What determines whether an elevated floor assembly is considered a mezzanine or a second floor assembly? To some it may seem simple enough – a mezzanine is open to the floor area below like an interior balcony and a second storey is not. Or even more simplistically, the elevated floor area must be called a mezzanine. In practice, it is more complicated than that.

A mezzanine is defined by the Building Code as “an intermediate floor assembly between the floor and ceiling of any room or storey and includes an interior balcony.” Whether the floor assembly is a mezzanine or second storey impacts the overall building classification as well as the requirements for that elevated floor assembly, including exits. Although there have been significant revisions to the National Building Code (NBC) since the 1995 edition that have made the requirements more concise and easier to follow, this is still one of those areas of the Building Code that continues to stump designers. There are four primary considerations in the design of a mezzanine.

1. Is this elevated floor assembly considered a storey in building height?
2. Is the mezzanine floor assembly required to be constructed as a fire separation or have a fire-resistance rating?
3. Is the floor assembly required to terminate at a vertical fire separation?
4. Is an open egress stair permitted from the mezzanine?

Although there have been significant revisions to the National Building Code (NBC) since the 1995 edition that have made the requirements more concise and easier to follow, this is still one of those areas of the Building Code that continues to stump designers

Is this elevated floor assembly considered a storey in building height?

If there are no other mezzanines in the storey, no other mezzanine above the one in question, the mezzanine does not exceed 40% of the open area of the room in which it is located and is relatively open to the room in which it is located without partitions or subdividing walls higher than 1070 mm above the mezzanine floor…then the floor assembly is a mezzanine and not a storey in building height (Sentence 3.2.1.1.(3), Division B NBC).

Alternatively, if there are no other mezzanines in the storey and there is no mezzanine above the one in question, if the mezzanine floor does not exceed 10% of the floor area in which it is located and it does not exceed 10% of the area of the suite in which it is located, it is considered to be a mezzanine (regardless of height of partitions above the floor assembly) (Sentence 3.2.1.1.(4), Division B NBC). Using this provision allows the most flexibility for both the mezzanine and the rooms below but may be more restrictive with respect to area of the mezzanine.

The addition of partitions in the room in which the mezzanine is located or on the mezzanine itself can result in the mezzanine no longer being considered a mezzanine which triggers other requirements related to exiting and vertical termination of the floor assembly. Sentence 3.2.1.1.(7) allows enclosed space on this relatively open mezzanine, but requires that the enclosed space does not exceed 10% of the open area of the room in which the mezzanine is located and is placed so that it does not obstruct visual communication between the open space above the mezzanine and the room in which the mezzanine is located. Note that Sentence 3.3.2.13.(3) permits open bookshelves in libraries.

The British Columbia Building Code (BCBC) (and potentially other provincially amended codes) has significantly different wording than the NBC for the 40% open mezzanines described above. In the BCBC, a mezzanine “need not be considered a storey in calculating the building height provided not less than 60% of the horizontal plane separating the mezzanine from the room or floor space in which it is located is open”. This is typically interpreted that closed rooms in the space the mezzanine opens into are permitted, as long as the ceiling of those rooms is below the horizontal plane of the mezzanine. This is the not the way the NBC is worded or applied, and seems to go against the intent of providing visual communication between those on the mezzanine level, and the open floor area they are a part of. The remainder of all BCBC requirements for mezzanines described in this document are similar to those in the NBC which are described below.

Other rules to be aware of that come up from time to time:

• If a mezzanine is partly or wholly superimposed above another level of mezzanine, the upper mezzanine is typically considered a storey in building height. (However, in practice either mezzanine can be considered the storey as indicated in the flow chart in the next section of this post).
• Platforms intended solely for periodic inspection and elevated maintenance catwalks are not treated as mezzanines and are considered separately in Sentence 3.2.1.1(6).
• If there are multiple mezzanines in a floor area, the aggregate area of mezzanines that are not superimposed must be considered for both the 10% and 40% rules indicated. However, these two rules need not be considered together. A few examples:
  • A mezzanine which is 9% of the floor area of the suite is permitted to co-exist with a mezzanine that does not exceed 40% of the open area of the room in which it is located (provided the 9% mezzanine is not located in the same room as the 40% mezzanine).
  • Two mezzanines that are enclosed and total 11% of the floor area of the suite would not be permitted. One of those mezzanines would be required to be considered a second storey.

Confusing right? This is one of those sections of the Building Code that no matter how many times you’ve read it, you have to re-read it for each new situation to make sure that it is properly applied.

As a rule of thumb, I use the following as a starting point: Is there a mezzanine superimposed above this one?

• If yes, then this mezzanine is a storey.
• If no, is the aggregate area of mezzanines 10% or less of the floor area of the suite in which it is located (total floor area if the building contains a single suite)?
  • If yes, it is not a storey.
  • If no, is the aggregate area of mezzanines in a room 40% or less of the open area of the room in which they are located?
    • If yes, then do the mezzanines open to the storey below meet the requirements of Sentence 3.2.1.1.(7)?
      • If yes, they are not a storey.
      • If no, they are a storey.

Ensure that the actual Building Code articles are used in conjunction with the above very simplistic analysis to ensure that the correct determination of building height is arrived at.

Is the mezzanine floor assembly required to be constructed as a fire separation or have a fire-resistance rating?

If the elevated floor assembly is required to be considered to be a storey in building height, it must be constructed in conformance with the fire separation requirements for floor assemblies stated in the building classification articles 3.2.2.20. to 3.2.2.90.

If the elevated floor assembly is permitted to be considered as a mezzanine, then it is permitted to be constructed in conformance with the fire-resistance rating requirements for mezzanines stated in the building classification articles 3.2.2.20. to 3.2.2.90.

Is the floor assembly required to terminate at a vertical fire separation?

If the floor assembly is required to be considered a storey in building height, it must terminate at an exterior wall, firewall, vertical shaft or a vertical fire separation having a fire-resistance rating not less than that required for the floor assembly. Thre are a few relaxations to this:

• The floor area is protected with the special measures of protection for interconnected floor spaces in Articles 3.2.8.3. through 3.2.8.8. (mechanical exhaust system, draft stops, protected floor space, etc).
• The requirements in Sentence 3.2.8.2.(6) can be met to allow for a relaxation.

If the floor assembly is permitted to be considered a mezzanine and not a storey in building height, it need not terminate at a vertical fire separation provided the following conditions are met (Sentence 3.2.8.2(1)):

• It serves an A1 major occupancy,
• it serves an A3 major occupancy in a building not more than 2 storeys in building height, or
• It serves a Group A, C, D, E or F major occupancy and is 500 m2 or less in area.

Other items to note:

• Mezzanines are required to terminate at vertical fire separations in B occupancies.
• Occupancy or garage fire separations may also be required around a mezzanine depending on its use and the partitions which are more than 1 070 mm high may affect whether the mezzanine can be considered a mezzanine or if it must be considered a storey in building height.
• If a mezzanine is required to terminate at a vertical fire separation, it requires fire rated exits to the exterior on the same basis as required for any other floor area (Article 3.4.2.2.).

Is an open egress stair permitted from the mezzanine?

Many mezzanine designers want an open stair from the mezzanine to the floor area below. This is only permitted under certain conditions:

• The mezzanine is not required to terminate at a vertical fire separation,
• The occupant load on the mezzanine does not exceed 60 people, AND
• The area and travel distance limits stated in Table 3.4.2.2. are not exceeded.

The area and travel distance limits are based on occupancy and match those for a single exit in an unsprinklered floor area. In my opinion, the occupancy to be used with this table is the occupancy of the mezzanine itself, but I have seen some Authorities Having Jurisdiction (AHJ’s) apply the occupancy of the overall space to the mezzanine. For example, if the mezzanine is used for office space but located in a medium hazard industrial warehouse, the area and travel distance limits for a D occupancy should be used. However, some AHJs would require that the area and travel distance requirements for an F2 occupancy be used. What is the interpretation of the AHJs you work in?

Travel distances on mezzanines are measured two different ways, depending on the egress provided in the space below:

• If the space is served by a single egress door, the travel distance is measured down the stairs to an egress door serving the space that the mezzanine overlooks.
• If the space is served by 2 or more egress doorways, the travel distance is measured to the top of the mezzanine egress stair.

If all of these conditions are not met, then a single open egress stair may not be used. In that case, Sentence 3.4.2.2.(3) requires only half of the required means of egress from the mezzanine to be means of egress leading to exits accessible at the mezzanine level on the same basis as floor areas provided the mezzanine is not required to terminate at a vertical fire separation. If a mezzanine is required to terminate at a vertical fire separation, exits are required on the same basis as for any second storey floor area (Sentence 3.4.2.2(1)).

Hopefully this helps provide a better understanding of mezzanine requirements and doesn’t raise more questions than it answers. The most important thing to remember for mezzanines is to go back to the Building Code each time and follow through the requirements for each individual mezzanine.

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