At the core of stair design for modern Alberta multifamily and single-family residences, open riser stairs offer transparency and a contemporary edge. However, the new requirements of the National Building Code - 2023 Alberta Edition (NBC(AE)), with direct implications from May 1, 2024, stipulate that the vertical opening between treads cannot permit the passage of a 100 mm diameter sphere (NBC(AE) 9.8.4.5.(1)). This singular parameter now governs design intent, manufacturing tolerances, finishing strategies, and on-site verification.

Open Riser Stairs: Achieving Modernity Without Compromising Safety

The open riser stair has become a hallmark of visually light, open-plan interiors, driving client demand in Alberta’s luxury and mid-market multifamily sector. Glass guardrails, steel stringers, and custom timber or engineered treads enable daylight to flow through building cores. Yet, every open riser presents an inherent hazard: what is architecturally “open” can be physically traversable for small children or even pets if not meticulously regulated. The 100 mm rule, unchanged in philosophy but newly enshrined in the 2023 NBC(AE) with precise language and updated references, tightly controls the risk envelope for any open stair.

The 100 mm Diameter Standard: Code Evolution and Jurisdictional Rationale

The Alberta code’s 100 mm (roughly 4 inch) test is not arbitrary. Originating from longstanding pediatric safety studies and harmonized with national and international codes, the test reflects the smallest dimension through which a child’s head (not just a hand or foot) might slip. This head entrapment principle has seen tragic origins in past decades where stairs, especially custom or “designer” installations, left riser gaps large enough to permit falls or entrapment events. The 100 mm sphere was established as a conservative threshold reflecting the anthropometry of most children under the age of six - the highest at-risk group for stair-related injuries.

Today, the regulation applies to all open riser stairs within dwellings, be they principal public stairways in a condominium lobby or the private stairs within a townhouse suite. Older allowances for greater openings (such as “no more than 150 mm high”) are no longer defensible under the Alberta Building Code’s explicit intent.

Translating the NBC-Alberta Edition Text to Construction Reality

The code’s language-“shall be of such construction that the opening between adjacent treads is not more than 100 mm”-has practical ramifications that ripple from detailing to framing and final site inspection.

  • Design Drawings: All project documentation must clearly specify maximum vertical openings of 100 mm between treads at every measurement point, accounting for the true path through which a sphere might pass (including sloped, angled, or shaped treads). The “sphere test” must consider the shortest possible gap, not just vertical height, as stair geometry and nosing profiles can open hidden passages for code inspectors’ testers.
  • Fabrication and Tolerances: At the shop-drawing phase, manufacturers of pre-fab steel or wood stair systems must prove that, after accounting for material expansion, site install tolerances, and applied finishes, no portion of the stair leaves an effective gap greater than 100 mm when measured along any plausible path perpendicular to the treads’ walking surface.
  • Site Installation: Actual field-built stairs often show wider variability than shop-built systems-settling of stringers, uneven tread placement, and post-install shrinkage can all open up “just-legal” gaps beyond 100 mm. Site staff must regularly check openings during installation, not just at handover, using a calibrated 100 mm gauge.
  • Inspection Protocols: Many Alberta municipalities direct their building inspectors to use 100 mm sphere probes or custom tools as part of standard occupancy inspections. Failing the test-even by a single millimeter-requires remediation before occupancy or project sign-off.

Multiple stages of quality assurance must treat this opening not as a nominal dimension, but as a hard stop: any passage above the 100 mm limit is a clear violation, subject to both regulatory action and significant liability exposure in the event of occupant injury.

Risks of Non-Compliance: Liability, Safety, and Project Delay

Tolerances in stair construction often tighten considerably compared to other aspects of interior finishing, because non-compliance with riser openings has both immediate and long-tail legal ramifications. A single open riser stair failing a 100 mm test can compromise:

  • Occupancy Permits: Municipal inspectors-whether in Calgary, Edmonton, Red Deer, or smaller Alberta centers-routinely refuse occupancy or impose costly conditional approvals if any stairway fails the code-mandated sphere test.
  • Warranty Claims: Post-possession, owners or condo boards discovering oversized stair gaps often escalate to Alberta New Home Warranty providers, triggering repair obligations, claims, or, in the case of major defects, buybacks or dispute resolution processes.
  • Personal Injury Liability: Plaintiffs lawyers and provincial regulators regularly cite non-compliant riser gaps in stair-related falls or child injuries, even when other code elements (like guards or handrails) are compliant. Insurance underwriters may exclude coverage or demand high deductibles for non-compliant stairs.
  • Costly Retrofits: Fixes for oversized open risers often require full or partial dismantlement of stair flights, extensive refinishing, or custom inlays-costs that can dwarf preventive design or QA efforts.

Strict adherence to the 100 mm rule is a high-value risk management strategy, not merely a regulatory requirement.

Uniformity and Dimensional Tolerances: Ensuring Comfort, Safety, and Function

The NBC(AE) does not prescribe only the spaces between treads; it also mandates the uniformity of stair riser heights and tread depths. Stairs are among the most habit-driven navigational elements in any building, and sudden dimensional changes increase the risk of missteps, slips, and falls.

Riser Height Tolerances

  • Adjacency Tolerance: Riser heights must not vary by more than 5 mm between adjacent treads. This guards against “trip-point” transitions where a user’s muscle memory anticipates a consistent lift.
  • Total Flight Tolerance: No more than 10 mm difference between the tallest and shortest risers across the entire flight. This cumulative tolerance accounts for both initial construction and dimensional changes due to site settling, shrinkage, or flooring overlays.

Uniform risers are not only about code compliance: they vastly reduce accident claims-particularly for children and the elderly.

Tread Depth (Run) Uniformity

  • Adjacency Tolerance: Maximum 5 mm variation in tread depth between adjacent treads.
  • Total Flight Tolerance: Up to 10 mm difference between deepest and shallowest treads in the flight.

Consistent tread depth presents a predictable walking pattern. On open riser stairs, the absence of the visual “read” of risers means users rely more on proprioception and floor finish contrasts. Designers and builders should, therefore, specify finishes and materials that do not visually obscure tread edges and ensure robust QA on stringer layout and tread placement.

Core Dimensional Requirements: Riser Heights and Tread Depths for Private Stairs

For private stairs within dwelling units, the code’s prescribed riser and tread ranges support both accessibility and comfort. The NBC(AE) directly specifies:

  • Riser height: Not less than 125 mm, not more than 200 mm
  • Tread depth (run): Not less than 210 mm, not more than 355 mm

Open riser designs must keep the overall combination of riser height and tread depth within these boundaries, factoring in finished material overlays (flooring, nosing, anti-slip additions) so the as-built dimensions satisfy code. Failing to do so can lead to forced reconstruction before occupancy or significant post-possession claims.

For multifamily projects (townhomes, condos) where dozens of units may include individual stairs, repetition can introduce cumulative error, propagating systematic non-conformance if not vigilantly checked at the factory and on-site.

Nosing Design: Minimizing Trip Hazards and Supporting Visual Clarity

Nosing-the protruding front edge of each tread-is tightly regulated in both its projection (not more than 25 mm past the face of the tread below) and detail (rounded/beveled between 6 mm and 13 mm horizontally from the front). On open riser stairs, proper nosing not only minimizes tripping risk, but also enhances stair “readability” for users by defining the toe line.

  • Projection and Overhang: Excessive nosing projection creates a leverage hazard, encouraging users’ feet to slip off the tread. By restricting to 25 mm, the NBC(AE) reduces this risk, while still permitting subtle shadowlines and design cues.
  • Edge Detailing: Nosing edges should be carefully machined, sanded, and finished to meet the 6-13 mm rounded or beveled requirement. Mechanical fastener heads, metal inlays, and coverings must respect this dimension. Sharp or square noses increase the likelihood of toe or heel trip incidents, especially on poorly lit stairs or in units with elderly occupants.
  • Material Considerations: Timber and engineered wood nosings must factor expansion and contraction (especially across Alberta’s dry/cold seasons) so that projection and edge radius remain compliant after settling. On concrete or steel treads, quality control must prevent chipping or sharp edge exposure at high-traffic zones.

For high-end multifamily developments, careful attention to nosing details supports project value by reducing post-possession repairs and warranty calls triggered by damaged nosings or user complaints.

Design & Engineering Considerations: Reconciling Code With Architectural Objectives

Navigating code constraints while pursuing architectural intent places a premium on coordination between the design, engineering, and construction disciplines.

  • Stair Geometry Coordination: Open riser stairs often include “floating” or cantilevered treads, curved stairs, or nonrectilinear configurations. Each introduces unique sphere test path geometries. Design teams must simulate not only vertical but angulated paths to ensure no contravention of the 100 mm rule.
  • Guardrail and Stringer Detailing: Integration of glass panels, spindles, or perforated steel guards must be situated so their cumulative geometry does not inadvertently widen the effective riser gap. Offset stringers or irregular edge finishing (such as post-weld grinding) can open up code-defying gaps even after initial passes.
  • Lighting Placement: Open riser stairs transmit light, but code-compliant illumination is crucial. Inadequate lighting can obscure treads, causing users to misjudge distances-especially where open risers alter shadow patterns. Specifying indirect lighting or tread-integrated illumination can enhance both aesthetics and safety.
  • Material Flexibility Allowance: Alberta’s relative humidity and temperature swings can cause significant wood shrinkage. Specification must call for pre-acclimatized treads, robust mechanical fasteners, and adjusted clearances so “as-placed” riser gaps do not later widen past 100 mm in January.

Manufacturing Excellence: Precision Fabrication for Compliance Assurance

Prefabricated stair manufacturers serving Alberta have adapted to the code’s anti-variability demands. A successful supplier will:

  • Digitally model each stair, including anticipated finish layers (laminates, paint, tread overlays), to guarantee after-finish compliance at every point;
  • Factory-test each completed flight with certified 100 mm probes, documenting results for both in-house QA and client approval;
  • Provide installation guides documenting measurement points and emphasizing the need for in-situ verification after any site adjustment, shimming, or finish carpentry;
  • Ship with pre-fit filler strips or blocking “kits” to be on hand in the event unpredictable site conditions open a gap.

Custom fabricators must be particularly cautious; visually irregular treads or staggered stringer arrangements demand special expertise in performing “worst-case path” sphere tests beyond just the code’s vertical diagram.

On-Site Execution: Builder and Installer Best Practices

Site installation of open riser stairs demands continuous vigilance. Variation can creep in through subfloor settling, small errors in stringer placement, or finish scope changes. Teams should:

  • Measure each opening with a calibrated 100 mm test gauge (not just a tape measure), ensuring all vertical and angulated paths are tested. Routine checks minimize rework costs and identify issues early.
  • Install stair flights before final finishes where possible to allow for post-install adjustments without damaging complete spaces.
  • When riser gaps approach the maximum allowable, consider “future proofing” by designing treads to accept supplementary filler strips or decorative riser bars that can be retrofitted without major aesthetic impact.
  • Maintain close collaboration with site supervisors and QA/QC managers to immediately flag any tolerance drift caused by subtrade errors (such as overzealous sanding or misaligned flooring transitions).

Final walk-throughs with project managers should always include sphere tests at every stair, with detailed photo documentation and as-built reports.

Minimizing Occupant Risk, Warranty Callbacks, and Brand Erosion

Open riser stairs often attract the attention of building owners, condo boards, and discerning tenants. Frequent concerns relate to perceived “children’s safety” or “pet entrapment” even for code-compliant installations. Proactive communication-supplying test documentation and demonstrating compliance-prevents friction and complaints.

  • Provide as-built compliance reports to clients and building managers, including photographs of the 100 mm sphere test at each flight.
  • Educate property managers and realtors on code rationale, emphasizing why gaps larger than 100 mm are impermissible irrespective of “industry custom” or anecdotal examples.
  • Document every code-related communication; non-compliance can trigger expensive warranty interventions, especially in projects serving families or pet owners.

For developers holding properties as rentals, compliant stairs reduce legal exposure to tenant injury claims and bolster asset value during resale or refinance events.

Detailed Case Scenarios: Applying NBC(AE) 9.8.4.5.(1) in Alberta Projects

Scenario 1: Mid-Rise Multifamily, Engineered Wood Treads

A concrete central core supports engineered timber open riser stairs in a 6-storey condo. Early design stipulated 105 mm gaps between raw structural treads to accommodate final finish overlays (laminate plus anti-slip edge). At final install, the overlays varied-some at 2 mm, others 4 mm-leaving several gaps over 100 mm when measured on the diagonal. Three units failed city inspection; the fix required in-situ installation of custom steel riser bars powder-coated to match tread color, plus expedited shop drawings and emergency on-site labor. Final retrofit cost exceeded $18,000 for the three flights, delaying occupancy certification by 11 days.

Scenario 2: Urban Infill Duplex, Site-Built Stairs

A custom homebuilder installed open riser stairs framed on-site, with treads fabricated from kiln-dried oak. Alberta’s late winter saw unusually low humidity; by possession in May, the oak had shrunk by 2 mm per tread, changing the measured gaps from 97 mm to 101-102 mm in several locations. The builder provided plug-in decorative steel “riser tracks” that clipped behind the tread nosing, tightening the gaps to 98 mm and passing re-inspection. The solution became a standard value-add in subsequent units.

Scenario 3: Rental Apartment, Metal/Glass Hybrid Stairs

Architects specified a transparent stair with glass treads and minimal steel stringers. As-built, the collection of nosing glass and steel support bars produced an angled gap where the glass cantilevered past the last support. The code official measured the worst-case opening-perpendicular to the anticipated child head path-and found a 104 mm “hidden gap.” A structural silicone bead and engineered acrylic riser insert (tested for impact resistance) brought the stair into compliant range without compromising the intended open aesthetic.

Compliance Strategies: Design, Construction, and Ownership Phase

  • Early Code Consulting: Engage code consultants at schematic design, not just construction drawing review, to verify all intended stair details will pass 100 mm sphere tests.
  • Mock-Ups: For custom or high-iteration stairs, construct full-scale on- or off-site mock-ups, testing gaps physically before duplicating assemblies across projects.
  • Material Pre-Testing: In Alberta’s climate, pre-season sample installation and measurement identifies likely shrinkage or expansion scenarios, enabling proactive adjustments.
  • QA/QC Audits: Define stair compliance as a critical path hold point on project schedules; do not proceed to subsequent finishing works without certified pass/fail reports.
  • Ongoing Training: Regularly update site and shop staff on evolving code requirements and best practices, using real-world inspection failures as teaching tools.
  • Warranty Provisions: For rental or condo developments, include inspection and riser gap verification as a standard component of annual maintenance programs, reinforcing both code compliance and occupant safety.

Future-Proofing Alberta Stairs: Designing Beyond Mere Code Minimum

Though 100 mm is today’s standard, some designers and building science experts advocate for even tighter openings-especially in family-oriented housing or projects marketing to demographics with children and pets. While not mandatory, reducing gaps to 90 mm or integrating passive barriers (like partial riser infills) can preempt stakeholder concerns and minimize future legal exposure if code requirements tighten further.

Similarly, technology-driven solutions-sensor illumination, anti-slip surface integration, and child-safety design cues-may in future supplement code-mandated physical limits, as industry trends and insurance mandates place ever-more focus on occupant injury prevention.

Summary Table: NBC(AE) Requirements and Key Strategies

  • Maximum opening between open riser treads: ≤ 100 mm (use sphere test at all plausible points)
  • Riser height (dwelling unit stairs): 125 mm - 200 mm; uniform to within 5 mm (adjacent), 10 mm (flight)
  • Tread depth: 210 mm - 355 mm; uniformity same as risers
  • Nosing projection: ≤ 25 mm; edge rounded/beveled 6-13 mm
  • Best practices: Engage code expertise early, prioritize mock-ups, sequence QA/QC before finishing, educate trades and owners

Imperatives for Developers, Builders, and Asset Managers

Failure to meet open riser stair requirements under the NBC(AE) 2023 is not merely a technical misstep: the economic risk, reputational cost, and occupant safety stakes all escalate rapidly. Tightly integrated collaboration at every stage-design, engineering, shop drawing, field build, and post-handover-is necessary to achieve both architectural vision and uncompromising compliance. As new versions of the code evolve and as Alberta’s urban density grows, the demand for rigor in stairway construction will only intensify.

Kingsway Builders delivers code-exacting multifamily construction solutions that meet every nuance of Alberta’s latest stair safety and design standards.