On a 30-foot-wide infill lot, the difference between a 3 m × 2 m picture window and a pair of clerestory portholes is usually one number on a table nobody reads. This is what OBC 9.10.14 and 9.10.15 are actually trying to do, how to measure it without arguing with the inspector, and where renovations quietly step over the line.
Fire does not respect property lines. A house that catches fire on one lot radiates heat, throws embers, and eventually vents flame out of any glass opening facing the neighbour. If the neighbour's wall is close enough to absorb that radiant flux, the cladding ignites, the siding drops burning bits into the soffit, and a single-house fire becomes a block fire. The National Building Code — which Ontario adopted and edited into the OBC — figured out in the middle of the 20th century that if you control two variables on the exposed side of each building (how much glass faces the neighbour, and how close to the lot line you let it sit), you break the chain. That is the whole logic behind Section 9.10.14 and the housing-specific version in 9.10.15. The table values come out of burn testing and radiant heat modelling of what a typical wood-frame envelope can take before it lights up across a gap.
Limiting distance (LD) is defined in OBC 9.10.14.2. and 9.10.15.2., and the measurement rules are where most rookie mistakes happen. The correct LD is the shortest perpendicular distance from the exposing building face to the property line, or to the centre line of a public way (a street, lane, or registered thoroughfare). If there is a second building on the same lot, the LD is to an imaginary line half-way between the two exposing faces — you get half and your neighbour-on-the-same-lot gets the other half. On a corner lot, each face has its own LD measured to its own line. For an irregularly shaped or skewed wall, do not measure to the closest corner stickout — the code says to project the face onto a vertical plane such that no portion of the actual wall sits between that plane and the reference line, and measure to that plane. On a lot where the side-yard setback varies along the wall length, inspectors sometimes allow the face to be split into portions with individual LDs per 9.10.15.2.(1)(b)(iii). Ask. Do not assume.
The exposing building face is one side of the building measured from finished ground level up to the uppermost ceiling. That includes the gable end above the soffit line if the gable faces the property line — the triangle of wall under the roof counts as face area. Where a building is divided into fire compartments by separations rated 45 minutes or more, each compartment's EBF is computed separately. Dormers, bay windows, and angled wall segments that all face the same general direction typically roll up into the same face, but the irregular-wall projection rule in 9.10.14.2.(3) and 9.10.15.2.(4) lets you break them into portions and run the table on each portion against its own LD — sometimes to your benefit, sometimes not. The point the rookies miss: the EBF is the wall area, not the opening area. A 40 m² wall with 8 m² of windows has an EBF of 40, not 40 minus 8. You drive the table with the full face and then ask how many m² of glass that face is allowed.
An unprotected opening is any opening in the wall that is not closed by a fire-rated assembly. That means windows (unless glazed with wired glass in steel frames or glass block per 9.10.14.4.(6)), doors, vents, and any skylight that is located in the exposing building face rather than the roof plane. Skylights in the roof itself are handled separately; a wall skylight low on a gable is an opening. A minor opening of 130 cm² or less (about a 115 mm × 115 mm square — think a dryer vent or a bathroom exhaust) is explicitly exempt by 9.10.14.6.(1) and 9.10.15.4.(9). Non-combustible cladding with small joints is not an opening. Where people get tripped up is on wood decks and their privacy screens — a deck itself is not an opening, but a wood deck with a closely-spaced railing close to the lot line can effectively become a fire-spread surface that inspectors will scrutinize, and the combustible-projection rules in 9.10.14.5.(6) / 9.10.15.5.(5) prohibit combustible projections within 1.2 m of the property line on multi-unit buildings. Houses get an exemption on that particular clause, but the soffit-protection rule in 9.10.14.5.(12) still applies to projecting eaves under 1.2 m.
Sentences 9.10.14.4.(7) and 9.10.15.4.(7) allow the maximum aggregate opening area to be up to twice the table value where the building is sprinklered AND all rooms adjacent to the exposing building face — including closets, bathrooms, and attached garages — are sprinklered, notwithstanding the NFPA 13D exemptions that would normally let you skip those spaces. That is the concession that makes urban infill lots livable. A 20 m² EBF at 1.5 m LD on a house is limited to 8% unprotected (1.6 m²) without sprinklers — barely enough for a bedroom window. With a compliant residential sprinkler throughout, the same wall is allowed 16% (3.2 m²), which is enough for the window a client actually wanted. The catch: it is an all-or-nothing concession. Skipping the sprinkler head in the bathroom because the installer quoted an upgrade defeats the entire concession. If you are planning infill with narrow setbacks, design the sprinkler coverage to include every room at the EBF before you draw the windows.
The first classic is a reno that adds a picture window to a wall the original builder sized at the table limit. The existing house was built to 12% unprotected at 3 m LD; the new opening pushes the aggregate to 18%, and the wall assembly behind it no longer carries the required FRR and cladding. Per 9.10.14.5. the required cladding drops to combustible-or-non-combustible only above 25% openings — exceed the table and the wall spec itself changes. The second classic is a deck or addition that shortens the limiting distance. A 1.8 m deck cantilevered off a house 3 m from the property line projects the face to 1.2 m, dropping permitted openings from 32% to 8% at a 20 m² EBF. The third, quieter one is a second building on the same lot — LD becomes half the distance to that building's face, and an addition on your side suddenly has to pay for glazing the neighbour's shed never had. Spatial separation is a two-body problem. When you change one side, draw both, measure both, and sanity-check the table before anyone cuts a rough opening.