Egress stairs: what the code actually requires.
An egress stair isn't a design choice — it's the protected path people use to get out of a building, and the Ontario Building Code sets its width, its geometry, and where it's allowed to go. Here's what governs the dimensions, and what changes when the stair lives outside the building.
Short answer: an egress stair is a required exit stair, so its width comes from the occupant load it serves (commonly 8.0 mm per person) measured against a 1100 mm minimum, with 125–180 mm rise, 280 mm minimum run, 2050 mm headroom, and no more than 3.7 m of vertical rise between landings. It has to be a conventional stair — a ship's ladder or alternating-tread stair cannot serve egress.
What makes a stair an "egress" stair
Plenty of industrial stairs are just access — you climb them to reach a platform, a machine, or a roof. An egress stair is different: it's counted in the building's means of egress, which means occupants are expected to use it to evacuate. That single distinction is what pulls in the width calculation, the protected-shaft requirements, the fire separation, and the geometry limits below. If the architect's exiting plan counts your stair, it's an egress stair, and nothing about it is optional.
The practical consequence buyers hit most often: you cannot solve a tight footprint with a steep-pitch device. A ship's ladder is a limited-access alternative stair, not an exit. If the space genuinely can't take a straight run, the answer is a switchback or an exterior stair — not a steeper pitch.
Egress stair dimensions
These are the figures that come up on nearly every egress stair drawing for a Part 3 building. Treat them as the typical case and confirm each against the Part and edition governing your project:
| Dimension | Typical requirement | What drives it |
|---|---|---|
| Clear width | 1100 mm min. | Occupant load served |
| Rise | 125 – 180 mm | Code band; uniform throughout |
| Run (tread) | 280 mm min. | Code band; uniform throughout |
| Headroom | 2050 mm min. | Structure above the flight |
| Vertical rise per flight | 3.7 m max. | Forces intermediate landings |
| Landing depth | ≥ stair width | Door swing may increase it |
| Guards / handrails | 1070 mm / 865–1070 mm | Industrial occupancy |
The two that catch people out are the last two rows. Landing depth is measured in the direction of travel and a door swinging onto the landing can eat it — which is why an egress stair footprint is always larger than the flights alone. And the 3.7 m rise limit is what turns a two-storey run into a switchback: you can't climb a full industrial storey in one uninterrupted flight.
Width comes from occupant load, not from the opening
The single most common egress mistake on a retrofit is sizing the stair to the hole in the floor. It works the other way around. The architect establishes the occupant load for the floor area, the exiting plan assigns a share of that load to each exit, and the stair width is load × the per-person factor for stairs — commonly 8.0 mm per person — compared against the minimum, with the larger of the two governing.
- Occupant load 100800 mm calc → 1100 mm governs
- Occupant load 1501200 mm calc governs
- Occupant load 2502000 mm calc governs
- Measured asclear width between obstructions
- Handrail projectioncounts against clear width
That last line matters for fabrication. "1100 mm wide" on an architectural plan and "1100 mm clear" on a steel shop drawing are not the same stair once handrails and stringer flanges are in. We build to the clear dimension and show the overall separately, so the reviewer sees both.
Exterior exit stairs
When there's no room to build a protected shaft inside an existing building, an exterior exit stair is often the cleanest path to compliance — the same reason it's the standard answer on warehouse and plant additions. It's permitted, but three things change:
- Spatial separation. The stair has to be positioned relative to unprotected openings in the wall it passes, so that a fire venting from a window doesn't take out the exit.
- Ice and snow. Open treads that shed rather than collect are the point of a grip-strut tread on an outdoor stair. A solid plate tread ices over.
- Finish. Hot-dip galvanizing is the default outdoors — a painted stair in road-salt and freeze-thaw exposure becomes a maintenance item within a few winters. See galvanized vs painted for the comparison.
An exterior egress stair is not the same thing as a fire escape in the legacy sense — modern exterior exit stairs are engineered to the same structural and geometric standard as an interior one, just built for weather. More on how we detail them on the outdoor steel stairs page.
Retrofits: check the edition that applies
On an existing building, the geometry that was legal when the stair was built may not match what's required for the work you're doing now. Egress width, riser and run bands, and guard heights all shifted between editions — we covered the specific deltas in OBC 2024 stair changes. The consolidated regulation is published on ontario.ca/laws, and the referenced structural load provisions come from the National Building Code of Canada. We cite the specific clause on every stamped drawing rather than asserting compliance in general terms, so a plans examiner can verify it in one pass. Full breakdown on the OBC stair code guide.
What we need to quote an egress stair
Faster than a back-and-forth: floor-to-floor dimension, the occupant load the stair serves (or the architect's exiting plan), interior or exterior, the landing conditions top and bottom, and whether the project is permit-bound. That's enough to size the flights, confirm whether a switchback is forced by the 3.7 m rule, and price the steel. The site checklist covers the rest, and egress staircases shows how we build them.