The Engineering Behind Cold-Formed Steel Framing on a Toronto Multiplex

A stamped shop drawing set for a cold-formed steel (CFS) multiplex lands on a developer's desk with stud schedules, connection details, and a Professional Engineer's seal on every sheet. Four decisions sit inside that set: gauge and mil thickness by wall type, screw versus welded connections, thermal bridging detailing, and galvanization rating. A developer has to be able to read those decisions before signing off, not just trust them. This article picks up where the Light Steel Framing for Toronto Multiplexes: The Complete Guide leaves off: steel has already been chosen, and now the drawing set needs a technical read.

How Do You Choose Gauge and Mil Thickness for Each Wall Type?

Cold-formed steel studs are specified two ways on the same sheet: mil thickness, which is the actual base metal thickness, and the older gauge designation, which most trades still call out on site. The two scales map to different structural roles because they are typically rolled from different steel grades.

Mil thickness	Gauge	Typical steel grade	Typical application
33 mil	20 ga	33 ksi	Non-load-bearing interior partitions
43 mil	18 ga	33 ksi	Non-load-bearing interior partitions
54 mil	16 ga	50 ksi	Load-bearing and exterior walls
68 mil	14 ga	50 ksi	Load-bearing and exterior walls
97 mil	12 ga	50 ksi	Load-bearing and exterior walls, higher loads

The practical split: 33 mil and 43 mil studs, in lower-strength 33 ksi steel, are sized for partitions that only carry their own weight. Anything carrying floor or roof load, or sitting on an exterior wall line, steps up to 54 mil and heavier in 50 ksi steel. On a shop drawing set, that means every wall tagged load-bearing should show a mil callout at 54 or above; if a load-bearing line is drawn at 43 mil, that is a flag to raise with the engineer of record, not something to wave through.

Screws or Welds: Why Do Most Toronto Crews Default to Screws?

CFS connections get made one of two ways: self-drilling screws through the stud flange, or field welds. Screw-fastened connections dominate on Toronto multiplex jobs, and the reason is largely regulatory rather than structural. Field welding structural steel in Ontario requires the contractor to hold company certification to CSA W47.1, Certification of Companies for Fusion Welding of Steel, administered through the Canadian Welding Bureau. Under that certification, individual welders are tested for the specific welding processes and positions they use in production before that work is accepted.

Screws carry no equivalent certification burden. A crew with standard framing tools and CFS experience can screw a connection to spec without a separate welding certification on file, which is why screw-fastened connections are the default on most Toronto-area jobs and welds get reserved for specific structural details the engineer calls out by name.

Where Does Thermal Bridging Come From, and What Actually Fixes It?

Steel conducts heat far more readily than the wood or insulation around it. In a standard steel-stud wall, heat does not stay put in the cavity insulation; it rides through the stud webs and flanges instead, a phenomenon called thermal bridging. A wall that looks fully insulated on paper can underperform its nominal R-value in practice because the steel itself is bypassing the insulation layer.

Two fixes are established in Canadian sheet-steel design guidance:

• Thermal-break clips. These isolate the exterior cladding or girts from the structural steel framing, breaking the direct metal-to-metal path that heat would otherwise ride along.
• Continuous exterior insulation. A layer of rigid insulation run uninterrupted across the exterior face of the studs means the insulation layer itself is never broken by a steel member, regardless of what happens inside the cavity.

A drawing set that shows cavity insulation alone, with no clip detail and no continuous exterior layer, has not actually solved thermal bridging; it has just insulated around it.

What Galvanization Rating Does a Toronto Climate Actually Need?

CFS framing is manufactured to CAN/CSA S136, ASTM A653/A653M, and CSSBI 61-18, and the coating spec on the mill certificate matters as much as the mil thickness.

Coating designation	Metric equivalent	Zinc coating	Typical use
G40 or equivalent	lighter coating	least zinc of the three	Interior dry areas
G60	Z180	approximately 180 g/m² (about 0.60 oz/ft²), both sides	Interior wet areas
G90	Z275	approximately 275 g/m² (about 0.90 oz/ft²), both sides; roughly 50% more zinc than G60	Exterior walls, higher-exposure and higher-moisture conditions

For Toronto's freeze-thaw climate, the coating that matters most is on the exterior envelope: G90 is generally the rating called up there, because the extra zinc buys more corrosion margin against repeated wet-dry and freeze-thaw cycling than G60 provides. G60 is appropriate for interior wet areas like bathrooms and mechanical rooms; lighter coatings are fine once the stud is fully inside the conditioned, dry envelope.

When Is a Stamped Engineer's Drawing Set Mandatory?

Part 9 of the Ontario Building Code, the prescriptive path most low-rise wood-frame housing is permitted under, contains no span or sizing tables for cold-formed steel. That means a steel-framed multiplex has no prescriptive route through the code the way a comparable wood-framed house sometimes does. The structural design has to go through Part 4, Structural Design, as a fully engineered design conforming to CAN/CSA-S136, the North American Specification for the Design of Cold-Formed Steel Structural Members.

Professional Engineers Ontario's own guideline is direct on this point: a Professional Engineer's sealed structural design is required whenever the work falls outside the code's prescriptive provisions, and CFS structural framing is exactly that case. So a stamped drawing set is not a value-add on a CFS multiplex; it is the only route to permit.

One detail worth checking on the seal itself: PEO requires the engineer's signature and the date the document was sealed to always be included on structural drawings. Initials alone are not an acceptable substitute for a full signature, and a sheet missing either element is not a compliant seal, regardless of what stamp appears on it.

What Does a Toronto Building Inspector Check at Framing Stage?

Toronto Building calls the structural framing inspection once framing is complete, including the rough-in of plumbing and HVAC. In the mandatory inspection sequence for small buildings and houses under Part 9, it falls after the foundation inspection and before the next-stage inspection covering insulation and fire separation.

The standard the inspector applies is simple and unforgiving: the inspector must be able to see the entire building element being inspected. Any framing, connections, or blocking that gets covered by drywall or sheathing before it has passed inspection has to be uncovered again, at the builder's expense, so it can actually be checked. On a CFS job that means clip details, blocking, and connection points all need to stay exposed until the framing inspection has signed off, not just until the crew feels done with that wall.

Why Not Every GTA Framing Crew Can Run a CFS Job

CFS framing calls for a level of tolerance and connection discipline that differs from wood framing, and trade availability in the GTA does not make finding that experience automatic. BuildForce Canada's Construction and Maintenance Looking Forward report for Ontario, covering 2025 to 2034, projects GTA construction employment rising roughly 14% in the residential sector and roughly 10% in the non-residential sector over that period. That's against a provincial market that already needs to hire an estimated 154,100 workers by 2034, just to replace an aging workforce and meet rising demand. Framing and structural trades sit inside that same tightening labour pool, which is part of why a developer should confirm a crew's specific CFS experience rather than assume any framing crew can pick up a steel job at the same pace as a wood one.

A Checklist Before the Shop Drawing Set Goes to Permit

Before a CFS shop drawing set moves to permit application, a developer can run it against a short list:

• Every load-bearing wall line is called out at 54 mil (16 ga) or heavier, in 50 ksi steel.
• Non-load-bearing partitions are not carrying a mil thickness or connection detail meant for load-bearing walls.
• Connection type is specified per detail: screwed by default, with any field welds tied to a contractor holding current CSA W47.1 certification.
• Thermal bridging is addressed with an actual clip detail or continuous exterior insulation layer, not cavity insulation alone.
• Galvanization is specified by wall condition: G90 for exterior and higher-exposure walls, G60 for interior wet areas, lighter coatings for interior dry areas.
• Every sheet is engineered under Part 4 and CSA S136, with the Professional Engineer's full signature and seal date present, not initials.
• The framing crew named on the job has documented CFS experience, not just general framing experience.

A set that clears all seven lines is one a developer can sign with confidence that the framing inspection, and the building behind it, will hold up to what the drawings promise.