Building size and construction type

Once the occupancy is known, the next question is how big and how tall the building may be, and what it must be built of. Codes link three things: the occupancy, the size of the building (its height in storeys and metres or feet, and its area) and the construction type, which describes how combustible the structure is and how long its main elements must resist fire. Sprinklers are the fourth factor, because a sprinklered building is allowed to be larger or less fire-resistant than an unsprinklered one. Getting this combination right is one of the most consequential decisions in a building's design, because it fixes the structural system, the fire-resistance ratings of floors, roofs and walls, and much of the cost.

Construction managers meet these rules in preconstruction cost studies, value engineering and product substitutions. A proposal to change from steel to wood framing, to drop the sprinkler system or to add a storey all depend on them. This lesson explains how the code sets size limits, defines construction types and treats mass timber, and how firewalls divide a large building into smaller ones for code purposes.

Measuring building size

Codes measure size with specific definitions, and the results can differ from the area on a real estate plan.

• Building area, in the NBC, is the greatest horizontal area of a building above grade within the outside surface of exterior walls, or within the outside surface of exterior walls and the centre line of firewalls. It is a footprint, not a total of all floors.
• Building height, in the NBC, is the number of storeys contained between the roof and the floor of the first storey. The first storey is the uppermost storey having its floor level not more than 2 m above grade. Basements are therefore not counted in building height.
• Grade is the lowest of the average levels of finished ground adjoining each exterior wall of a building, with some exclusions for localized depressions such as vehicle or pedestrian entrances.

These definitions matter on sloping sites, where a basement may be partly above ground on one side, and on buildings with mezzanines, which may or may not count as storeys depending on their size and openness. A construction manager who adds a mezzanine for storage during construction may unknowingly add a storey for code purposes.

Construction requirements in the NBC

The NBC does not use numbered construction types. Instead, Subsection 3.2.2 contains a series of articles, each covering a combination of major occupancy, building height, building area per street faced and sprinklering, and each stating the construction required. Three broad categories of construction appear in those articles.

• Noncombustible construction, in which the building is built of materials that meet the noncombustibility test, with specified exceptions for limited combustible components such as some interior finishes, roofing and concealed elements.
• Combustible construction, in which wood and other combustible materials may be used for the structure, subject to the fire-resistance ratings the article requires.
• Encapsulated mass timber construction (EMTC), introduced in the 2020 NBC, in which the structure is mass timber protected by encapsulation materials such as gypsum board, allowing taller timber buildings than ordinary combustible construction.

Each article in Subsection 3.2.2 is identified by occupancy and conditions, such as a Group D building up to a stated number of storeys, sprinklered, with a stated maximum building area. It states whether the building must be noncombustible, whether combustible construction is permitted, and the fire-resistance ratings for floors, mezzanines, roofs and loadbearing walls, typically 45 min, 1 h or 2 h. A designer finds the article that fits the proposed building and reads off the requirements. Facing more streets, which gives fire fighters access to more sides of the building, can allow a larger area in some articles.

Articles typically allow larger buildings when they are sprinklered and when they are noncombustible. For example, small unsprinklered combustible buildings with a modest area per floor may need no fire rating at all for floors, while larger buildings need 45 min or 1 h, and high buildings and some occupancies must be noncombustible with 2 h floors. The tables in each article give exact areas, which vary by occupancy and edition, so always read the article itself.

How sprinklers change the limits

Automatic sprinklers control most fires before they spread, so the NBC allows larger buildings and sometimes lower ratings when a building is sprinklered throughout. Many articles in Subsection 3.2.2 exist in pairs: one for an unsprinklered building and one for the same occupancy sprinklered, with a larger permitted area or more storeys for the sprinklered version. Sprinklers are also required in many buildings regardless of size: for example, Group A and B buildings above stated sizes, high buildings, and, in recent editions and many provinces, residential buildings above a stated height. Provinces have added their own sprinkler requirements, especially for care occupancies and retirement homes after fatal fires.

Firefighting access also matters. The area permitted by many articles depends on how many streets the building faces, defined as being within a stated distance of a street or access route with the required width and clearances. Facing two or three streets can allow a larger area, because fire fighters can attack the fire from more sides.

Mass timber and tall wood buildings

Mass timber, including cross-laminated timber (CLT), glued-laminated timber (glulam), nail-laminated timber and laminated veneer lumber, has changed what codes allow. Large solid timber members char at a predictable rate in a fire, and the char layer insulates the wood behind it, so mass timber can achieve fire-resistance ratings that light wood framing cannot. Encapsulating the timber with gypsum board delays the onset of charring and reduces the fuel the structure contributes to a fire.

The 2020 NBC introduced encapsulated mass timber construction, allowing buildings up to 12 storeys for residential and business occupancies when sprinklered and when the mass timber is protected by encapsulation with a stated rating, with limits on exposed timber. The 2025 NBC keeps the 12-storey limit for those occupancies but raises the building height limit from 42 m to 50 m. Provinces have moved faster: British Columbia allowed EMTC up to 18 storeys in its 2024 code, and Ontario followed with similar changes. Construction managers on mass timber projects should expect requirements for encapsulation inspection, protection of timber from moisture during construction, and fire safety measures during construction, because the structure is combustible before encapsulation is complete.

Firewalls

A firewall divides a building into two or more parts that are treated as separate buildings for the purposes of building area and construction requirements. A firewall must have a high fire-resistance rating and remain standing even if the structure on one side collapses, so it is usually concrete or masonry, extends through the roof or to a parapet, and is designed to resist the lateral forces a collapsing structure would place on it. Firewalls are a common way to build large warehouses, shopping centres and long apartment blocks in cheaper construction types.

In the NBC, a firewall generally requires a 2 h rating, or 4 h where the building on either side has a high fire load such as Group E mercantile or Group F-1 or F-2 industrial, and must be constructed of masonry or concrete, with openings limited and protected by closures. A firewall may also have to extend above the roof, typically by a parapet of a stated height, unless the roof on both sides meets alternative requirements.

Construction type in practice

Construction managers deal with construction type every day, often without naming it. A steel deck roof on unprotected steel joists in a Type IIB or noncombustible unrated building is cheap, but adding a storey may require protecting the steel with fireproofing or gypsum board. Wood blocking, furring and roofing in a noncombustible building are permitted only within the code's limits. Combustible insulation, plastic piping and cladding containing plastics need specific approval in noncombustible buildings. Substituting a product on site, such as a foam-plastic insulated panel for a mineral-wool panel, can change compliance with the construction type and with exterior wall fire spread rules.

The construction type also drives the fire-resistance ratings that must be maintained throughout construction: rated floor assemblies need listed designs, rated columns need protection to their full height, and penetrations need firestopping. Lesson 6 covers these ratings in detail.