Rooftop Enclosures: Penthouse vs. Mechanical Floor

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July 10, 2023 by Ben Seeto

The classification of sheltered mechanical, electrical, and plumbing (MEP) equipment as a “penthouse” in lieu of a “mechanical floor” provides numerous code advantages at the top level of a building. The main benefits include:

  • Height & Area. Penthouses are considered part of the “story” below such that they are not included when evaluating the building’s height (number of stories or in feet) nor are they included in the building’s area evaluation.
  • Means of Egress. Penthouses are considered a “normally unoccupied” space and are not subject to the means of egress requirements for an “occupied floor”.
  • Shaft Termination. Top of shaft enclosures are not required in penthouses which provides flexibility with the termination of shafts that otherwise cannot be recognized in a typical story, specifically with specialty exhausts that prohibit the use of fire dampers such as lab ventilation hazardous exhaust.

For a rooftop structure to be classified as a “penthouse” and realize the above benefits, there are various use limitations, and all the following requirements will need to be met:

  • Height. Unless the building is of Type I construction, the penthouse is limited to a maximum height of 18 feet above the roof deck. Where the penthouse is used to enclose a tank or elevator, that requirement is extended to 28 feet. Penthouses in Type I buildings are permitted to exceed this threshold.
  • Area. The area of the penthouse is limited to 1/3 the area of the supporting roof deck.
  • Use. Penthouses are limited to mechanical or electrical equipment only. Neither storage nor the use of space for a small office are permitted.
  • Construction. Penthouses are required to be constructed of materials consistent with the building’s construction type.

A building’s continued operation and interior environment rely on the building’s mechanical, electrical, and plumbing systems, which require large equipment. The square footage to locate the equipment may be hard to come by, and when it is found, it comes at the expense of valuable interior building area. Looking to the roof of the building to locate the equipment is a frequent alternative.

From the perspective of the building code, there are three options for rooftop structures to house such equipment:

  1. Equipment directly on the roof and open to sky;
  2. A story housing mechanical equipment; or
  3. A penthouse.

 

 

 

Application of any information provided, for any use, is at the reader’s risk and without liability to Code Red Consultants. Code Red Consultants does not warrant the accuracy of any information contained in this blog as applicable codes and standards change over time. The application, enforcement and interpretation of codes and standards may vary between Authorities Having Jurisdiction and for this reason, registered design professionals should be consulted to determine the appropriate application of codes and standards to a specific scope of work.