Steel Building Mezzanines: Planning the Second Level

A steel building mezzanine puts unused height to work. Warehouses, truck shops and manufacturing plants often have six metres or more of clear space above the floor, and a mezzanine turns part of it into offices, storage or a parts room without making the footprint bigger. Done well, it is one of the cheaper ways to add usable area. Done late, it can mean heavier columns, new footings and a second trip to the building department. Here is how to plan one properly.

What a mezzanine is used for

A mezzanine is an intermediate floor that sits within a storey, usually open to the space below on at least one side. In commercial and industrial steel buildings, the common uses are:

• Offices and meeting rooms above a shop or warehouse floor, often with windows looking over operations.
• Parts and light storage in truck and equipment service shops, which frees floor space for bays.
• Staff areas such as lunchrooms, locker rooms and washrooms.
• Mechanical and electrical platforms for air handlers, compressors or panels kept off the production floor.
• Inventory and pick areas in distribution buildings, sometimes served by a lift or conveyor.

The use matters because it sets the floor load, the exits and the fire requirements. Decide it first, and be honest about how the space might change. An office mezzanine that later fills with stacked parts boxes is carrying loads it may not have been designed for.

Floor loads: the number that drives the design

Every mezzanine is engineered for a live load, the weight of people, furniture, stored goods and equipment. The building code sets minimum values by use. Office floors sit at the lighter end, while storage and light manufacturing floors are designed for considerably more. Heavy items such as racking, safes, filing systems or machinery add concentrated loads that need their own check.

Give your designer a realistic list: what will sit on the floor, how high material will be stacked, and whether a forklift, pallet jack or scissor lift will ever run on it. The engineer turns that into a design load, and that number sizes the deck, the beams, the columns and the footings. Your engineer and municipality confirm the values that apply to your building.

Framing options for a steel building mezzanine

There are three common ways to build one inside a pre-engineered steel building.

Integrated with the main frame

The mezzanine beams connect to the building columns, and interior columns carry the rest. This is efficient when the mezzanine is planned with the building, because the frame is designed for the extra load from the start.

Freestanding structure

The mezzanine has its own columns and bracing and does not lean on the building frame. This suits additions to an existing building, or cases where the owner wants the option to move or remove it later.

Floor deck choices

Steel deck with a concrete topping is the usual choice for offices, washrooms and heavier storage, since it is stiff, quiet underfoot and fire resistant. Steel grating or plate suits equipment platforms. Wood-based decking is lighter and cheaper but has limits in some uses and fire situations, so check it with your designer before assuming it is allowed.

Either way, the columns under the mezzanine need footings sized for their loads. A slab poured for a warehouse floor usually can't take a mezzanine column without a pad beneath it. Our guide to steel building foundations covers how those footings are designed.

Clear height, columns and layout

A mezzanine needs headroom both above and below it. Work out the clear height you need underneath, for people, racking or vehicles, then add the depth of the floor structure, then the headroom above to the underside of the roof steel, lights and sprinklers. That stack often decides the eave height of a new building, which is why a mezzanine belongs in the first drawings.

Layout points that are easy to miss:

• Column placement. Columns under the mezzanine should fall where they won't block bays, doors or forklift aisles.
• Cranes. A mezzanine under a crane runway cuts into hook coverage. See our guide to overhead cranes in steel buildings if you plan both.
• Getting material up. A pallet gate, loading edge or freight lift saves carrying stock up the stairs.
• Services. Plan plumbing, ductwork and electrical runs to and from the mezzanine before the floor is framed.

Sketching the building in our 3D building designer is a quick way to test eave height and bay layout before engineering starts.

Stairs, guards and exits

Stairs, guards and handrails on a mezzanine are regulated items, not finishing details. The code sets requirements for stair width, rise and run, landings, guard height and handrails, and the number of exits depends on the mezzanine's size, use and occupant load. A larger mezzanine may need a second stair. Open edges need guards, and storage mezzanines often add kick plates and safety gates at loading points. Ladders are generally only acceptable for limited-access equipment platforms, not for spaces people work in.

Code, permits and fire protection

Under the Ontario Building Code and the National Building Code, a mezzanine is treated as part of the storey it sits in only within limits. Above a certain share of the floor area below, or if it is enclosed, it can count as an additional storey. That can change construction requirements, fire separations, sprinkler needs and exits for the whole building. The exact thresholds depend on the code edition and building details, so your designer and the municipality confirm how your mezzanine is classified.

A new mezzanine, or one added to an existing building, normally needs a building permit with stamped structural drawings. Our articles on steel building permits in Ontario and Canadian building codes explain how that process works. If the building is sprinklered, coverage usually has to extend under the mezzanine, and enclosed office space may need extra heads, so involve the fire protection designer early.

Adding a mezzanine to an existing building

Many owners want a mezzanine years after the building went up. It is often possible, but start with an engineering review. The existing columns, frames and slab were designed for the original loads, so a freestanding mezzanine on new footings is usually the simplest route. Expect saw-cutting the slab for new pads, a permit, and possibly sprinkler and electrical changes. If you are building new and think you may want a mezzanine later, ask for the frame and footings to be designed for it now. That costs far less than a retrofit.

Mezzanine cost depends on area, floor load, deck type, stairs and finishes. Treat any early figure as a general planning range until the structure is engineered.

Plan your mezzanine with one team

Meta Steel Buildings designs mezzanines as part of industrial and commercial steel buildings across Ontario and the rest of Canada. One team handles consulting, 3D design, Canadian-made steel fabrication, foundations, permits and erection, so the mezzanine, frame and footings are engineered together. We're PEO registered and WSIB covered. Tell us the use, size and clear heights you need, then request a quote or call (888) 230-3363.

Frequently Asked Questions

Can you add a mezzanine to an existing steel building?

Often, yes. An engineer first reviews the existing frames, columns and slab. Because the original building usually was not designed for the extra load, a freestanding mezzanine with its own columns and new footings is the most common solution. A building permit is normally required, and sprinkler and electrical changes may be needed.

Does a mezzanine count as a storey under the building code?

It depends on its size and whether it is open or enclosed. A mezzanine is treated as part of the storey it sits in only within limits set by the code. Larger or enclosed mezzanines can count as an additional storey, which affects fire separations, sprinklers and exits. Your designer and municipality confirm the classification.

How much weight can a steel mezzanine hold?

As much as it is engineered for. The design load comes from the intended use, with office floors designed for less than storage or light manufacturing floors, plus checks for heavy items such as racking or equipment. Give your designer a realistic list of what will be on the floor so the deck, beams, columns and footings are sized correctly.