How far can a stair tread nosing project past the riser under the Ontario Building Code?

Per OBC 9.8.4.2.(2), tread depth must be at least the run and no more than the run plus 25 mm. That makes the maximum nosing projection 25 mm (1″). Zero projection is allowed only where the tread depth equals the run — typical for exterior integral-nosing concrete or stone treads. Interior wood stairs almost always use a 19–25 mm projection for toe clearance.

What is the minimum effective depth of a wooden stair stringer after it is notched?

OBC 9.8.9.4.(1)(a) requires a minimum 90 mm of effective depth, measured perpendicular to the bottom of the stringer at the point of minimum cross-section (under the notch for the tread/riser). Overall stock must be at least 235 mm. A 2×12 (286 mm) cut for a typical 190–195 mm rise leaves roughly 100–115 mm — passes. A 2×10 (235 mm) cut for the same rise is borderline and often fails once you account for bottom-edge shrinkage and a chalk-line variance.

How high should a handrail be on a stair in Ontario?

Per OBC 9.8.7.4.(2), required handrails shall be 865 mm to 1 070 mm high, measured vertically from the top of the rail to a straight line drawn tangent to the tread nosings (or the surface of a ramp or landing). The 865 mm minimum matches U.S. 34″ practice; the 1 070 mm maximum matches the guard height that prevails where a stair sits next to an open drop. Most carpenters land on 900–915 mm (about 36″).

Do I need a horizontal handrail extension at the top and bottom of a stair in Ontario?

Per OBC 9.8.7.3.(2), handrails on stairs other than those serving a single dwelling unit must extend horizontally at least 300 mm past the top and bottom risers. Stairs and ramps serving a single dwelling (including a house with a legal secondary suite) are exempt from the extension requirement. That means your main-floor-to-second-floor stair inside a house does NOT need the 300 mm tails, but a stair in an apartment corridor, commercial entrance, or any stair not within a dwelling unit does.

Why does a 2×4 fail as a handrail even if the height is correct?

OBC 9.8.7.5.(2) requires that handrails be continuously graspable along their entire length, with no obstruction on or above them that would break the handhold. The guidance note A-9.8.7.5.(2) expands this: the profile must let the hand close around the rail to arrest a slip. A flat 2×4 measures 38 × 89 mm — the short dimension (38 mm) passes, but the 89 mm width is too wide for most adult fingers to wrap around. The code fix is a shaped profile: a 32–51 mm round dowel, a milled handrail stock (42 × 57 mm with a routed graspable upper bead), or a 2×4 with the top corners heavily rounded and the sides dadoed so the gripped portion stays inside the 32–51 mm band.

OBC Stair Stringer & Handrail Details — Free Ontario Building Code Tool

Cutting Stringers and Hanging Handrails to OBC 2024

The overall rise, run, and headroom is one calculation — handled by the stair calculator next door. This page is the other half of the job: the tread nosing, the notched stringer, and the handrail. The numbers below come from Ontario Building Code 2024, Division B, Section 9.8.

Nosing projection — the 0–25 mm window and why it matters

OBC 9.8.4.2.(2) sets the relationship between run and tread depth in one line: tread depth shall be not less than its run and not more than its run plus 25 mm. The nosing projection is just the arithmetic difference — tread depth minus run — so the legal range is 0 to 25 mm beyond the riser face. Zero is only realistic on exterior concrete where the nosing is integral; on any interior wood stair you want 19–25 mm so the ball of the foot has a real place to land on the descent. Drop below that and toes catch risers; push past 25 mm and you fail inspection. A separate rule in 9.8.4.8.(1) requires the top of the nosing to have a rounded or beveled edge extending 6 mm to 14 mm horizontally — the reason the ⅜″ (≈10 mm) round-over bit exists. A square-sawn nosing is a shin-catcher and a fail; a beveled or bullnosed profile takes 10 seconds and sidesteps the question.

Stringer cut geometry — how framers break the 90 mm rule without realizing

OBC 9.8.9.4.(1)(a) is the most-missed rule on site: after a stringer is notched for the treads and risers, there must be at least 90 mm of intact wood remaining, measured perpendicular to the bottom edge at the point of minimum cross-section (directly under the nosing). The arithmetic is effective_depth = stock_depth × cos θ − rise × sin θ, where θ is the stair pitch. A 2×12 (286 mm) with 195 mm rise and 260 mm run gives θ = 37°, so effective depth = 286 × 0.80 − 195 × 0.60 = 112 mm. Passes. Swap to a 2×10 (235 mm) with the same rise/run: 235 × 0.80 − 195 × 0.60 = 71 mm. Fails by 19 mm. That's why 2×10 stringers only work on shallow flights — deck stairs with 170 mm rise, or short interior flights. The fix is always the same: upsize to a 2×12 (or an LVL stringer for long flights). Never shim the stringer up to fake the depth — bearing fails in shear at the notch, and you've built an unsafe stair that reads fine on the tape.

Graspability — why a 2×4 handrail fails even if the height is right

OBC 9.8.7.5.(2) says handrails must be continually graspable along their entire length with no obstruction on or above them to break a handhold. Appendix note A-9.8.7.5.(2) sets the profile window: round rails 32–51 mm in diameter (1¼–2″); any oval or rectangular profile must allow the hand to close around it. A flat 2×4 on edge is 38 × 89 mm — the 38 mm side is fine, but the 89 mm side is too wide for adult fingers to wrap. The grip test: grab the rail one-handed and try to hold your weight; if your fingertips don't meet your thumb, it's a shelf, not a handrail. The fixes are a 1½″ (38 mm) round dowel, purpose-milled handrail stock, or a 2×4 with heavy corner round-overs AND side dadoes so the gripped upper 38 mm has real finger clearance below. A 2×4-on-edge with a ¼″ round-over does not pass — the note is explicit that the obstruction below the grip breaks the handhold.

Handrail extensions at top and bottom — where they're required vs not

OBC 9.8.7.3.(2) requires at least one handrail to extend horizontally not less than 300 mm beyond the top and bottom of each flight, with an exemption for stairs serving a single dwelling unit. The piece framers miss is what "top and bottom" means in a switchback: each flight is its own unit, so a two-flight corridor stair needs four 300 mm tails. The extension exists because the last step is the highest-slip moment — the ankle unbends and stride changes, and a rail stopping at the last riser leaves no grab-point. Practical terminations: a horizontal 300 mm return into the wall (cleanest), a 90° turn-down (common on commercial stairs), or a loop back to the balustrade. The dwelling exemption is narrow — once the stair enters a common corridor or a commercial occupancy, the 300 mm tails are back on.

Carriage stringers on wide stairs — when 2 stringers becomes 3

OBC 9.8.9.4.(1)(d) caps wooden stringer spacing at 900 mm o.c. in dwelling stairs and 600 mm o.c. in all others. Sentence (2) stretches dwelling spacing to 1 200 mm o.c. only where closed risers support the front of each tread. A 36″ (915 mm) interior dwelling stair with open risers exceeds 900 mm — three stringers (outer pair plus one carriage). Same width with closed risers sneaks under 1 200 mm on two. A 42″ commercial stair is well over the 600 mm public cap — outer pair plus one carriage minimum; 48″+ wants two. The classic mistake is a 40″ open-riser 2-stringer basement stair that squeaks on every descent. A 1× "ribbon" under tread centres is not a carriage — 9.8.9.4 is written around stringer geometry, not tread reinforcement.

Uniformity tolerance — what ±5 mm actually means with a Skil saw

OBC 9.8.4.4 is deceptively gentle: adjacent risers or runs may vary by 5 mm, and the full flight by 10 mm end to end. 5 mm is 3/16″ — roughly the width of a pencil line. The trap is that variation compounds: if each riser is "a little tall" by 2 mm and the bottom riser gets the leftover, you can be 8 mm off before nosings go on. Prevention: (1) lay out the rise on ONE stringer and use it as the pattern for the rest — never start each from scratch; (2) cut the bottom of every stringer from the same line after layout; (3) measure as-built total rise before cutting anything and re-divide if rough rise deviates. The circular saw adds ~3 mm of cut drift; careful framers over-cut with the circular saw and finish each notch with a jigsaw to the chalk line. Slower, but uniform — and uniform is what keeps people on their feet, because the brain calibrates by step three and stops looking, so the single odd riser is always the one that trips people.

Scope: the tool above checks nosing geometry, wooden stringer cut-outs, and handrail details. Winder and spiral stairs (OBC 9.8.4.3, 9.8.4.6, 9.8.4.7), tapered-tread exit stairs falling under Part 3, and handrail structural attachment design per 9.8.7.7 are out of scope. For rise/run/headroom against Table 9.8.4.1, see the OBC Stair Calculator. Guards (9.8.8) are handled by the OBC Guard Height & Load Checker.

About OBC Stair Stringer & Handrail Details

Free Ontario Building Code 2024 checker for stair tread nosings, wooden stringer cut-outs (90 mm effective depth rule), and handrail height / extension / graspability. Complements our OBC stair calculator.

How to use

Pick the section tab — Nosing details (tread depth vs run geometry under 9.8.4.2), Stringer cut (the 90 mm effective-depth rule under 9.8.9.4(1)(a) and 900/600/1,200 mm o.c. carriage spacing under 9.8.9.4(1)(d) and (2)), or Handrail (height, graspability profile, and extensions under 9.8.7).
For nosings: enter run and tread depth, and choose the nosing edge profile — square sawn (FAIL), rounded with 6–14 mm radius, or beveled. OBC 9.8.4.2.(2) requires tread depth ≥ run and ≤ run + 25 mm — so legal projection is 0–25 mm. OBC 9.8.4.8.(1) requires a rounded or beveled top edge extending 6–14 mm horizontally; the standard 10 mm (⅜″) round-over bit hits dead-centre.
For stringers: pick stock size (2×10 = 235 mm, 2×12 = 286 mm, LVL, or custom), enter the rise, stair width, and occupancy (private dwelling vs public). Tool computes effective depth using the trig formula: stock × cos(pitch) − rise × sin(pitch), and auto-checks the 90 mm minimum at the point of minimum cross-section (under the notch). A 2×12 at 195 mm rise leaves ~112 mm — passes; a 2×10 at the same rise leaves only ~71 mm — FAILS by 19 mm.
Verify carriage spacing against the wall thickness — OBC 9.8.9.4.(1)(d) caps wooden stringer spacing at 900 mm o.c. for stairs serving not more than one dwelling unit and 600 mm o.c. for all other stairs. 9.8.9.4.(2) stretches dwelling spacing to 1,200 mm o.c. ONLY where closed risers (a nailed riser board under each nosing) support the front of each tread. A 36″ open-riser dwelling stair needs 3 stringers; a 42″ closed-riser dwelling stair gets away with 2.
For handrails: pick location (private dwelling interior, private dwelling exterior, public/apartment/commercial), enter rail height above the nosing line, profile shape (round, oval, milled handrail stock, flat 2× on edge), and the gripped dimension. OBC 9.8.7.4.(2) requires 865–1,070 mm height; A-9.8.7.5.(2) requires the gripped portion to fall in the 32–51 mm graspability window — a flat 2×4 (89 mm wide) FAILS even at correct height.
Set whether 300 mm horizontal extensions are required at top and bottom — OBC 9.8.7.3.(2) exempts stairs serving a single dwelling unit (including a house with a legal secondary suite). Apartment corridors, commercial entrances, and any stair NOT within a dwelling need at least one rail extending 300 mm past the top and bottom risers; switchback flights need four 300 mm tails total.
Read the riser uniformity check before committing the cuts — OBC 9.8.4.4 permits adjacent risers to vary by up to 5 mm and the whole flight by 10 mm. Lay out from one master stringer, cut the rest to that pattern, and measure rough total rise BEFORE cutting in case the floors are out of level. Circular saws drift roughly 3 mm — finish each notch with a jigsaw to the chalk line to stay inside the tolerance.

Examples

2x12 SPF stringer, 195 mm rise, 260 mm run

Pitch ≈ 37°. Effective depth = 286 × cos 37° − 195 × sin 37° = 286 × 0.80 − 195 × 0.60 = 229 − 117 = 112 mm. Passes the 90 mm rule with 22 mm to spare. Three stringers required for 36″ open-riser dwelling stair (900 mm o.c. cap).

1.5 inch round wood handrail, 36 inch high

Round profile 38 mm diameter — passes the 32–51 mm graspability window. Mounted 915 mm (36″) above the nosing line — passes 865–1,070 mm height window. Stair within a dwelling unit → no 300 mm extensions required at top and bottom.

Frequently asked questions

How far can the tread nosing project past the riser?

Per OBC 9.8.4.2.(2), tread depth must equal the run, and may not exceed run + 25 mm. So nosing projection is 0 to 25 mm (0 to 1″). Zero is only realistic on integral-nosing concrete or stone treads. Interior wood stairs almost always use a 19–25 mm projection so the ball of the foot has somewhere to land.

What's the minimum effective depth of a notched stringer?

OBC 9.8.9.4.(1)(a) requires 90 mm of intact wood after the tread/riser cut — measured perpendicular to the bottom edge at the point of minimum cross-section (under the nosing). Stock minimum is 235 mm. A 2×12 with 195 mm rise leaves ~112 mm — passes. A 2×10 (235 mm) with the same rise leaves only ~71 mm — FAILS by 19 mm. 2×10 stringers only work on shallow flights (decks at ~170 mm rise).

How high should a handrail sit?

Per OBC 9.8.7.4.(2): 865–1,070 mm above the nosing line — the straight tangent line drawn through the tops of all nosings. Most installs land at 900 mm (~36″). Handrails serving stair flights only must be continuously graspable per 9.8.7.5.(2) with no obstruction breaking the handhold.

Do I need 300 mm horizontal extensions at top and bottom?

On stairs serving a single dwelling unit (including a house with a legal secondary suite) — NO. 9.8.7.3.(2) exempts dwelling stairs. On any other stair (apartment corridor, commercial, exterior stair not within a dwelling) — YES, at least one handrail extends horizontally 300 mm past the top and bottom risers. In a switchback, each flight is its own unit, so a two-flight corridor needs four 300 mm tails.

Why does a 2x4 fail as a handrail even at the right height?

OBC 9.8.7.5.(2) requires the rail to be continuously graspable along its entire length. Appendix A-9.8.7.5.(2) sets the profile window at 32–51 mm round (1¼–2″); rectangular shapes must let the hand close around them. A flat 2×4 measures 38 × 89 mm — the 38 mm side is OK, but the 89 mm width is too wide for adult fingers to wrap. Use a round 1½″ dowel, milled handrail stock, or a 2×4 with heavy round-overs AND side dadoes.

When does a stair need a middle carriage stringer?

OBC 9.8.9.4.(1)(d) caps wooden stringer spacing at 900 mm o.c. for dwelling stairs and 600 mm o.c. for all other stairs. 9.8.9.4.(2) stretches dwelling spacing to 1,200 mm o.c. ONLY where closed risers (nailed risers under each nosing) support the front of each tread. A 36″ open-riser dwelling stair → 3 stringers. A 42″ closed-riser dwelling stair → 2 stringers.

What uniformity tolerance applies to riser height?

OBC 9.8.4.4 permits adjacent risers to vary by up to 5 mm and the whole flight by 10 mm. 5 mm is roughly the width of a pencil line. To stay inside it: lay out the rise on ONE master stringer, use it as the pattern for the rest, cut bottoms from the same line, and measure rough total rise BEFORE cutting in case the floors are out of level. Circular saws drift ~3 mm — finish each notch with a jigsaw to the chalk line.

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