A structural steel mezzanine is an intermediate floor built inside a building, usually framed with steel columns and beams, decked with steel deck and concrete, grating, checker plate or panels, and reached by a steel stair. In Ontario, a mezzanine is treated as part of the building under the Ontario Building Code: it has to carry the loads of its use, have guards at open edges, provide proper means of egress, and stay within area limits if it is not to be counted as a storey. Most also need a building permit. This guide covers the structure, the stairs and access, and the planning steps for a steel mezzanine in a warehouse, plant, shop or commercial unit.
What a structural steel mezzanine is
The 2024 Ontario Building Code defines a mezzanine as an intermediate floor assembly between the floor and ceiling of any room or storey, including an interior balcony. In practice, steel mezzanines are used for:
- Storage above a warehouse or shop floor
- Offices overlooking a production area
- Equipment platforms for mechanical units, conveyors or processes
- Retail and display floors in commercial units
There are two broad types:
- Structural steel mezzanines, designed by an engineer for the building and the load, with columns, beams, joists and decking fabricated to suit the space. This is what most custom projects need.
- Pre-engineered or modular mezzanine kits, sold in standard bay sizes and load ratings. They can suit simple storage platforms, but they still need to fit the building's code requirements and the floor they sit on.
Storage racking with walkways built into it is a different system again, designed by the racking supplier.
Code points to settle first
Settle these with your designer and the building department before structural design starts, because they decide the size, guards, stairs and fire protection.
Does it count as a storey?
The space above a mezzanine can be left out of the building height calculation if the mezzanine stays within limits. Under Article 3.2.1.1. (with matching rules for Part 9 buildings in Article 9.10.4.1.):
- Mezzanines with an open space above (no partitions or walls higher than 1,070 mm above the mezzanine floor, with a limited exception for a small enclosed space) can total up to 40% of the open area of the room they are in.
- Mezzanines that don't meet those conditions can total up to 10% of the floor area they are located in, and the mezzanine area in a suite can't exceed 10% of that suite.
- A second mezzanine level stacked over the first generally counts as a storey.
If the mezzanine counts as a storey, the building's height changes, and fire separation requirements for floor assemblies apply. Even when it doesn't count, the code's construction articles can require a fire-resistance rating for mezzanines depending on the building's construction type. Your designer works this out.
Egress
Article 3.4.2.2. requires the space above a mezzanine to be served by means of egress on the same basis as a floor area, unless the mezzanine meets set limits. Those limits include an occupant load of not more than 60, and a maximum area and travel distance by occupancy, for example:
| Occupancy of space | Maximum area | Distance limit |
|---|---|---|
| Business and personal services | 200 m² | 25 m |
| Mercantile | 150 m² | 15 m |
| Medium-hazard industrial | 150 m² | 10 m |
| Low-hazard industrial | 200 m² | 15 m |
Source: 2024 Ontario Building Code, Division B, Table 3.4.2.2. Even when a mezzanine qualifies, at least half of the required means of egress must meet the general rule if the mezzanine isn't required to end at a fire separation. In short, a large or busy mezzanine may need a second stair.
Guards
Article 3.3.1.18. requires a guard at least 1,070 mm high at a mezzanine where the drop is more than 600 mm. Openings through the guard must not pass a 100 mm sphere in most buildings, or a 200 mm sphere in buildings of industrial occupancy, unless larger openings are shown not to be a hazard. In workplaces, Ontario's Industrial Establishments regulation (Reg. 851) also requires a guardrail at the open side of a mezzanine, with a mid rail and a toe board where tools or objects could fall on workers below.
Loads
Table 4.1.5.3. of the Building Code sets minimum live loads by use. Interior mezzanines that can't be used as a viewing area by an assembly of people are designed for the load of the occupancy they serve, and storage areas carry a minimum of 4.8 kPa or the actual stored load if that is higher. Tell your engineer exactly what will go on the mezzanine: pallets, shelving, equipment, a forklift-loaded edge. That information sets the beam sizes, decking and column loads.
The structure: columns, beams and decking
A typical steel mezzanine frame includes:
- Columns, usually HSS or W-shapes, on base plates anchored to the floor
- Beams spanning between columns, often W-shapes or channels, with joists or secondary beams between them
- Bracing or moment connections to keep the frame stable sideways
- Decking suited to the use
- Edge members for the guard posts and any load gate or pallet drop area
What the columns sit on
This is the most commonly underestimated part. Each column delivers a concentrated load to the floor. A warehouse slab on grade may not be designed for it, and cutting in a new footing or pad under each column is common. An engineer should check the slab thickness, reinforcement and soil, and design the base plates and anchors. For base plate design, see column base plate design under CSA S16.
Decking options
| Decking | Suits | Notes |
|---|---|---|
| Steel deck with concrete topping | Offices, heavy storage, rolling loads | Heaviest and most solid; acts like a normal floor |
| Bar grating | Equipment platforms, plant areas | Lets light, air and sprinkler water through; small items can fall |
| Checker plate | Platforms where nothing should fall through | Needs close support or stiffeners; see steel checker plate |
| Engineered panels on steel joists | Light storage and offices | Check the panel's load and fire properties |
Sprinklers, lighting and ventilation under a solid deck often need to be extended, which belongs in the planning, not as a surprise after installation.
Stairs, ladders and platform access
Access is a code item, not an add-on:
- Stairs. A mezzanine used by people regularly needs a stair that meets the Building Code's rise, run, width, handrail and guard rules. In Part 3 buildings, stairs have a rise of 125 mm to 180 mm and a run of 280 mm to 355 mm. Stairs serving industrial occupancies other than storage garages may have open risers, which allows grating treads. See our guide to Ontario Building Code rules for steel stairs and, for plant settings, industrial steel stairs and roof access stairs.
- Second stair. Required where the mezzanine is too large, too busy or too far from an exit to meet the egress limits above.
- Ladders. Ladders suit only occasional access to equipment platforms, not occupied mezzanines. In workplaces, Reg. 851 requires permanent platforms with a fixed stair or access ladder where frequent access is needed to elevated equipment. Fixed ladders are covered in fixed steel ladders and cages in Ontario.
- Material handling openings. A forklift loading edge needs a gate or barrier that keeps the guard function when it is closed. Plan it with the frame so the edge beam and posts can carry it.
Planning a steel mezzanine, step by step
- Define the use and load. What goes up there, how much it weighs and how people and goods get up and down.
- Measure the space. Clear height below and above, column grid of the building, obstructions such as sprinklers, lights, ducts and doors.
- Check the code limits with your designer: area, storey count, egress, fire resistance and guards.
- Check the floor. Engineer reviews the slab and soil for column loads.
- Engineer the structure and stairs, and prepare permit drawings.
- Apply for the building permit.
- Shop drawings from the fabricator, reviewed against the engineering.
- Fabrication, delivery and installation, often in a working building, so plan access, sequencing and any shutdowns.
For the factors that set the price of the frame, see steel framed building cost: beams and columns.
Steel mezzanines from Procut
We supply and fabricate structural steel beams, columns, posts, plates and channels, and miscellaneous steel including stairs, railings, ladders and light supports, with CWB certified shop welding and shop drawings prepared in house. Engineering services are available for projects that need them, and we supply and install structural steel and deliver across the GTA and Ontario. Send your layout and intended use, and we will prepare a free take-off and quote.
Send your drawings or layout, or see our services for structural steel fabrication and installation.
Frequently asked questions
Do I need a permit for a steel mezzanine in Ontario?
In most cases, yes. Adding a mezzanine is construction that adds floor area and changes how the building is used, so it normally needs a building permit with drawings showing the structure, guards, stairs and egress. Check with your municipality's building department before ordering.
Does a mezzanine count as a storey?
Not always. Under the 2024 Ontario Building Code, the space above a mezzanine can be left out of the building height calculation if the mezzanine stays within set area limits, such as 40% of the open area of the room where the space above stays open, or 10% of the floor area otherwise. A mezzanine that doesn't meet the conditions counts as a storey, which can trigger fire separation and other requirements.
How high must a mezzanine guard be?
Under Part 3 of the Ontario Building Code, a guard at least 1,070 mm high is required at a mezzanine where the drop is more than 600 mm. Openings must not pass a 100 mm sphere, or a 200 mm sphere in buildings of industrial occupancy, unless larger openings are shown not to be a hazard.
Can a steel mezzanine sit on my existing warehouse slab?
Sometimes. Mezzanine columns put concentrated loads on the floor, and many slabs on grade can't take them without new footings or pads under each column. An engineer should check the slab and soil before the design is finalized.
