Do Steel Beams in Houses Need Fireproofing?

In a single-family house, a steel beam generally doesn't need fireproofing under the 2024 Ontario Building Code. Article 9.10.8.10. says the floor fire-resistance ratings don't apply to a dwelling unit with no other dwelling unit above or below it, so neither do the ratings for the beams and columns holding those floors up. The big exception is a house with a secondary suite: there, loadbearing steel elements that support the floor between the two units must be protected by at least 15.9 mm Type X gypsum board.

Basement apartments are common in GTA renovations, so this question comes up on a lot of beam jobs. Here's how the rules work for each kind of house.

Why the code cares about steel beams in a fire

Steel doesn't burn, but it loses strength and stiffness as it heats up. An unprotected beam in a hot fire can sag or lose capacity, and the floor above goes with it. Fire protection, whether gypsum board, plaster, concrete or a specialist coating, slows the rise in steel temperature so the beam keeps carrying its load for the required time.

The code's approach for houses is to require that protection only where a fire in one place could trap or endanger people somewhere else: between separate dwelling units, or between a home and another occupancy. Inside a single home, it relies on smoke alarms and exits instead.

Single-family houses: usually no rating required

Part 9 sets fire-resistance ratings for floors in Table 9.10.8.1. and requires loadbearing walls, columns and arches in the storey below a rated floor to have at least the same rating (Sentence 9.10.8.3.(1)). But Article 9.10.8.10. switches Table 9.10.8.1. off for:

• A dwelling unit that has no other dwelling unit above or below it, which covers a typical detached house, semi or townhouse unit
• A dwelling unit that is not above or below another major occupancy, such as a shop
• A house with a secondary suite, where the floor framing is protected on the underside by a continuous smoke-tight barrier of at least 15.9 mm Type X gypsum board

So in a typical single-family house, when you replace a bearing wall or a wood beam with steel, there's no code requirement to fireproof the beam. Many owners still box it in drywall for appearance, which is a finish choice. If you're opening up a wall for a beam, our guide to load-bearing wall removal permits in the GTA covers what the building department will look at.

Houses with a secondary suite

This is where steel beams do need protection. A secondary suite under the 2024 OBC is a self-contained dwelling unit in a building that contains only one other dwelling unit and common spaces, with both units forming a single real estate entity. The classic example is a basement apartment.

For these houses, the code includes Ontario-specific rules:

• Steel supporting the floor between units. Sentence 9.10.8.3.(2) requires light-frame walls, columns, arches and beams, as well as loadbearing steel elements, that support floors between dwelling units in a house with a secondary suite, including their common spaces, to be protected by at least 15.9 mm thick Type X gypsum board.
• The floor itself. Sentence 9.10.9.16.(4) allows the floor-ceiling framing between the units to be protected by a continuous smoke-tight barrier of at least 15.9 mm Type X gypsum board on the underside, instead of a rated fire separation. Sentence 9.10.9.16.(5) allows the rating of that fire separation to be waived where the house is sprinklered.

In practice, a steel beam in the ceiling of a basement apartment gets boxed in 15.9 mm (5/8 in) Type X board, and steel posts holding up that beam get the same treatment. The boxing should tie into the Type X ceiling so the smoke-tight barrier is continuous.

Existing houses and Part 11

Turning an existing house into a two-unit house is a renovation, and Part 11 of the OBC provides compliance alternatives for existing buildings. Table 11.5.1.1.-C includes alternatives for the fire-resistance rules above, for example accepting a 15 min horizontal fire separation in a house with a secondary suite where smoke alarms are installed in every unit and common area and are interconnected, or waiving the rating where the building is sprinklered throughout. Whether an alternative applies to your house, and what it means for the beam, is for the building department to confirm. Existing two-unit houses are also covered by retrofit requirements in the Ontario Fire Code.

When a full fire-resistance rating applies

Once a building moves beyond a single house or a house with a secondary suite, for example three or more units, or homes above or below a store, Table 9.10.8.1. and the fire separation rules apply in full. Then:

• Floors in residential buildings up to 3 storeys generally need a 45 min rating, and residential suites have to be separated from each other by fire separations
• Columns and loadbearing walls below a rated floor need at least the same rating as the floor (Sentence 9.10.8.3.(1))
• Structural steel members in construction required to have a rating must be protected to provide that rating (Article 9.10.7.1.)

The protection has to match a recognised design. MMAH Supplementary Standard SB-2 gives fire performance ratings for generic assemblies, including tables for protected steel columns and individually protected steel beams using concrete, plaster and gypsum board systems, and tested proprietary designs are also used. This is designer territory. For larger buildings and commercial work, see our guide to structural steel fireproofing in Ontario.

Attached garages

An attached garage that serves only the house it's attached to is considered part of that dwelling unit, so no fire separation is required between them (Sentence 9.10.9.18.(3)). The code does require an air barrier system between the garage and the house to keep out gas and exhaust fumes (Sentence 9.10.9.18.(4)).

A steel beam in a garage ceiling under a bedroom therefore doesn't need a fire rating in a single house, but the ceiling has to stay gas-tight around it. Where the beam passes through the ceiling or wall finish, the joints need to be sealed as part of that air barrier.

How steel beams are boxed in drywall

Gypsum board is the usual way to protect or finish a steel beam in a house. The general approach:

1. Framing around the beam. Wood blocking or nailers bolted to the beam, or light steel framing, give the board something to fasten to. Many beams are fabricated with holes in the flanges for exactly this.
2. Type X board on all exposed sides, typically the sides and bottom of a beam that sits below the ceiling line.
3. Continuity with the ceiling. In a two-unit house, the beam boxing and the Type X ceiling should form one continuous smoke-tight barrier, with joints taped and penetrations sealed.
4. Posts treated the same way where they support the beam that needs protection.

Where a specific fire-resistance rating is required, the board type, number of layers, fastening and framing have to follow the rated design your designer specifies, not just a typical drywall detail. Intumescent coatings, which swell when heated to insulate the steel, are another option where the steel is meant to stay visible, but they must be products tested for the rating needed.

If you're planning a beam in a house that has or may later have a second unit, tell your designer early. Beam depth, the boxing and the ceiling height all interact, and it's easier to plan for the drywall than to fit it later. Our guide to the cost to install a steel beam in a basement covers the rest of the job.

Steel beams ready for finishing

We fabricate steel beams and posts for houses with the holes, cap plates and base plates your drawings call for, including holes for wood nailers so the beam can be boxed in, and we supply and install basement beams and posts. Send us your drawings for a quote, or see our steel products.