01 · Introduction
Why egress windows are different from every other window.
Most window rules in the Ontario Building Code are about light, energy, or fire. Egress is about life safety. When a fire blocks the usual way out of a house, the people on that floor need a second way — and the Code says that second way has to be reachable, openable, and large enough to climb through, without any tools, in the dark, under stress.
Article 9.9.10.1 sets out exactly what counts as an emergency escape opening. The numbers are specific, the operating requirements are strict, and the rules shift when the bedroom is in a basement. This article walks through each requirement, explains the reasoning behind it, and flags the details that cost time on permit review — especially for basement conversions and secondary suites.
This is also one of the most important rules to get right on paper, because an inspector can measure every dimension of an egress window on site. If the clear opening falls short by even a few millimetres, the window is replaced. That's an expensive mistake at the wrong end of a project.
What's in this guide
02 · Key Terms
The vocabulary the Code uses for escape.
Before measuring anything, it helps to be clear on what the Code actually means by each term. A few of these — especially "clear opening" and "openable portion" — are where most measurement errors start.
Egress window
An exterior window sized and operated to let a person escape a bedroom floor during an emergency. Governed by OBC 9.9.10.1.
Clear opening
The unobstructed, openable area through which a person can actually pass. Sashes, hardware, and the frame thickness are all deducted.
Openable portion
On a sliding window, the movable half. The 380 mm dimension and 0.35 m² rule apply to this portion only — not the whole unit.
Sill height
The vertical distance from the finished floor inside the room up to the bottom of the clear opening. Capped at 1000 mm except in basements.
Window well
The excavated space outside a basement window that lets the window open and provides room to climb out. Must give 550 mm of clearance in front.
Suite
A single dwelling unit — a house, a secondary suite, or an apartment. Each suite needs its own compliant egress path per floor level containing a bedroom.
03 · The Rules
OBC 9.9.10.1 — what every egress window must do.
The Code distils emergency escape into three numerical limits and three operating requirements. All six must be met, every time, with no substitutions or trade-offs.
| Requirement | What the Code says |
|---|---|
| Clear opening area | Minimum 0.35 m² (≈ 3.77 ft²) of unobstructed open area |
| Minimum dimension | No single dimension less than 380 mm (15″) — width or height |
| Sill height (non-basement) | Maximum 1000 mm (≈ 39″) above the finished floor |
| Operable | Openable from the inside without tools or special knowledge |
| Self-supporting | Must maintain the required opening without any additional support |
| Window-well clearance | At least 550 mm (≈ 22″) of clearance in front of the window, if the window opens into a well |
Summary of OBC 2024 Article 9.9.10.1. Refer to the Code for authoritative text.
Note — one per floor level, not one per bedroom
The Code requires at least one compliant egress opening per floor level containing a bedroom — not one per bedroom. A door to the exterior on the same floor satisfies the requirement for every bedroom on that floor. That's why a second-floor addition may not need egress windows if there's an interior stair to a main-floor exterior door.
04 · The clear opening
Why 0.35 m² is not a window size.
The 0.35 m² minimum refers to the clear opening — the hole a person fits through — not the overall size of the window unit, and not even the glass area. A window advertised as 36″ × 24″ does not give you 0.56 m² of clear opening. The frame, the sash, and whatever part of the window stays shut when it's open all come off that number.
The Code also sets a minimum dimension of 380 mm. That means neither the width nor the height of the opening can be less than 380 mm, even if the total area clears 0.35 m². Both conditions apply — area and dimension.
Fig. 1 — For sliding or double-hung windows, only the openable half counts toward the 0.35 m² minimum. A unit that would pass as a casement may fail as a slider of the same nominal size.
Getting to 0.35 m² — the practical math
A few configurations that clear the bar, and why:
- Casement, ~24″ × 36″ sash — roughly 0.56 m² clear opening with the sash swung fully open. Comfortable margin.
- Slider, ~48″ × 30″ overall — openable portion is half the width, so roughly 24″ × 30″ or 0.46 m². Passes, but barely.
- Slider, ~36″ × 24″ overall — openable portion is only ≈ 0.28 m². Fails on area.
- Awning (bottom-hinged) — the sash only swings open partially, and the opening is obstructed by the sash itself. Rarely meets egress on its own.
Common mistake
Specifying "a 0.35 m² window" on the drawings. That's not a valid specification — an entire 0.35 m² window unit will have a clear opening closer to 0.18 m². Always call out the openable clear opening explicitly, and ask the supplier for the clear-opening dimensions, not the rough-opening dimensions.
05 · Sill height & window wells
What changes when the bedroom is in the basement.
For bedrooms above grade, the egress window sill can be no more than 1000 mm above the finished floor — so a reasonably agile adult can reach it. The basement is the one major exception: the Code waives the 1000 mm sill limit there, because raising a basement window that high would put the opening entirely below grade.
Basements come with their own complication: the window usually opens into a window well, and the Code regulates that too. Any window well serving an egress window must provide at least 550 mm of clearance in front of the window — enough space to open the sash and climb out.
Fig. 2 — A compliant basement egress arrangement: 550 mm minimum clearance in the well, sill height unrestricted, and any protective well cover must be releasable from below without tools.
Basement checklist
A compliant basement egress window needs: (1) the standard 0.35 m² clear opening with 380 mm minimum dimension, (2) openable without tools from inside, (3) a window well with ≥ 550 mm clearance in front, (4) any protective cover over the well must be openable from below without tools, and (5) if the sash swings into the well, the well must be wide enough that the swing doesn't block the escape path.
Fall protection on higher egress windows
A wrinkle for above-grade bedrooms: if the egress window sits with its sill less than 900 mm above the floor and the window opens onto a drop greater than 1800 mm to exterior grade, OBC 9.8.8.1 also requires fall protection. The two requirements look like they conflict — the window has to be easy to open for escape, but hard to open for child safety — but the current OBC (per OBOA guidance) accepts Window Opening Control Devices compliant with ASTM F2090 as a solution that does both: a child can't push the window open, but an adult can release the mechanism in one motion.
06 · Egress vs. natural light
Two rules, one window.
Natural light (OBC 9.7.2.3) and egress (OBC 9.9.10.1) both regulate bedroom windows — but they measure completely different things. One counts glass; the other counts hole. A window can comfortably satisfy one rule and fail the other.
| Natural light (9.7.2.3) | Egress (9.9.10.1) | |
|---|---|---|
| What is measured | Transparent glass area | Unobstructed openable area |
| Minimum value | 5% of bedroom floor area | 0.35 m² with no dimension < 380 mm |
| Applies to | Every bedroom & principal room | One per floor level with a bedroom |
| Frame counted? | No — glass only | No — clear opening only |
| Fixed windows OK? | Yes — they still let light in | No — window must open |
The two rules compared side by side. A fixed picture window can satisfy the 5% daylight rule but fails egress entirely.
A short bedroom sanity check
For a typical 10 m² basement bedroom, you need 0.5 m² of glass (5% rule) and 0.35 m² of clear opening (egress). A single 600 × 900 mm casement window provides roughly 0.4 m² of glass and 0.43 m² of clear opening when fully swung — compliant on both counts with a modest buffer. A 1200 × 600 mm horizontal slider gives the same glass but only ≈ 0.28 m² clear opening (half the sash). Passes daylight, fails egress.
07 · A worked example
Designing egress for a basement secondary suite.
A common Ontario project: converting a basement into a legal secondary suite with one bedroom. The bedroom is 3.2 × 3.5 m (≈ 11.2 m² floor area). The suite has a separate entry door at the side of the house at grade — but it's at the other end of the basement from the bedroom, and does not count as being on the same floor as the bedroom for egress purposes because the path passes through the suite's main living space.
Step-by-step
1. Does the bedroom need an egress window? Yes. The only exterior door on the floor is at the far end of the suite, separated from the bedroom by other rooms. An egress window in the bedroom is the safer and simpler compliance path.
2. Natural light minimum: 11.2 m² × 5% = 0.56 m² of glass required.
3. Egress minimum: 0.35 m² clear opening, no dimension < 380 mm.
4. Pick the window: A 760 × 1200 mm casement gives ≈ 0.69 m² of glass and ≈ 0.78 m² clear opening. Comfortable margin on both.
5. Size the well: Excavate the well to give at least 550 mm clear in front of the swung-open sash. If the sash opens into the well, widen the well so the 550 mm clearance is still available beside the open sash.
6. Sill height: Basement — no maximum sill height applies, but locate the window so the clear opening is reachable from inside (a step or sill ledge below the window is a common detail).
Common pitfalls
Specifying a slider of the nominal size used for the casement. Sliders deliver roughly half their nominal area as clear opening. A 1200 × 600 mm slider that looks equivalent to the casement above actually fails egress.
Forgetting the minimum dimension. A long, shallow window that gives 0.35 m² in raw area may still fail if either the width or the height of the clear opening is under 380 mm. Both conditions apply.
A window well too narrow for the swing. If the sash opens into the well and the well is 600 mm deep, the clear space in front of the open sash may drop below 550 mm. Either widen the well or choose a casement that opens away from the well.
Missing the exterior-door exemption. If the bedroom's floor level already has a door to the outside (for example, a main-floor bedroom with a patio door in the living room), no egress window is required on that floor. Don't over-specify.
Landscaping over the well. A well cover is allowed, but it must open from inside the well without tools. A heavy garden planter sitting on top counts as an obstruction and will fail inspection.
Forgetting the fall-protection question. On a second-storey egress window with a low sill, confirm whether OBC 9.8.8.1 fall protection applies, and specify an ASTM F2090 WOCD on your window schedule if it does.
08 · Disclaimer & references
Using this guide responsibly.
IMPORTANT NOTICE
This article is provided for informational and preliminary design purposes only. It is not a substitute for the Ontario Building Code itself, for the judgement of a qualified designer or engineer, or for the approval of the Authority Having Jurisdiction.
The requirements summarized here are based on the 2024 edition of the Ontario Building Code, which became mandatory for all permit applications in Ontario on April 1, 2025. The Code is subject to amendment — always verify the current in-force text before relying on any figure for permit submission. Local municipalities may impose additional or more restrictive requirements.
Fall-protection and Window Opening Control Device (WOCD) questions are an evolving area of Code interpretation. The guidance summarized here reflects OBOA Building Code Advisory Services commentary at time of publication. For specific projects, confirm the acceptance of ASTM F2090 WOCDs with your local building official.
This guide focuses on single-family houses and suites falling under Part 9 of the OBC. Part 3 buildings (larger multi-unit residential, hotels, institutional uses) have separate and more complex egress requirements that this article does not cover.
AD Polyline makes no warranty, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of this content. Use at your own discretion and consult your designer for project-specific advice.
References. Ontario Building Code (O. Reg. 163/24), Division B, Part 9, Article 9.9.10.1 — Egress Windows or Doors for Bedrooms; Article 9.7.2.3 — Natural Light; Article 9.8.8.1 — Protection Against Falling; OBOA Building Code Advisory Services guidance on ASTM F2090 Window Opening Control Devices.
Planning a basement suite or addition?
AD Polyline handles design, permits, and construction-ready documentation for residential projects across Ontario — including the secondary-suite conversions where egress usually decides whether a project works at all. If you're stuck on a basement bedroom that won't hit code, we can help.
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