The numbers below come from Ontario Building Code 2024 Division B, Section 9.8.8 and Part 4 Article 4.1.5.14. These are what an inspector reaches for when they pull a tape on your top rail and put a 100 mm test ball against your spindles.
The most common field mistake is guessing the trigger. OBC 9.8.8.1.(1) requires a guard on every walking surface where the elevation difference to the adjacent surface within 1.2 m is more than 600 mm — 23-5/8", not the 30" rule people remember from the US IRC. A deck that is 24" above the lawn does not need a guard; 26" does. The 1.2 m lookout distance also matters — a 400 mm drop to a landscape bed still triggers the rule if the grade falls 900 mm within that 4 ft. Exceptions in 9.8.8.1.(2) are narrow: loading docks, repair-garage floor pits, and access provided for maintenance only. Residential doors under 9.8.8.1.(3) need either a guard or a swing-limiter whenever the exterior floor is more than 600 mm below the interior floor — the classic garden-door-with-no-deck-yet scenario. Second-storey openable windows fall under 9.8.8.1.(4) and need a guard or a 100 mm opening limiter unless the sill is over 900 mm above the floor.
OBC 9.8.8.3 gives you three numbers: 900 mm, 1 070 mm, and 1 500 mm. The default in 9.8.8.3.(1) is 1 070 mm (42"). The relief in 9.8.8.3.(2) covers all guards within a dwelling unit (or house with a secondary suite): 900 mm (35-7/16"). The relief in 9.8.8.3.(3) covers exterior guards serving ≤ 1 dwelling only when the walking surface is 1 800 mm or less above grade. That last condition catches a lot of people. A walkout deck three feet off the ground is a 900 mm guard. A second-storey deck ten feet off the ground is a 1 070 mm guard. A third-storey balcony more than 10 m above grade jumps to 1 500 mm under 9.8.8.3.(3.1). One more quirk: 9.8.8.3.(4) measures guard height on a stair flight from the top of the guard to a line tangent to the tread nosings, not from the walking surface — so a straight top rail 900 mm above each nosing is continuous across the flight. Landings measure from the walking surface.
OBC 9.8.8.5.(1) reads simply: openings through guards shall be sized to prevent the passage of a 100 mm sphere — 3-15/16", just under 4 inches. Inspectors carry a plastic ball. The same rule applies between the bottom rail and the walking surface. The practical layout on a dwelling deck, given a 1-1/2" square pressure-treated 2×2 spindle, is 3-1/2" on-centre: 3.5" centres minus 1.5" spindle equals a 2.0" clear gap (51 mm) — well under the 100 mm limit, with 49 mm of margin for a twisted spindle or a layout drift. Going to 4" o.c. gives 2-1/2" (63.5 mm), still passing but eating most of your margin. 4-1/2" o.c. gives 76 mm and usually fails under a pressed ball. 3-1/2" o.c. is standard because it is forgiving. Stairs get one more sphere rule in 9.8.8.5.(2): the triangle formed by the riser, tread, and bottom rail of the flight guard shall not pass a 150 mm sphere — so the bottom rail must follow the stair slope tight to the nosing, not run parallel to the stringer with a growing gap at the bottom.
9.8.8.6.(1) says: no member, attachment, or opening located between 140 mm and 900 mm above the walking surface shall facilitate climbing. 140 mm is roughly a toddler's first step on a stiff shoe; 900 mm sits above the centre of gravity of a three-year-old. A guard that keeps an adult in won't keep a determined small kid in — a child who can get a toe at 300 mm and a hand at 900 mm can swing a leg over. This is why horizontal cable rails fail almost every residential inspection — each cable is a ladder rung. Horizontal pipe rails, ranch-style 1×4 flat rails, and lattice with climbable openings all get rejected. Even the bottom rail itself can fail if it sits 3/4" off the deck and gives a toe hold. The clean detail is vertical spindles on a bottom rail set flush with the decking, with a top-rail profile that offers nothing grippable in the climb zone.
Table 9.8.8.2 row 1 sets the dwelling-unit loads: a concentrated 1.0 kN (≈ 225 lb) at any point on the top of the guard, inward OR outward, or a uniform 0.5 kN/m — whichever governs. Infill resists 0.5 kN over 300 × 300 mm. The top of the guard takes 1.5 kN/m vertical. Common-area guards (row 3) jump to 0.75 kN/m uniform. None of these loads act simultaneously per 9.8.8.2.(4). The 225 lb reads small until you work out a corner post: 225 lb at the top of a 36" post is a 675 in-lb moment at the base. A row of screws through a 2× rim joist into air cannot carry that moment — you need fasteners into the rim AND perpendicular blocking. The field-proven detail is a Simpson DTT2Z tension tie bolted through the post base into blocked rim — it turns a fastener-head problem into a steel-strap problem. Per 9.8.8.2.(5), dwelling guards may use MMAH SB-7 Guards for Housing and Small Buildings as a deemed-to-comply detail instead of engineered loads — that is what most residential decks actually follow.
Three failures show up on nearly every deck inspection that goes wrong. First: loose 2×2 spindles. Toe-nailed with 2-1/2" finish nails, a 2×2 fails the in-plane element load test in 9.8.8.2.(2) — the 0.1 kN push rotates the spindle because toe nails have no shear resistance in the load direction. Fix with (2) #8 × 3" screws through the face of the rails into the spindle end, or a Simpson BC-style bracket. Second: missing bolt at the corner post. Corner posts nailed on instead of bolted fail — both directions of the 1.0 kN load converge at the corner and nails cannot carry that moment. Use (2) 1/2" through-bolts with washers both sides, or a DTT2Z. Third: handrail mounted to a guard post. A handrail bracket screwed to a guard post fails two rules at once — the bracket creates a climbable element in the 140–900 mm zone, and the rail breaks continuity at the post per 9.8.7.2. Mount the handrail to the opposite wall when you can, or to brackets that don't project into the climb zone. Decks with screw-attached spindles, bolted corners, and clean handrail runs sail through inspection.