage are required to be enclosed with rated construction similar to any other vertical opening in a building. In an open parking garage, enclosure of vertical openings, including stairs, is not required. In both cases, the vehicle ramps connecting levels are not required to be enclosed.

Height and Area
For an open parking garage used exclusively for parking (with an exception for a small office and waiting area at grade), the number of tiers and area is permitted to comply with IBC 406.5.4, which is an increase over the allowable height and area limits in IBC Chapter 5. The numbers in IBC Table 406.5.4 can be further increased when the garage has open sides on at least ¾ of the building perimeter. Enclosed garages cannot take this approach and must comply with the limits set forth in Chapter 5.

Fire Resistance Ratings of Exterior Walls
Both open and enclosed parking garages are required to comply with the exterior wall rating requirements of IBC 602. However, per Footnote C to IBC Table 602, open parking garages with a fire separation distance of 10 feet or greater are not required to have a fire-resistance rating. Enclosed parking garages do not have this exception and would require a 1-hour exterior wall unless the fire separation distance is 30 feet or greater.

Conclusion

​An open parking garage must meet the minimum requirements for both area of openings and perimeter of openings to allow for natural ventilation. If a parking garage does not meet these requirements, it is considered an enclosed garage and must be provided with mechanical ventilation, sprinkler protection, and it must also meet the other requirements listed above.