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Determining Building Height

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If someone were to point to a building and ask you “What is the height, in storeys, of that building?”, how would you answer? Would you just count the number of window levels from outside of the building? Would you need to enter the building to count the number of floors as you climbed to the top? The number of storeys of a building may seem like a straightforward concept; however, there are many factors involved in determining a building’s height when applying the building code.

The 2015 edition of the National Building Code of Canada (NBCC) defines building height as “the number of storeys contained between the roof and the floor of the first storey”. The same definition is used in the Alberta amended version of the NBCC, and the 2019 edition of the Ontario Building Code (OBC).

To determine the number of storeys in the building, other defined terms need to be understood:

    • Storey: Storey is simply the space between the top of any floor and the top of the floor next above it, or ceiling when no floor is above.
    • First Storey: The first storey is the uppermost storey having its floor not more than 2 m above grade. The OBC uses 1.8 m instead of 2 m.
    • Grade: NBCC defines grade of a building as the lowest of the average levels of finished grade adjoining each exterior wall, except for localized depressions. In other words, the average grade of each side of a building between exterior walls would be individually calculated, and the lowest of these averages would be considered the grade for the entire building. Alternatively, some provincial building codes, such as the OBC, define grade differently and considers grade to be the overall average level of grade from all exterior walls.

 image of a building for Determining building height blog

Using these defined terms, a “walk-out basement” may not be a basement at all; it could be a walk-out first storey depending on where the grade is considered and at what height the next storey above begins in relation to that defined grade.

If that isn’t confusing enough, the Building Code has several conditions where space is not required to be considered a storey.

Mezzanines (intermediate floor assemblies between the floor and ceiling of a room or storey) are not required to be considered a storey if they meet specific conditions. Specifically, the aggregate area of mezzanines, without partitions or walls above 1.07 m, cannot exceed 40% of the open area of the room in which they are located. If partitions extend above 1.07 m, the mezzanine cannot exceed 10% of the open area of the room in which they are located. Additionally, each level of the mezzanine that is superimposed above the first level of the mezzanine is required to be considered a storey.

A service space (a space to conceal the installation of building service facilities such as chutes, ducts pipes shafts or wires) where a person is able to enter and perform maintenance and other operations is not required to be considered a storey. However, these types of spaces are required to have enhanced fire safety such as sprinkler protection, egress signs, fire alarm devices, emergency lighting, and vestibules at exit doors. This type of service space is often referred to as an interstitial space and is most commonly used between floors in hospitals.

Platforms and elevated maintenance catwalks that are not used for storage and are noncombustible are not required to be considered a storey. The Building code does not limit the area of such platforms and catwalks.

A roof-top enclosure used for elevator machinery, a stairway, or a service room is also not required to be considered a storey.

Finally, space under tiers of seating in an arena type building is not required to be considered a storey provided the space is used for dressing rooms, concessions or similar uses.

Based on the various requirements and permissions, it can often be difficult to determine how many storeys are in a building. The number of storeys is used in determining the construction requirements for that building, and an additional storey could result in the building requiring to be changed from combustible construction to noncombustible construction, requiring sprinkler protection, and/or more robust fire-resistance for floors, mezzanines and supporting structure.

Sometimes, building designers try and manipulate grade so that a building can be considered fewer storeys than intended by the codes, which can result in less stringent construction and fire protection requirements. More often, designers mistakenly misinterpret the Building Code and the exceptions to building height, resulting in buildings that are misclassified.

An example of this misclassification is a large industrial building that was considered a two-storey building by the designer. At first glance, the building could easily be mistaken as two storeys since the building contained two main operating floors with concrete floor assemblies. However, after a more thorough review, it was determined that the building was five storeys in height plus a mezzanine. This was mainly due the building having several large, superimposed mezzanines that the NBCC criteria considered storeys.

Other examples could be something as simple as having a roof top enclosure for a service room, and including a single washroom beside the service room. By definition, the added washroom changes the space from a rooftop enclosure, which is permitted to not be considered a storey, to a storey, thereby increasing the building height.

It can be advantageous to make sure that a building’s height and construction requirements are in accordance with the intent of the building code prior to beginning construction. Improper building classification could result in delayed construction and/or increased construction costs.

PLC Fire Safety Engineering has the knowledge and expertise to help owners and designers confirm the fire and life safety requirements for a building prior to design completion and construction. PLC has also developed many alternative solutions during, and post, construction to achieve the equivalent level of fire and life safety the building codes require.

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