If you build in BC, you know the rule: 100-millimetre sphere. It can’t pass through any opening in a required guard. Pickets, balusters, cables, glass joints, none of them can admit the gauge.
The rule itself is straightforward. The reason guards fail it isn’t ignorance of the rule. It’s that the spacing was set on site by eye, the end conditions weren’t planned, or the bottom rail was treated as cosmetic instead of as infill. The 100 mm rule is the most-failed guard requirement we see in Metro Vancouver, and almost every failure is preventable at the shop drawing stage.
What the rule actually says
The BC Building Code requires that the openings in a guard prevent a 100-millimetre sphere from passing through. The intent is well-documented: it’s a child safety provision, derived from historical incidents where a young child’s head passed through a wider opening and the child either fell through or was trapped.
The “opening” isn’t just the gap between balusters. It’s any opening in the infill, including:
- The space between vertical balusters or pickets
- The space between horizontal cable runs
- The gap between the bottom rail and the walking surface
- The gap at the end of a guard against a wall or post
- The space between glass panels or in any glass joint
- The openings in any decorative or perforated infill
If a sphere can pass anywhere along the guard, the guard fails.
Bay layout: the math has to work
The most common failure mode we see is uneven spacing at the end of a run. A long balcony guard divided into even bays for the first three-quarters of the run ends with an oddly-sized last bay because the run length wasn’t divisible by the chosen spacing.
The fix is to do the math on the shop drawings before fabrication. Total run length, post locations, number of bays, and resulting clear opening, all calculated on paper before anything is cut. The end condition gets the same attention as the middle of the run. If a few millimetres need to come out of every bay to make the end work, we adjust uniformly across the whole guard, not just at the end.
This is part of what shop fabrication buys you. A custom shop-fabricated guard arrives with the spacing pre-determined and verified against the inspector’s gauge. A site-built guard relies on the framer’s eye and the framer’s tolerance for the math, which is variable.
The bottom of the guard
Pickets running cleanly from the top rail down to a bottom rail, and the bottom rail sitting 100 mm or more off the deck, admits the sphere underneath. The infill is fine. The gap nobody designed is the problem.
We detail the bottom rail close to the deck surface. There’s usually a small clearance for drainage and to avoid trapping debris against the rail, but the clearance is less than 100 mm. On exterior decks where leaf debris collects, the clearance is just enough for water and small debris to wash through, not enough to admit the sphere.
Inspectors check this routinely. A guard that looks clean from a distance and fails at the bottom rail is a frequent inspection write-up.
End conditions matter as much as the middle
The gap between the last picket and the post, or the gap between the guard and the wall it terminates against, has to satisfy the 100 mm rule. A guard with even spacing through the middle but a 120 mm gap at the wall return fails.
We detail the end conditions on the shop drawings the same way we detail the middle: with explicit picket-to-post and picket-to-wall dimensions. The picket adjacent to a post might be welded to the post or set with a specific clear opening; either way, the dimension is on the drawing and consistent in the field.
Cable rails: deflection, not rest
For cable infill guards, the sphere rule is enforced with the cable deflected by hand pressure, not at rest. A cable rail that satisfies the spacing at rest can fail under inspection if the cable deflects more than its spacing allows.
The fix is shorter post spacing, tighter cable tension within the manufacturer’s specification, or intermediate pickets that the cables run through without anchoring. Designing for the deflected condition from the start is what keeps the rail compliant in actual use.
Glass infill: the joint is the gap
Most modern glass railing systems are designed to comply with the sphere rule, with glass panels set tight to posts or running continuously between structural fixings. The conversation gets more nuanced when:
- A glass panel terminates against an existing structure with an irregular gap
- A custom glass infill design includes decorative gaps between panels
- A frameless glass system uses standoffs that create small gaps at the connection points
We check every glass joint on the shop drawings to confirm no gap admits the sphere. On a custom glass design where decorative gaps are part of the intent, the design has to be reconciled with the code from the start, we won’t fabricate a non-compliant guard because the rendering looked nice.
Stairs are a separate problem
On a stair guard, the sphere rule applies measured perpendicular to the rake of the stair, so the vertical infill spacing has to satisfy 100 mm in the rake plane. Additionally, the BC Building Code has a specific rule for the triangular opening formed at the bottom of a stair guard between the bottom rail, the tread nosing, and the stringer. That opening is governed separately because of the geometry created by the sloped stair.
For mono stringer stairs and other architectural stair configurations, the guard detailing has to address both the standard sphere rule on the verticals and the triangular opening at the base. Shop drawings on a custom architectural stair handle both.
Decorative and perforated infill
Custom laser-cut steel panels, expanded mesh, perforated steel, and other decorative infill types can satisfy the sphere rule as long as no opening in the pattern admits the gauge. We’ve used laser-cut privacy screens and perforated panels as guard infill on contemporary Vancouver projects, with the pattern designed so every opening is less than 100 mm.
The same rule applies to glass with bird-strike patterns, decorative grids, or any custom infill. The pattern designer has to verify the maximum opening dimension against the code, and we verify it again on the shop drawings before fabrication.
What we put on the shop drawings
Every guard we fabricate has the following dimensioned on the shop drawings:
- Centre-to-centre baluster spacing
- Clear opening between balusters
- Bottom rail height above the walking surface
- End conditions at posts and walls
- Picket count per bay
- Total run length and bay-by-bay verification
If the drawing isn’t dimensioned, the field won’t be either. The discipline at the drawing stage is what determines whether the inspection passes.
What inspectors actually check
The inspector arrives with a sphere gauge, a sphere or disc of the correct diameter, and slides it along the guard, checking every opening. They check the verticals between pickets, they check under the bottom rail, they check the end conditions, and on cable rails they push the cables to deflect them.
If the gauge passes through anywhere, the guard fails and the work doesn’t pass the inspection. The fix is to modify the guard until the gauge no longer passes, which sometimes means adding pickets, lowering the bottom rail, or re-tensioning cables. All of which costs significantly more after install than getting the layout right before fabrication.
How to specify it correctly
If you’re specifying a guard on a Vancouver project, residential or commercial, make sure the shop drawings show the exact spacing, the bottom rail dimension, the end conditions, and any decorative pattern’s maximum opening. Don’t leave it to “field condition.” A custom shop fabricating to detailed shop drawings can deliver a guard that passes the sphere test reliably on the first inspection. A site-built guard relying on the framer’s judgment is rolling dice on whether the spacing happens to work out.
If you have a guard scope on a current project, new construction, renovation, or replacement of one that failed inspection, send us the drawings and we’ll detail it so the inspector’s gauge has nowhere to go.
