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Code guide

Baluster Spacing in Ontario: The 100 mm Rule

Baluster spacing in the 2024 Ontario Building Code: openings must stop a 100 mm sphere, 150 mm at stair triangles, the no-climb zone and the 0.1 kN spread test.
sphere that must not pass through a guard
100 mm
sphere that must not pass through a guard
stair triangle under the bottom rail
150 mm
stair triangle under the bottom rail
no-climb zone above the walking surface
140 to 900 mm
no-climb zone above the walking surface
spread test on adjacent balusters
0.1 kN
spread test on adjacent balusters

Last updated

Under the 2024 Ontario Building Code, the openings in a required guard must stop a 100 mm sphere from passing through (OBC Div. B 9.8.8.5.(1)OBC Div. B 9.8.8.5. Openings in Guards(1) Except as permitted in Sentences (3) and (4), openings through guards shall be of a size that prevents the passage of a spherical object having a diameter of 100 mm. (See Note A-9.8.8.5.(1) and (3)) (2) Except for guards that serve industrial occupancies, the triangular openings formed by stair risers, stair…), so the clear gap between balusters must be less than 100 mm. On stairs, the triangle under the bottom rail must stop a 150 mm sphere (9.8.8.5.(2)OBC Div. B 9.8.8.5. Openings in Guards(1) Except as permitted in Sentences (3) and (4), openings through guards shall be of a size that prevents the passage of a spherical object having a diameter of 100 mm. (See Note A-9.8.8.5.(1) and (3)) (2) Except for guards that serve industrial occupancies, the triangular openings formed by stair risers, stair…), and nothing between 140 mm and 900 mm above the walking surface may make the guard easy to climb (9.8.8.6.OBC Div. B 9.8.8.6. Guards Designed Not to Facilitate Climbing(1) Guards required by Article 9.8.8.1., except those in industrial occupancies and where it can be shown that the location and size of openings do not represent a hazard, shall be designed so that no member, attachment or opening located between 140 mm and 900 mm above the floor or walking surface protected by the…).

This page covers baluster spacing and the related opening rules for guards on stairs, decks, balconies and landings in Part 9 buildings. For guard heights, see our guard height guide. Your building department decides how the code applies.

Baluster rules at a glance

Item Rule in short Where to look
Openings in a required guard must stop a 100 mm sphere 9.8.8.5.(1)
Stair triangle (riser, tread, bottom rail) must stop a 150 mm sphere 9.8.8.5.(2)
Industrial guards top rail plus rails stopping a 535 mm sphere 9.8.8.5.(3)
Guards that are not required stop 100 mm or pass 200 mm 9.8.8.5.(4)
No-climb zone 140 mm to 900 mm above the walking surface 9.8.8.6.(1)
Spread test 0.1 kN on adjacent elements; gap still under the limit 9.8.8.2.(2)
When a guard is required more than 600 mm drop within 1.2 m 9.8.8.1.(1)

This is our summary of the articles, not a copy of the code. Read the full text before you build.

The 100 mm sphere

The rule is written as a test: a 100 mm diameter sphere must not pass through any opening in a required guard (9.8.8.5.(1)OBC Div. B 9.8.8.5. Openings in Guards(1) Except as permitted in Sentences (3) and (4), openings through guards shall be of a size that prevents the passage of a spherical object having a diameter of 100 mm. (See Note A-9.8.8.5.(1) and (3)) (2) Except for guards that serve industrial occupancies, the triangular openings formed by stair risers, stair…). It applies between balusters, between the bottom rail and the deck, and anywhere else in the guard. The size is chosen to keep a small child's head and body from slipping through.

Because the sphere must not pass, the gap must be less than 100 mm. Leaving a margin, such as 90 mm, allows for lumber that shrinks or twists.

Stairs: the 150 mm triangle

On a stair, the bottom rail of a guard usually runs parallel to the nosings, which leaves a triangle between the rail, the riser and the tread. That opening must stop a 150 mm sphere (9.8.8.5.(2)OBC Div. B 9.8.8.5. Openings in Guards(1) Except as permitted in Sentences (3) and (4), openings through guards shall be of a size that prevents the passage of a spherical object having a diameter of 100 mm. (See Note A-9.8.8.5.(1) and (3)) (2) Except for guards that serve industrial occupancies, the triangular openings formed by stair risers, stair…). The balusters themselves still follow the 100 mm rule.

The no-climb zone

Required guards must be designed so that no member, attachment or opening between 140 mm and 900 mm above the floor or walking surface makes the guard easy to climb, unless it can be shown that the location and size of the openings are not a hazard (9.8.8.6.OBC Div. B 9.8.8.6. Guards Designed Not to Facilitate Climbing(1) Guards required by Article 9.8.8.1., except those in industrial occupancies and where it can be shown that the location and size of openings do not represent a hazard, shall be designed so that no member, attachment or opening located between 140 mm and 900 mm above the floor or walking surface protected by the…). This is why horizontal rails, cable railings and decorative patterns with footholds in that zone often draw questions at inspection. Vertical balusters avoid the issue. Industrial occupancies are exempt.

Balusters must stay tight

Spacing is not just measured when the guard is new. When two adjacent vertical elements are each pushed with a load of 0.1 kN in opposite directions, the opening must still meet the 100 mm limit (9.8.8.2.(2)OBC Div. B 9.8.8.2. Loads on Guards(1) Except as provided in Sentences (2), (3), (5) and (6), guards shall be designed to resist the specified loads prescribed in Table 9.8.8.2. (2) The size of the opening between any two adjacent vertical elements within a guard shall not exceed the limits required by Sentence 9.8.8.5.(1) when each of these elements…). Thin or loosely fastened balusters that flex apart fail this test. For guards in houses and decks serving up to two dwelling units, the design loads are applied across three balusters where they fit in a 300 mm width (9.8.8.2.(3)OBC Div. B 9.8.8.2. Loads on Guards(1) Except as provided in Sentences (2), (3), (5) and (6), guards shall be designed to resist the specified loads prescribed in Table 9.8.8.2. (2) The size of the opening between any two adjacent vertical elements within a guard shall not exceed the limits required by Sentence 9.8.8.5.(1) when each of these elements…).

Guards that are not required

A guard is required where the drop is more than 600 mm within 1.2 m of the walking surface (9.8.8.1.OBC Div. B 9.8.8.1. Required Guards(1) Except as provided in Sentence (2) and except at the leading edge at the top of a flight, every surface to which access is provided, including but not limited to flights of steps and ramps, exterior landings, porches, balconies, mezzanines, galleries and raised walkways, shall be protected by a guard on each side…). If you add a guard where the code does not require one, its openings must either stop a 100 mm sphere or let a 200 mm sphere pass (9.8.8.5.(4)OBC Div. B 9.8.8.5. Openings in Guards(1) Except as permitted in Sentences (3) and (4), openings through guards shall be of a size that prevents the passage of a spherical object having a diameter of 100 mm. (See Note A-9.8.8.5.(1) and (3)) (2) Except for guards that serve industrial occupancies, the triangular openings formed by stair risers, stair…). That avoids openings big enough for a child to get into but too small to get out of.

Common mistakes

  1. Measuring centre to centre. The 100 mm rule is the clear gap between balusters.
  2. A gap of exactly 100 mm. The sphere must not pass, so stay under it.
  3. Forgetting the gap under the bottom rail on a deck.
  4. Horizontal or cable rails in the 140 to 900 mm zone without approval.
  5. Balusters that flex apart under a small push.

How to look it up yourself

Open Division B, Part 9 of the 2024 Compendium. Subsection 9.8.8. covers guards. Search terms that appear in the code text:

  • "Openings in Guards" (9.8.8.5.)
  • "Not to Facilitate Climbing" (9.8.8.6.)
  • "Loads on Guards" (9.8.8.2.)
  • "Required Guards" (9.8.8.1.)

Read the article in the code viewer alongside the climbing article.

BCIN exam angle

The 100 mm and 150 mm spheres and the 140 to 900 mm climbing zone are classic House exam questions. Study the House chapter on stairs, ramps, handrails and guards, then try the free quiz on the House exam page.

Based on the official 2024 Building Code Compendium on ontario.ca.

© King's Printer for Ontario, 2024. Reproduced with permission.

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Frequently asked questions

What is the maximum baluster spacing in Ontario?
Openings through a required guard must prevent a 100 mm sphere from passing through (Division B, Sentence 9.8.8.5.(1)). In practice, the clear gap between balusters must be less than 100 mm, so it is wise to leave a margin, such as 90 mm.
What about the gap under a stair railing?
The triangular opening formed by the riser, the tread and the bottom rail of a required guard on a stair must stop a 150 mm sphere, except in industrial occupancies (Division B, Sentence 9.8.8.5.(2)).
Can a deck railing have horizontal rails in Ontario?
Required guards must not have members, attachments or openings between 140 mm and 900 mm above the walking surface that make them easy to climb, unless it can be shown the openings are not a hazard (Division B, Article 9.8.8.6.). Horizontal rails in that zone usually fail this test, so ask your building department before using them.
Do balusters have to resist being pushed apart?
Yes. When two adjacent vertical elements are each pushed with a load of 0.1 kN in opposite directions, the opening between them must still meet the 100 mm limit (Division B, Sentence 9.8.8.2.(2)).
What if the guard is not required by the code?
Openings in a guard that is not required, outside industrial occupancies, must either stop a 100 mm sphere or let a 200 mm sphere pass, so nothing can get stuck halfway (Division B, Sentence 9.8.8.5.(4)).