01 · Introduction
Why the Code limits how much glass you can put on a wall.
Windows let light in — and, in a fire, they let flame out. The Ontario Building Code caps the area of glass on any given exterior wall based on how close that wall sits to the property line and how big the wall is. This article explains the three variables behind Table 9.10.15.4 and shows how to use them on a real house.
The technical term is spatial separation. If your neighbour's house catches fire, the Code wants to slow any radiant heat or flame from jumping across the property line. The closer your wall sits to the line, and the larger that wall is, the smaller the permitted area of glass. Close walls with lots of glass have always been the weak link in fire spread between houses, so the Code tightens down hard as the distance shrinks.
AD Polyline's OBC Glazing Limit Calculator reads off Table 9.10.15.4 instantly and flags whether a proposed window layout clears the limit. This article explains the three inputs the tool asks for — and what they mean in the Code.
What's in this guide
02 · Key Terms
The three inputs, defined.
Table 9.10.15.4 of the Code takes two inputs and gives you a maximum permissible glazing percentage. The calculator adds a third optional input so you can test your proposed design against the limit. Getting these definitions right is what separates a clean permit submission from a review comment.
Limiting Distance (LD)
The horizontal distance from an exposing building face to the property line, the centre of a street or lane, or an imaginary line between buildings on the same property. Measured at right angles to the face.
Exposing Building Face (EBF)
The exterior wall area facing one direction on any side of a house — measured from finished grade to the underside of the uppermost ceiling. Gables count.
Unprotected Opening
Any doorway, window, or opening that is not protected by a closure with a required fire-protection rating. Ordinary windows are unprotected openings.
Glazed Opening
The transparent portion of any window or door — the glass itself. Table 9.10.15.4 specifically governs glazed openings in exterior walls of houses.
Closure
A physical element (shutter, fire-rated window, fire door) that seals an opening. Required for walls with LD < 1.2 m except where specifically waived.
Property line
The legal boundary of a lot. Under certain registered agreements, limiting distance may be measured to a point on adjoining land rather than to the property line itself.
03 · Limiting Distance
How to measure it — and where to measure from.
Limiting distance is measured horizontally and at right angles to the exposing building face. It is not a straight line to the nearest corner of another building; it's the perpendicular distance from your wall to the reference line.
Fig. 1 — Limiting distance is the perpendicular from the exposing face to the property line, not a diagonal to the nearest point.
What the line is measured to
The reference line depends on the site condition:
- Adjacent property — measure to the property line.
- Street, lane, or public way — measure to the centre line of that public way, not to your curb.
- Two houses on one lot — measure to the imaginary line midway between the two exposing faces (or as otherwise defined in 9.10.15).
- Irregular or skewed wall — measure from a vertical plane located so that no part of the actual wall lies between that plane and the reference line (OBC 9.10.15.2(2)).
Why measurement matters
Going from LD = 1.19 m to LD = 1.20 m moves you from a wall that needs fire-rated closures to one that doesn't. The table is genuinely stepwise — small survey corrections can change what's allowed.
04 · Exposing Building Face
Calculating the wall area the Code cares about.
The exposing building face is simply the gross area of the wall facing one direction — width × height — measured from finished ground level to the uppermost ceiling. On a two-storey house with a gable, the gable wall includes the triangle up to the roof line.
Fig. 2 — EBF is the full gross wall area on one side of the house, from finished grade up through any gable.
Splitting the wall into portions
The Code also lets you calculate glazing limits for individual portions of an exterior wall (OBC 9.10.15.2(1)(b)(iii)). This is useful when different parts of a wall sit at different distances from the property line — for example, a recessed entry or an angled bay. Each portion is evaluated against its own limiting distance, but the total EBF still sets the row in Table 9.10.15.4.
05 · Reading the Table
How Table 9.10.15.4 gives you the answer.
Once you have the two numbers — limiting distance (metres) and EBF area (square metres) — you look them up on a grid. The row is your EBF bracket; the column is your LD bracket. The intersection is the maximum permissible glazing as a percentage of the EBF.
| EBF (m²) ↓ LD (m) → | < 1.2 | 1.2 | 1.5 | 2.0 | 2.5 | 3.0 | 4.0 | 6.0 | ≥ 8 |
|---|---|---|---|---|---|---|---|---|---|
| 10 | 0 | 8 | 12 | 21 | 33 | 55 | 96 | 100 | 100 |
| 15 | 0 | 8 | 10 | 17 | 25 | 37 | 67 | 100 | 100 |
| 20 | 0 | 8 | 10 | 15 | 21 | 30 | 53 | 100 | 100 |
| 25 | 0 | 8 | 9 | 13 | 19 | 26 | 45 | 100 | 100 |
| 30 | 0 | 7 | 9 | 12 | 17 | 23 | 39 | 88 | 100 |
| 40 | 0 | 7 | 8 | 11 | 15 | 20 | 32 | 69 | 100 |
| 50 | 0 | 7 | 8 | 10 | 14 | 18 | 28 | 57 | 100 |
| 100 | 0 | 7 | 8 | 9 | 11 | 13 | 18 | 34 | 56 |
| Over 100 | 0 | 7 | 7 | 8 | 9 | 10 | 12 | 19 | 28 |
Abbreviated excerpt of OBC Table 9.10.15.4 — maximum % of glazed openings permitted. The full table extends to LD = 25 m. Always consult the current Code for the authoritative values.
How to read the table
Find the row with the EBF area closest to yours without going over. Find the column with the LD closest to yours without going over. The cell value is the maximum permissible glazing as a % of your EBF. Convert to m² by multiplying that % by your EBF area.
Three rules worth memorizing
LD < 1.2 m — no glazing allowed. Column 1 of the table is all zeros. Any opening in this wall must either be eliminated or protected by a fire-rated closure (OBC 9.10.15.4(4)).
LD ≥ 1.2 m — the "LD squared" shortcut. For a single portion of wall, the Code permits glazing area up to (limiting distance)² in square metres — useful as a sanity check. A wall 3 m from the line can have up to 9 m² of glazing regardless of the wall's total area (subject to the table).
Small openings are exempt. A single opening no larger than 130 cm² (roughly 20 in²) is not considered an unprotected opening under OBC 9.10.15.4(5) — covering things like dryer vents and small bathroom ventilators.
06 · When to use the tool
Where the calculator saves you time.
Tight lots
Checking whether a side-yard window on a narrow urban lot clears the limit before detailing the elevation.
Additions
Confirming that new bumpouts toward a side lot line won't trigger new glazing restrictions on existing walls.
Garage setbacks
Sizing garage windows when a detached garage sits close to a lot line (different rules apply to accessory buildings).
Skewed walls
Running the quick calculation for a wall that isn't parallel to the property line, using the vertical plane method.
Permit pre-check
Verifying proposed % of unprotected openings against the limit before final drawings.
Design iteration
Comparing window schedules quickly when a client asks for more glass on the side elevation.
07 · A worked example
A two-storey side wall with LD = 1.91 m.
Take a realistic case: a two-storey house on a city lot where the side wall sits 1.91 m from the property line. The wall is 8.0 m wide and roughly 5.6 m tall (ground to the underside of the top-floor ceiling) for an EBF of ≈ 44.9 m².
Step-by-step
1. Measure LD: 1.91 m, perpendicular to the face → falls in the 1.5-to-2.0 m bracket; read the 1.5 column (closest value not exceeding 1.91).
2. Calculate EBF: 8.0 m × 5.6 m = 44.8 m² → use the 40 m² row (closest not exceeding 44.9).
3. Look up the cell: Row 40 × Column 1.5 gives 8% maximum glazing.
4. Convert to area: 44.9 m² × 8% = ≈ 3.59 m² of permissible glazing.
5. Compare to proposed: If your side-elevation glazing totals 3.57 m² (≈ 7.95%), you're under the limit — compliant.
Special cases worth knowing
Detached garages and accessory buildings. Under OBC 9.10.15.4(3), glazing limits do not apply to the exposing face of a house facing a detached garage or accessory building, provided the garage serves only the house and is on the same lot. The opposite face (garage → house) may also be exempt under 9.10.15.5.
Sprinklered buildings. For residential buildings with full sprinkler protection, the maximum aggregate area of unprotected openings can double, subject to conditions. This rarely applies to single-family houses but is common in residential Part 3 work.
LD registered beyond the property line. Section 9.10.15.2(4)–(5) allows limiting distance to be measured to a point on adjoining land if the landowners and municipality enter into a registered mutual agreement. Rare, but useful on constrained infill sites.
Skewed or irregular walls. Per 9.10.15.2(2), use a vertical reference plane placed so that no part of the actual wall lies between that plane and the reference line. This conservatively handles angled and jogged walls.
08 · Disclaimer & references
Using this guide responsibly.
IMPORTANT NOTICE
This article and the associated OBC Glazing Limit Calculator are provided for informational and preliminary design purposes only. They are not a substitute for the Ontario Building Code, for the judgement of a qualified designer, architect, or engineer, or for approval by the Authority Having Jurisdiction.
The table values summarized here are based on OBC 2024 Table 9.10.15.4 and represent the portion of the table most commonly used for small residential buildings. Always consult the current in-force version of the Code for the complete table and applicable sentences, including any municipal or local amendments.
This tool applies to houses as defined in the Code — it does not cover Part 3 buildings, commercial occupancies, or assembly occupancies, which have separate and more complex spatial separation requirements. Sprinklered buildings, fire compartmentation, and non-standard wall constructions may permit alternative results under specific sentences that the tool does not automatically apply.
Limiting distance measurement can be affected by registered agreements, lot configuration, setback variances, and municipal interpretation. Before committing to a design, verify LD on an Ontario Land Surveyor's plan, and coordinate with your designer of record.
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.
References. Ontario Building Code (O. Reg. 163/24), Division B, Part 9, Article 9.10.15.4 — Glazed Openings in Exposing Building Face; Article 9.10.15.2 — Area and Location of Exposing Building Face; Article 9.10.15.5 — Construction of Exposing Building Face of Houses.
Tight lot? Tricky elevations?
AD Polyline handles design, permits, and construction-ready documentation for residential projects across Ontario — including the spatial-separation headaches that come with infill and laneway work. Get in touch and we'll tell you honestly whether we can help.
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