Structural steel needs fireproofing in Ontario only when the Building Code requires that member to have a fire-resistance rating. Steel doesn't burn, so in many buildings it can be left bare. But it loses strength as it heats, so where the code requires a floor, roof or column to hold up for a set time in a fire, the steel has to be protected to achieve that rating. Whether that applies depends on the building's occupancy, height, area and sprinklers, plus a list of specific exceptions.
This article covers how that decision works under the 2024 Ontario Building Code (OBC) for commercial, industrial and larger residential buildings. For houses, see do steel beams in houses need fireproofing?
How steel behaves when it's heated
Steel is noncombustible: it adds no fuel to a fire. The problem is temperature. As steel heats up, it gets weaker and more flexible, and it expands.
UK steel industry guidance (SteelConstruction.info) summarizes it this way: strength loss is generally accepted to begin at about 300°C and increases rapidly after 400°C; by 550°C steel retains approximately 60% of its room-temperature yield strength and 45% of its stiffness. Room fires can heat unprotected steel past those temperatures.
So when heated, steel structural members tend to:
- Lose load capacity, until the load they're carrying exceeds what's left.
- Deflect and sag, because stiffness drops faster than strength.
- Expand, pushing on walls, connections and the members around them.
- Buckle, especially slender columns and the compression flanges of beams.
How fast a member heats depends on how much steel there is compared with the surface exposed to fire. Ontario's Supplementary Standard SB-2 captures this as the M/D ratio: the mass of a column in kilograms per metre divided by its heated perimeter in metres. A heavy, compact section heats more slowly than a light, thin one, so it needs less protection for the same rating.
Does structural steel have a fire rating?
Not by itself. A fire-resistance rating is a time (45 min, 1 h, 2 h and so on) assigned to a material, assembly or structural member. Under Article 3.1.7.1 of the OBC, a required rating is established in one of two ways:
- By test, to CAN/ULC-S101, "Standard Method of Fire Endurance Tests of Building Construction and Materials." Proprietary systems such as spray-applied products and intumescent coatings are rated through tested, listed designs.
- By assignment, from MMAH Supplementary Standard SB-2, "Fire Performance Ratings." SB-2 gives minimum thicknesses of concrete, masonry, plaster and Type X gypsum board for protected steel columns, cover thicknesses for individually protected steel beams, and a method for concrete-filled hollow steel columns.
In other words, the rating belongs to the steel plus its protection, or to the assembly it's part of.
When the Ontario code requires a rating
For buildings designed under Part 3, Subsection 3.2.2 classifies the building by major occupancy, height, area and whether it is sprinklered. Each classification article then states what construction is allowed and which assemblies need ratings. A few examples from the 2024 OBC show how much it varies:
| Building (OBC article) | What it requires |
|---|---|
| Group D (business and personal services), up to 6 storeys, within area limits (3.2.2.58) | Noncombustible construction; floors 1 h; roof 1 h, waived if 1 storey; loadbearing columns rated at least as high as the assembly they support |
| Group F, Division 2 (medium-hazard industrial), up to 2 storeys, sprinklered (3.2.2.81) | Columns supporting a rated assembly need a 45 min rating or must be of noncombustible construction |
| Group F, Division 3 (low-hazard industrial), 1 storey, within area limits (3.2.2.89) | Heavy timber or noncombustible construction; no ratings listed for floors, roof or columns |
Two general rules tie it together:
- Supporting construction (Article 3.1.7.5). Loadbearing walls, columns and arches in the storey below a rated floor or roof generally need at least the same rating as the assembly they support. A 1 h floor usually means 1 h columns under it.
- Part 9 buildings (Article 9.10.7.1). In small buildings, structural steel members used in construction required to have a fire-resistance rating must be protected to provide that rating, subject to the same exceptions.
Notice the Group F, Division 2 example: bare steel columns can be acceptable there because they're noncombustible. That's why a one- or two-storey steel warehouse or shop often has no fireproofing at all, while a mid-rise office does. The architect or code consultant does this classification early in design, and the drawings show which members need protection and what rating.
Steel that is exempt from fire protection
Article 3.2.2.3 of the OBC lists steel that doesn't need fire protection even in buildings where the frame does. In summary, it includes:
- Steel lintels over openings not more than 2 m wide in loadbearing walls and not more than 3 m wide in non-loadbearing walls, and wider lintels if they're supported at intervals of not more than 2 m by members with the required rating.
- The bottom flanges of shelf angles and plates that aren't part of the structural frame.
- Steel in elevator hoistways around doorways and supporting guides and similar equipment, where it is entirely enclosed in the hoistway and not part of the structural frame.
- Steel members of stairways and escalators that aren't part of the building's structural frame.
- Steel members of porches, exterior balconies, exterior stairways, fire escapes, cornices, marquees and similar appurtenances outside an exterior wall.
- Loadbearing members wholly or partly outside the building face in buildings up to 4 storeys in certain occupancies, if they are kept at least 1 m from unprotected openings or shielded from radiated heat.
Each item has conditions in the full text, so check the exact wording for your case.
The main fireproofing methods
Where protection is required, the design team chooses a method that suits the rating, the exposure and the look:
- Spray-applied fire-resistive materials. Cementitious or fibrous coatings sprayed onto beams, columns and deck. Common for concealed steel above ceilings. Rated through listed designs.
- Intumescent coatings. Thin paint-like coatings that swell into an insulating char when heated. Used where steel is exposed architecturally. Rated through listed designs, with the thickness depending on the section and the rating.
- Gypsum board enclosures. Layers of Type X gypsum board around columns or beams. SB-2 gives the minimum board thickness for each rating based on the column's M/D ratio.
- Concrete or masonry encasement. The traditional method, with minimum cover thicknesses in SB-2.
- Concrete-filled HSS columns. SB-2 allows a rating to be assigned to concentrically loaded hollow steel columns filled with plain, fibre-reinforced or bar-reinforced concrete, within the limits of its formula.
- Rated assemblies. Floor and roof assemblies with a rated ceiling membrane can protect the steel above as part of a tested design.
What it means for fabrication
Fireproofing is applied after the steel is erected, but decisions made in the shop affect it:
- Primer compatibility. Spray-applied products and intumescent systems each have their own requirements for the surface they go on. Some need bare steel, some need a specific primer. Tell your fabricator what fire protection is planned before the steel is painted.
- Clips and attachments. Brackets, hangers and connection plates should be welded on in the shop where possible. Field welding after fireproofing damages it and has to be repaired.
- Member sizes. Because protection depends on M/D, the engineer may pick a heavier section to reduce coating thickness. Order exactly the section on the drawings.
Steel after a fire is a separate topic. A commonly quoted rule of thumb is that hot-rolled steel that is still straight with no obvious distortion is probably fit for use, but reuse is an engineer's call after an inspection.
Steel supplied ready for the protection your project needs
We fabricate structural steel to your shop drawings, with clips and connection plates welded in our CWB-certified shop and the finish your specification calls for. Tell us the fire protection system when you ask for a quote so the coating plan is right from the start. Planning a mezzanine or a frame addition? See our steel mezzanine guide, and for how the steel codes fit together, Canadian structural steel codes.
Send us your drawings for a quote, or see the services we offer, from shop drawings to delivery and installation.
Frequently asked questions
Does structural steel have a fire rating?
Not on its own in any useful sense. Steel is noncombustible, but a fire-resistance rating belongs to a protected member or a tested assembly. In Ontario, ratings come from tests to CAN/ULC-S101 or from Supplementary Standard SB-2, which ties steel ratings to the protection around it.
What happens to steel in a fire?
It doesn't burn, but it loses strength and stiffness as it heats. UK steel industry guidance says strength loss begins at about 300°C, increases rapidly after 400°C, and that by 550°C steel keeps roughly 60% of its room-temperature yield strength and 45% of its stiffness.
Is fireproofing required on all steel beams?
No. Protection is needed only where the building code requires the member, or the floor or roof it supports, to have a fire-resistance rating. Many one-storey buildings and some sprinklered buildings allow unprotected steel, and Article 3.2.2.3 lists further exceptions.
Does a steel lintel need fire protection?
Not in many cases. The Ontario code exempts steel lintels over openings up to 2 m wide in loadbearing walls and up to 3 m wide in non-loadbearing walls, and wider ones if they are supported at intervals of not more than 2 m by members with the required rating.
