What are the maximum gap sizes allowed in pool fencing under Ottawa bylaws?

Under Ottawa's bylaws and the Ontario Building Code, the maximum gap size allowed in pool fencing is 100 millimetres (approximately 4 inches) — meaning no opening in the fence, gate, or barrier may permit passage of a sphere 100 millimetres in diameter, whether that opening is between pickets, beneath the bottom rail, between the fence and a post, or at any other point along the enclosure. This 100-millimetre rule is the single most frequently cited pool fencing requirement by Ottawa building inspectors and By-law officers, and it applies uniformly to all fence types, materials, and styles.

The 100-millimetre standard comes from Section 3.10.2 of Division B of the Ontario Building Code, which governs swimming pool enclosures for all residential pools in Ontario with water depths exceeding 600 millimetres. The dimension was chosen because it corresponds approximately to the width of a small child's head and torso — if a sphere of that diameter cannot pass through an opening, a young child cannot squeeze through it. The test is strictly physical: inspectors carry a 100-millimetre sphere or template and attempt to pass it through every suspect opening in the enclosure. If it fits through anywhere, the fence fails.

The 100-millimetre rule applies to several distinct areas of the pool enclosure, each with its own common failure points in Ottawa installations:

Between vertical pickets or balusters: This is the most straightforward measurement — the clear space between adjacent vertical members must not exceed 100 millimetres. For aluminum pool fencing, the industry standard picket spacing is 95 millimetres on centre, which typically produces a clear gap of approximately 80 to 85 millimetres after accounting for the picket width. Wrought iron fences often have wider spacing to accommodate thicker balusters, so the clear gap must be measured rather than assumed. Wood fence boards can shrink over time as the wood dries, especially in Ottawa's low-humidity winters, widening gaps that were compliant at installation. If you have a wood pool fence, check picket gaps annually in spring — boards that have dried and shrunk by even 10 to 15 millimetres can push a previously compliant gap over the 100-millimetre limit.

Between the bottom of the fence and grade: The gap beneath the fence is one of the most common compliance failures in Ottawa because of our freeze-thaw cycle. Frost heave pushes fence posts upward, soil settles beneath the bottom rail, and grading changes from landscaping or erosion can all create bottom gaps that exceed 100 millimetres. On sloped terrain — common in Ottawa neighbourhoods like Westboro, Rockcliffe, and parts of Kanata built on the Champlain Sea clay ridge — the gap between a level bottom rail and the downhill grade can be well over 100 millimetres at the low point. Solutions include stepping or racking the fence panels to follow the grade, adding a bottom rail or trim board, or building up the grade beneath the fence with compacted granular material. Fixing bottom gaps typically costs $10 to $30 per linear foot depending on the method used.

Between the fence and adjacent structures: Where a pool fence meets a house wall, a deck post, a retaining wall, or any other structure, the gap at the junction must not exceed 100 millimetres. This is a surprisingly common failure point in Ottawa because fence installers sometimes leave generous gaps at these transitions to accommodate seasonal ground movement or to avoid drilling into masonry. A rubber or metal flashing strip to seal the gap costs $15 to $40 per junction and takes minutes to install.

Gate clearances require special attention. The gap between the gate and the gate post, and the gap beneath the gate, must both comply with the 100-millimetre maximum. Because gates swing on hinges, the effective gap changes as the gate moves through its arc. Ottawa inspectors measure the gap with the gate in the fully closed (latched) position. Spring-loaded self-closing hinges can create a wider gap at the hinge side as the gate is pulled open, but the compliance measurement is taken at the latched position only. Gate bottom clearance is particularly prone to failure on interlock or flagstone pool decks in Ottawa, where frost heave can raise individual pavers and reduce the clearance to the point where the gate drags — prompting the homeowner to raise the gate, which then creates an over-100-millimetre gap on the opposite side of the swing arc.

Chain-link fencing has its own gap considerations. Standard residential chain-link mesh has a diamond opening of approximately 50 millimetres (2 inches), which comfortably passes the 100-millimetre sphere test — the diagonal of the diamond opening is larger than 100 millimetres, but the sphere cannot pass through the diamond because the opening narrows as the sphere pushes against the wire. However, chain-link mesh can stretch over time, especially under the weight of snow and ice loading in Ottawa winters, enlarging the diamond openings. Severely stretched chain-link mesh can fail the sphere test. Additionally, the horizontal rails at the top and bottom of chain-link fences create a climbing ladder — while not a gap issue per se, this climbability deficiency is often flagged alongside gap measurements during Ottawa pool fence inspections.

For decorative fences with ornamental designs — scrollwork, circles, arches, or other openings — every single decorative opening must individually pass the 100-millimetre sphere test. Some ornamental iron and aluminum fence designs feature circles or ovals within the panel as decorative accents. If any such opening exceeds 100 millimetres in its smallest dimension, the fence fails. Ottawa fence suppliers who specialize in pool enclosures stock designs that incorporate decorative elements within the 100-millimetre limit, but generic ornamental fencing purchased from big-box retailers may not be pool-compliant.

How Ottawa Inspectors Test Your Pool Fence Gaps

When an Ottawa building inspector or By-law officer evaluates your pool enclosure, they test every opening they can identify with a 100-millimetre sphere or go/no-go gauge. The inspection is thorough: they check the middle of spans (where fence sections can bow outward, widening gaps), the bottom at multiple points along each run (where grade variation creates different clearances), every gate in closed position, every junction with walls or structures, and every decorative opening. A single failure point means the entire enclosure fails, even if 99 per cent of the fence is compliant. The typical re-inspection fee if you fail and need to correct deficiencies is $75 to $150 in Ottawa.

Wondering whether your existing pool fence gaps meet code? Ottawa Pool Installation connects homeowners with local pool enclosure specialists who can measure every opening, identify any non-compliant gaps, and make corrections before your swimming season starts.