Cutting Stringers to the Ontario Building Code

The numbers below come from Ontario Building Code Division B, Section 9.8, and they are what a framing inspector actually measures when they pull a tape to your stringer.

Private vs public vs service stairs

OBC Table 9.8.4.1 splits stairs into three categories and each gets its own rise limits. A private stair is an interior or exterior stair serving a single dwelling unit or a house with a secondary suite — rise 125 to 200 mm, minimum width 860 mm per 9.8.2.1(4). A public stair is anything else that is not a service stair — rise 125 to 180 mm, run 255 to 355 mm, minimum width 900 mm per 9.8.2.1(1). A service stair serves a service room or service space only (mechanical rooms, crawl spaces, unoccupied attics) and has no maximum rise, just a 125 mm minimum. The limits differ because private stairs are walked daily by a household who have the geometry memorized, while public stairs see strangers and handle emergency egress in the dark. When a basement suite crosses the line into "secondary dwelling unit," the stair still has to meet private geometry — no more old 225 mm risers just because the basement was always that way.

The rise-run relationship and the 2R + T rule

Human stride length is a biomechanical constant and the code numbers are built around it. The old carpenter's rule is 2R + T between 600 and 630 mm (roughly 24 to 25 inches). At a 175 mm rise the ideal run lands around 280 mm (2 × 175 + 280 = 630). Push the rise to the private-stair maximum of 200 mm and the math says run should drop to about 215 mm — but a shorter tread with a taller riser feels cramped even when it passes code. A stair built to the extreme limits of Table 9.8.4.1 always feels steep; it meets the rule but loses the 2R + T target. Floor-to-floor height drives everything: an 8 ft ceiling with a 2x10 floor is often around 2720 mm total rise, which divides into 14 risers at 194 mm and a 270 mm run — tight but legal. Add a thicker floor assembly and you either add a riser or push past the 200 mm ceiling, which is not allowed inside a dwelling.

Uniformity tolerance and why every step must match

OBC 9.8.4.4 allows a maximum 5 mm variation between adjacent risers and 10 mm across the whole flight, with identical tolerances on the run per 9.8.4.4(3). This is not a paperwork rule. The brain calibrates stride length after two or three steps, and after that the feet stop looking — a single riser that is 15 mm off trips people. The right way to build this in is to measure total rise before cutting anything, subtract finish floor thickness at top and bottom, divide into equal risers, and cut the stringers to those exact increments. The common site mistake is laying out from the bottom with a framing square and letting the top riser "land wherever" — that top riser ends up 20 mm tall or 20 mm short and the flight fails inspection. One allowed exception under 9.8.4.4(2): where the bottom riser meets a sloped driveway or garage floor, the riser height may vary across the stair width by up to 1 in 12 since the walking surface itself is not level.

Headroom under 9.8.2.2

Clear height is measured vertically from a line tangent to the tread nosings up to the lowest obstruction. The minimum is 1950 mm inside a dwelling unit per 9.8.2.2(3), 2050 mm everywhere else per 9.8.2.2(2), and a special allowance of 1850 mm under beams and ducts inside secondary suites per 9.8.2.2(4). The tight spot on almost every basement stair is the underside of the floor framing at the top three or four treads — a 2x10 joist hanging below the drywall eats 250 mm of headroom right where your head is. Before cutting stringers, snap a line from the tread nosing plane to the soffit and physically measure. When a basement finish comes up short, the fixes in order of cost are: drop the ceiling in the stair zone, re-divide the rise to add one more riser (pushing the top nosing forward and away from the soffit), or flip the stair direction so the tight header is at the bottom instead of the top. Framing a sloped soffit that follows the stringer is a last resort that eats hours.

Landings and turns under 9.8.5 and 9.8.6

Per 9.8.3.3, a single flight cannot rise more than 3.7 m without a landing breaking it up — that is the hard ceiling on how tall a stair can be before it becomes two flights. Landings are required at the top and bottom of every flight per 9.8.6.2(1), with limited exceptions for exterior secondary entrances of three risers or fewer. Landing length must equal the stair width, but 9.8.6.3(2) caps that requirement at 1100 mm when the landing does not turn or turns less than 90 degrees — a straight run landing does not have to be comically deep. Winders — pie-shaped treads that turn the stair — are permitted in dwelling units under 9.8.4.6 with tight rules: the winder set turns no more than 90 degrees total, each winder turns between 30 and 45 degrees, adjacent winders turn through the same angle, and multiple winder sets between floors must be at least 1200 mm apart. Tapered treads under 9.8.4.3 need at least 150 mm run at the narrow end and must meet the Table 9.8.4.1 run at a point 300 mm in from the narrow side. Straight stairs with a flat landing are almost always cheaper, safer, and faster to frame — use winders only when floor plan really demands it.

Handrails and guards — scope note, 9.8.7 and 9.8.8

Handrails are required on at least one side of any interior stair of more than 2 risers, or more than 3 risers exterior, per 9.8.7.1(3). Mounting height is 865 to 1070 mm measured vertically from a line tangent to the tread nosings per 9.8.7.4(2). The rail must be continuously graspable from the bottom riser to the top riser under 9.8.7.2(1) — no big newel posts interrupting the run, no awkward transitions where the hand loses contact. Guards are required wherever the walking surface is more than 600 mm above the adjacent surface within 1.2 m, per 9.8.8.1(1). Guard height inside a dwelling unit or on exterior walking surfaces not more than 1800 mm above grade is 900 mm per 9.8.8.3(2) and (3); everywhere else the minimum is 1070 mm per 9.8.8.3(1), and exterior stairs or landings more than 10 m above grade need 1500 mm per 9.8.8.3(3.1). Baluster spacing must prevent passage of a 100 mm sphere per 9.8.8.5(1) — the classic "4-inch ball rule," enforced with a literal test ball by many inspectors. One important scope note: the calculator above checks stringer geometry (rise, run, headroom, total rise) only. Handrail mounts, guard attachment loads under 9.8.7.7 and Table 9.8.8.2, and baluster layout must be verified separately from drawings and field measurements before final inspection.