The blog cover image is for reference purposes only and does not depict the actual Medallion Fence product.

Short answer: most aluminum guards in Ontario must:

A 900 mm guard is allowed in a few specific cases. And unlike BC and Alberta, Ontario applies the climbing rule to every required guard. It doesn't matter whether the deck is 700 mm off the lawn or ten storeys up.

That's the summary. The details are where inspections get failed.

Ask a room of GTA deck builders how tall a guard has to be and most will say 36 or 42 inches. Both answers can get a final inspection rejected. Forty-two inches is 1066.8 mm, and the code asks for 1070 mm. Those 3.2 mm have sent more than one crew back to site with a new set of posts.

Aluminum guards rarely fail in Ontario because an extrusion was too weak. They fail on millimetres, on the wording of the climbing clause, and on missing engineering. Here is how the current code reads, where Ontario parts ways with the West, and the questions homeowners keep asking that rarely get a straight answer.

The Code in Force Right Now

Ontario is on the 2024 Ontario Building Code, O. Reg. 163/24. It took effect January 1, 2025. Since April 1, 2025, every new permit application has been reviewed under it.

The 2024 OBC is the 2020 National Building Code of Canada plus Ontario's own amendments. If your details still quote 2012 article numbers, it's worth a second look.

Where to find the rules:

When a Guard Is Required

Article 9.8.8.1 requires a guard on the open side of a deck, balcony, porch, landing, stair or raised walkway in either of these cases:

• The walking surface is more than 600 mm above the adjacent surface.
• The adjacent surface within 1.2 m of the walking surface slopes steeper than 1 in 2.

Two details catch people.

First, measure to the low point within 1.2 m, not straight down from the rim joist. A 500 mm deck next to a window well or a retaining wall can still need a guard.

Second, the slope clause catches decks on sloped lots. The drop at the edge looks small, but the yard falls away quickly beyond it.

That makes a decorative aluminum rail with 150 mm gaps on a low front porch a real hazard. It isn't harmless trim.

Guard Height: 900 mm or 1070 mm

Article 9.8.8.3 sets the minimums:

• 1070 mm is the default for all guards.
• 900 mm is allowed for guards inside a dwelling unit.
• 900 mm is allowed for exterior guards serving a single dwelling unit, where the walking surface is not more than 1800 mm above finished ground.
• 900 mm is allowed on flights of stairs other than required exit stairs. It is measured vertically from a line through the outside edges of the nosings.

Where it goes wrong in the field:

The 100 mm Rule, Including the Openings People Forget

Article 9.8.8.5 says openings through a required guard must stop a 100 mm sphere. That covers every opening, not just the space between pickets:

• the gap under the bottom rail
• the gap between the end post and the house wall or cladding
• the gap at a gate or a section joint

The stair triangle is the one exception. That's the opening formed by the tread, the riser and the bottom rail of a stair guard, and the limit there is a 150 mm sphere. Older guides still say 100 mm at the triangle, and they're out of date.

Watch the imperial conversions:

• A 6 inch triangle from American product literature is 152.4 mm, which is over the 150 mm limit.
• A US "4 inch" picket space is 101.6 mm, which fails the 100 mm rule.

The 2024 code also added a test that matters for aluminum. Sentence 9.8.8.2.(2) says the gap between two adjacent vertical elements must stay within the 100 mm limit when each is pushed with 0.1 kN in opposite directions, in the plane of the guard.

Long, slender pickets in a wide section flex. A gap that measures 96 mm at rest can open past 100 mm with a light push. Ask suppliers for clear spacing under that load, not nominal on-centre spacing.

Climbability: Where Ontario Is Stricter

Article 9.8.8.6, and 3.3.1.18.(4) for Part 3 buildings, says no member, attachment or opening between 140 mm and 900 mm above the walking surface may facilitate climbing. In Ontario this applies to every required guard outside industrial occupancies. Deck height doesn't matter.

That's the biggest difference from the national model. The national code, which BC and Alberta build on, applies the climbing rule only where the guard protects a level more than 4.2 m above the level below. Vancouver writes its own by-law and adopted the same approach in September 2025. That opened the door to horizontal infill on lower decks there.

Ontario officials generally accept the safe harbours in explanatory Note A-9.8.8.6.(1):

• protrusions more than 450 mm apart, both horizontally and vertically
• horizontal offsets of 15 mm or less
• openings no bigger than 45 mm wide by 20 mm high
• protrusions sloped steeper than 2 in 1

What this means for aluminum design:

• Horizontal pickets, cable infill, and decorative rings or scrolls in the 140 to 900 mm zone generally won't pass on a required guard.
• Some municipalities accept decorative panels if permanently fixed safety glass backs them.
• Keep the top of the bottom rail at or below 140 mm so it sits under the zone. Then confirm the gap beneath still passes the sphere test.

The part almost nobody discusses is what sits next to the guard. An AC condenser, a built-in bench or a planter beside a railing works like a step.

Kitchener's October 2025 guidance gives several options:

• keep the feature at least 760 mm from the climbable parts of the guard
• raise its top to 900 mm or more
• surround it with its own non-climbable guard

Not every jurisdiction reads it this way. BC's Building Code Appeal Board has ruled that the climbing clause covers the guard itself, not objects placed beside it. In Ontario, assume your inspector will look at the bench.

Loads, and Why Aluminum Always Needs Paperwork

Table 9.8.8.2 sets these loads for guards inside a dwelling unit and exterior guards serving not more than two dwelling units:

• Top of guard: 0.5 kN/m, or a 1.0 kN point load anywhere.
• Infill and individual elements: 0.5 kN over a 300 mm by 300 mm area, engaging three balusters where the spacing allows.
• Vertical load on the top: 1.5 kN/m.

Other Part 9 guards take 0.75 kN/m or 1.0 kN at the top, with the same vertical load. The loads aren't applied at the same time.

Part 3 buildings use Article 4.1.5.14:

• 0.75 kN/m or 1.0 kN at the top
• 0.5 kN on a 100 mm by 100 mm area for individual elements
• half the outward load applied inward
• 1.5 kN/m vertical

Wind on glass or solid infill comes on top of that.

Building departments differ on what they want to see. Markham, for example, wants proof of the engineered design before installation. London keeps a list of accepted railing systems. Find out which approach your building department uses before you order.

In practice, the extrusion is rarely the weak link. The anchorage is.

Aluminum Details That Get Missed

Ontario vs BC vs Alberta

All three share the 600 mm trigger, the 900 and 1070 mm heights and the 100 mm sphere. The practical result is that a horizontal aluminum system with a clean test report can be fully legal on a Calgary deck and still fail a final in Mississauga.

Questions Homeowners Ask

For Architects and Specifiers

Five lines worth tightening in a guard spec:

So What Does a Compliant Aluminum Guard Look Like?

Here is what a compliant Ontario guard looks like:

• It's 1070 mm high unless you can point to the exact exemption that allows 900.
• Its openings stop a 100 mm sphere even when someone pushes on the pickets.
• Nothing between 140 and 900 mm, on the guard or beside it, gives a small child a foothold.
• Its posts are bolted into structure that was designed to take the load.