The National Building Code - 2023 Alberta Edition establishes a clear minimum effective depth for wooden stair stringers in residential applications. Section 9.8.7.2.(1) calls for a minimum effective depth of 90 mm (3.5 inches), measured perpendicularly from the bottom of the stringer at its narrowest cross-section. Furthermore, the overall depth of the stringer should not be less than 235 mm (9.25 inches). These metrics are foundational to both the safety of stair assemblies and their ability to resist deflection, creep, and structural failure over time.

Effective depth is not simply the nominal depth of the member but specifically the remaining depth beneath the throat of the cutout after the treads and risers have been notched in a cut stringer. This distinction is critical: excessive notching severely compromises the bending strength of the member, a fact regularly observed in forensic case studies where stringer failures precipitate accidents and costly remediation.

Effective Depth in Construction: The Nitty-Gritty

The field experience consistently underscores the importance of verifying effective depth not just at one location, but along the entire run of the stringer, particularly in cut stringers for open-riser and finished hardwood stairs. The minimum 90 mm is required even at the weakest, most notched portions, not simply the unaltered bulk of the lumber. In multifamily projects, where stairs may serve as primary vertical circulation, missing this requirement exposes both the project and the developer to substantial risk. Alberta’s building inspectors and municipal permit authorities will reference this section in any post-construction deficiency review or site inspection.

Multi-floor projects, especially those involving mid- to late-stage value engineering or design changes, increase the chance of dimension miscalculations. Code-compliant plans may err in field execution, especially when working with thinner 2x10 or 2x12 SPF that have already been bowing during storage or transport. Every millimeter lost to variation, cupping, or splitting can bring a stringer perilously close to the effective depth minimum.

In fire-separated exits, main stairways, or secondary access stairs, the effective depth serves not merely as a static structural criterion but as a critical parameter in a dynamic safety system: deep enough stringers are less susceptible to sudden shear or collapse under crowded egress, live loads, or accidental impact. In renovations, it becomes vital to verify that existing wood stairs in older buildings also comply with these depth requirements, as grandfathering may not be accepted if major changes are undertaken.

Design Detail: Profile Considerations and Notching

Designing stair stringers involves more than selecting the correct size of lumber. The profile of the treads and risers, their placement, and the style of stringer (cut, closed, box, or housed) all influence the throat depth at any given run. For typical residential treads of 254 mm (10 inches) run and 178 mm (7 inches) rise, a standard 2x12 stringer (actual dimension ~286mm) frequently meets code - but only if notch cuts are precise and the wood is free from defects. A careless or overly aggressive saw cut for tread seating can shave precious millimeters from the most critical section and push the stringer beneath code minima. This risk is amplified with field-fabricated stringers and less experienced framing crews.

The 235 mm minimum overall depth is similarly non-negotiable: shallow stringer profiles resulting from site improvisation or the use of undersized lumber (such as 2x10) simply do not meet code, regardless of notching care. Commercial assemblies, or high-traffic stairs, may also require deeper or thicker stringers to accommodate greater live loads and deflection limitations.

Maximum Unsupported Span: Navigating Prescriptive and Engineered Limits

The maximum unsupported run for stair stringers is a chief determinant of both performance and builder liability. It is essential to distinguish between prescriptive limits for residential wood stringers and those for engineered or commercial assemblies. Load, span, and stringer type (cut vs closed, housed vs boxed) must be jointly considered.

The NBC stops short of a simple, universally prescriptive maximum span, instead requiring stringer design based on loading, deflection, and effective depth. As a design aid, industry reference documents fill this gap:

  • Cut Stringers: Standard practice, referencing IRC and the American Wood Council’s deck construction guide, is to limit unsupported spans to 1,829 mm (6 feet) where cut notches significantly reduce the load-bearing section. Cut stringers are almost universally used for site-built wood stairs, and the notched area is always the critical limit.
  • Closed Stringers: With the full section intact and no tread/riser notching, closed (or housed) stringers can extend much further. Typical practice allows up to 4,039 mm (13 feet, 3 inches), although this must always be confirmed with span tables or engineering calcs and code review.

These values provide a safe baseline but require contextual adjustment for real-world scenarios involving higher-than-residential occupancy loads, heavier tread finishes (tile or stone), or non-SPF framing. In multifamily construction, where utilitarian stairwells may stretch further for efficiency's sake, it becomes necessary to verify load, span, and vibration criteria using an engineer’s stamped drawings. Reliance solely on prescriptive tables from American sources is not code-compliant in Alberta if NBC span/load criteria are not also observed.

Practical Span Considerations: Site vs. Drawings

The risk of cross-referencing AWC or IRC prescriptive values without regard for local code amendments or real loading patterns is substantial. In Canadian climates, greater wood movement due to humidity and temperature swings can increase the risk of deflection, induce joint creep, or exacerbate fastener withdrawal - all effects intensified by longer spans. Snow-affected back stairs or unheated egress stairs may need even shorter spans or robust support at mid-span landings.

Builders frequently encounter site realities where drawn stair runs need to be adjusted for mechanical, fire, or accessibility reasons, resulting in longer-than-planned spans or more complex load paths. Proactively accounting for the maximum allowable unsupported run at the design stage avoids the project risk and cost of post-inspection remedial work. In areas with high live load or risk of wheelchair traffic, engineered stringers are often adopted well below the theoretical span maximums, for reasons of robustness, vibration control, and long-term maintenance reduction.

Code Compliance in Practice: Engineering for Non-Standard Conditions

Where stringer runs exceed the basic cut or closed wood member limits, or where atypical floor-to-floor heights are designed, Alberta’s NBC requires such assemblies to be designed and certified by a professional engineer. The result is an increase in both documentation burden and construction scrutiny, but with the compensating benefit of clear load path analysis, deflection checks, and durability optimization. Typical engineered solutions include intermediate landings providing support, switchbacks reducing run, or the use of factory-built steel or engineered wood stringers with tested properties.

Notably, engineered wood solutions (I-joists, LVL, or specialty Ply-Lam products) have moved rapidly into multifamily stair construction due to their ability to span much greater distances with minimal deflection or risk of splitting. Code compliance here is not prescriptive but dictated by manufacturer load data and site-specific engineering. For example, Ply-Lam housed stringers in the 14" depth and 1.5" thickness range may span up to 20 feet unsupported, provided all connection detailing, fire rating, and vibration criteria are met. Yet, these premium products come with added costs and supply chain lead times that must be managed at inception.

Thickness, Spacing, and Support: Code-Mandated Construction Details

Stringer depth and span interact with other critical NBC-mandated parameters: thickness, spacing, and support conditions, each of which is essential for achieving a robust stair assembly that passes inspection and minimizes project liability.

Thickness: Prescriptive or Engineered

Code stipulates a minimum thickness of 25 mm (1 inch) if the stringer is supported along its length (through continuous wall or intermediate blocking). For unsupported stringers - the norm in most site-built stairs - thickness must be no less than 38 mm (1.5 inches), which translates to standard 2x lumber.

In higher-occupancy stairs, or where unusual support conditions (such as recessed hangers, exposed backs, or integrally-framed landings) are used, builders often opt for thicker, engineered stringers. This includes doubled, tripled, or LVL stringers to resist higher live loads and stricter deflection limits. Route, lamination orientation, and moisture content of engineered wood products must all be reviewed, as thickness alone does not guarantee structural continuity if manufacturing or handling defects are present.

Stringer Spacing: Mitigating Tread Deflection and Localized Load

  • For stair runs serving a single dwelling unit, code limits maximum stringer spacing to 900 mm (35.4 inches). This spacing will typically see three stringers (left, right, and center) for residential stairs up to approximately 900 mm wide, which is common in multifamily and townhouse builds.
  • For broader stairs (serving more than one dwelling, or in exit stairs), that maximum shrinks to 600 mm (23.6 inches), which generally requires a fourth stringer or, in wide egress stairs, even more. The aim is to reduce span for each tread, control bounce, and manage increased occupant loading during emergencies.

Vibrations and tread deflection - sources of both code violations and customer complaints - are primarily controlled by this stringer spacing. In luxury multifamily developments where finish quality (tile, stone, hardwood overlay) is a selling point, tighter stringer spacing both insulates against call-backs and increases stair life. Expert trades know that site conditions (warped plates, inconsistent edge bearing, lap joints) can overwhelm even code-compliant spacing, so careful on-site measurement and confirmatory nailing or screwing through the stringer/tread joints is essential.

Support at Top and Bottom: Connection Points Are Failure Points

Even stringers that are perfectly sized and spaced can fail through poor support conditions. The NBC directs that stair stringers must be adequately supported and secured at both the top and bottom, achieved by:

  • Bearing fully on framing plates or hangers at the head of the stair. Ideally, notching for top support should not encroach on the effective depth requirement at the narrowest cross-section.
  • Anchoring to concrete or adequately-sized stringer cleats at the stair base. Slip, movement, or detachment here can result in catastrophic failure, especially under high loads or cumulative seasonal shrink/swell cycles.
  • Ensuring that fasteners meet manufacturer’s (or code) requirements for minimum number, gauge, and edge clearances. Ledger boards and connector hardware must be compatible and free from rust or site damage.
  • Continuous nailing, screwing, or bolting through the entire support length - not gluing, toe-screwing, or placing on hangers alone unless specifically engineered as such.

Field observation and expert review of stair failures consistently identify inadequate or compromised support as a chief cause of collapse, even when stringer size and span are otherwise code-compliant. In multifamily retrofits, care must be taken not to erode bearing at landing connections or base slabs as new finish heights are installed. Early coordination with engineering and finishing trades can eliminate costly rework and inspection delays.

Material Choices: SPF, Engineered Wood, and Their Real-World Consequences

Traditionally, Spruce-Pine-Fir (SPF) has been the go-to for residential wood stair stringers in Alberta. Its cost-effectiveness, local availability, and light weight are unmatched, but so are its variability and susceptibility to twist, check, or creep over time. SPF must be carefully graded (Select Structural or No.1/No.2) and free from defects over its length. The trade-off is that SPF stringers often require close adherence to span limits and careful handling during delivery and storage, as even minor splits at the end grain can propagate beneath load.

Engineered wood products, such as Plywood Laminated Stringers (Ply-Lam), I-joists, and Laminated Veneer Lumber (LVL), provide superior load carrying capacity, uniformity, and span potential. They resist warping and splitting, are dimensionally stable, and offer predictable engineering values. Ply-Lam stringers, for example, offer effective depths beyond standard SPF while maintaining thinner total thickness, accommodating design aspirations for airy or open stairwells with fewer visible supports.

Use of engineered wood is not without considerations:

  • Cost and Lead Time: Engineered options are more expensive (up to 2-3x SPF) and subject to supply chain constraints, particularly in periods of high demand or limited batch runs. Early procurement and trade coordination are essential for projects with tight schedules.
  • Moisture Resistance: Engineered stringers must be protected from site wetting during framing and enclosed quickly. Prolonged exposure can delaminate certain ply products or rust proprietary connectors, compromising both structure and warranty.
  • Connection Detailing: Unique properties and connection requirements mean that standard nailing schedules or hanger types may not be applicable. Always confirm with material supplier and review engineer’s details prior to sign-off.
  • Fire and Code Ratings: Newer engineered products may need specific fire resistance assembly ratings or have limits on unprotected spans depending on the occupancy and fire separation requirements. Ensure labels and certification are on file for municipal review.

For high-end multifamily, townhome, or luxury projects, engineered stringers can facilitate modern, open stair designs and greater rises without introducing bounce or excessive deflection. Their uniformity lends itself well to high-tolerance finishing, reducing rework and warranty issues. When used, sequencing and trade education are key to avoiding errors during install - especially when stairwells double as mechanical chases or are sequenced before mechanical rough-in.

Compliance, Documentation, and Inspection: Navigating Project Workflow

Building in Alberta under the 2023 code places premium importance on up-to-date documentation and process control. With stringer compliance regulated by local authorities and regularly revisited by field inspectors, best practice requires:

  • Documentation Conformance: Maintain a clear record of structural drawings, calculations, engineering stamps (if beyond prescriptive limits), and supplier documentation. Municipal inspectors will expect clear evidence of compliance, especially where engineered products or non-standard spans are used.
  • On-Site Verification: Institute robust field QA/QC routines: verify all stringers for throat depth, thickness, and support at time of rough framing inspection - not after finishing. Keep calipers/tapes on site for trade education and to satisfy inspectors conducting spot checks.
  • Permit Finalization: Secure all required permits pre-construction, including any amendments where stair assembly scope changes (rise, run, stringer type, material). For projects using third-party supplied engineered stringers, include specification sheets and fire/life safety certification in submittals.
  • Continual Code Review: Stay current with code amendments, province-wide clarifications, and city-specific interpretations. Alberta’s code landscape evolves rapidly; assumptions from even one cycle prior (e.g., deck stair code allowance) may be out of date at permit issuance.

In large multifamily projects or developments with phased occupancies, coordination with adjacent trades and municipality on timing and inspection sequence is vital. Stringers are almost never replaced after finishing, so “first-time-right” installation is essential for both schedule and liability containment. For design/build and GC-led projects, early alignment between designer, framer, and permit authority can prevent cascading changes and claims downstream.

Finally, the growth of alternative stair technologies, including steel framed assemblies, modular staircase kits, and hybrid timber/steel systems, does not obviate the need for code review; in fact, it increases documentation burden. Even with complete proprietary systems, stringer depth, support, span, and connection clarity must be attested to at permit and site review - there are no “off the shelf” exceptions where NBC 9.8.7.2.(1) is concerned.

Real-World Failures: Lessons from the Field

Case studies in Alberta frequently show stair stringer compliance issues as the root cause of both aesthetic and catastrophic failures. Examples include:

  • Cut stringers with insufficient throat depth, resulting in mid-span splits under unusual loading (e.g., moving-day heavy traffic or equipment delivery).
  • Reduction in effective depth caused by site alteration - electricians or HVAC trades notching stringers for service runs after framing inspection has passed.
  • Over-spanned cut stringers in long basement stairs, producing excessive bounce and leading to tread loosening, squeaks, and eventual tread breakage.
  • Use of undersized SPF stringers in rental upgrades, where owner-builders misinterpret code to allow 2x10s for all stair runs, triggering failure at inspection or, worse, after lease-up.
  • Retrofit stairs with compromised connections at the bottom due to altered slab height, resulting in stair collapse during routine use.

Alberta’s inspection authorities are increasingly attuned to the subtleties of stair stringer construction as a flashpoint for both safety and building envelope issues (e.g. air or water leakage via poorly integrated assemblies). Proactive code compliance, expert review, and documentation can turn a historic risk area into a project strength, enhancing both asset durability and ownership value.

Integrating Best Practice: Risk Mitigation and Value Creation

Code minimums represent the lowest acceptable performance bar - proactive multifamily developers and GCs in Alberta treat these as a baseline from which to add value.

  • Exceeding Minimums: Specify and budget for deeper/thicker stringers where possible, particularly on main stairs or highly trafficked egress routes. Slight premium at framing yields long-term durability and reduced maintenance costs.
  • Engineer Early, Not Lately: When in doubt on span, load, or design, engage a structural engineer during design, not after framing inspection. Early verification speeds both procurement and inspection signoff.
  • Educate Across Trades: All rough-in trades must understand the inviolability of stringer depth and span: no site-fabricated notches or alterations are permitted post-framing. Visible signage, toolbox talks, and field signage help avoid unintended site modifications.
  • Detail for Connections: Invest in robust connection details at both ends and at all supports - ledgers, hangers, cleats, and their fixings must be sized and installed in accordance with manufacturer and code recommendations. This eliminates a frequent cause of code failure.
  • Frequent Field Verification: Adopt standard operating procedures requiring tape/measure verifications for every stringer, and photo-document all assemblies before closing. Digital records simplify both municipal review and warranty dispute defense.

Integrated project management teams typically revise checklists for field personnel, implement digital QA platforms, and coordinate with municipal inspectors prior to significant changes in stair detailing. Seamless integration of code-mandated requirements with practical construction knowledge results in not just passing inspections but enhancing building reputation, warranty cost control, and occupant safety.

Summary Table: NBC 9.8.7.2.(1) - Alberta Residential Stair Stringer Compliance (2023)

  • Minimum Effective Depth: 90 mm (3.5 in) at narrowest section
  • Minimum Overall Depth: 235 mm (9.25 in)
  • Minimum Thickness (supported): 25 mm (1 in)
  • Minimum Thickness (unsupported): 38 mm (1.5 in)
  • Maximum Span (cut): 1,829 mm (6 ft)*
  • Maximum Span (closed): 4,039 mm (13 ft 3 in)*
  • Stringer Spacing (single dwelling): 900 mm (35.4 in)
  • Stringer Spacing (other stairs): 600 mm (23.6 in)
  • Support: Secure at both ends per manufacturer/code
  • *Must always be verified by calculation for non-standard loads

Looking Forward: Staying Ahead of Evolving Standards

The introduction of the 2023 Alberta code and its focus on both effective depth and span reflects an ongoing evolution toward safer, more reliable, and longer-lasting stair assemblies. With increasing awareness from municipal authorities, project lenders, and insurance underwriters about the risk outcomes associated with stair stringer design and installation, real differentiation is found in strict compliance, proactive engineering, and superior field management.

The best multifamily and residential projects in Alberta distinguish themselves not only by meeting the NBC on paper but by integrating its requirements with a field-proven mentality, avoiding rework, failures, or costly litigation. Material selection, procurement management, and team education are all secondary benefits of a disciplined approach to stringer design and install; few other framing decisions offer such a direct correlation to both project safety and value preservation.

Conclusion

By rigorously meeting or exceeding the minimum effective depth and maximum unsupported span requirements for residential stair stringers detailed in NBC 9.8.7.2.(1), Alberta builders position themselves for enhanced safety, smoother inspections, and long-term building performance. The interplay of thickness, spacing, support, and material selection must be addressed in both design and field execution; documentation and continuous education are mandatory. When properly managed, project teams not only pass code but generate added value for all project stakeholders, from developers and investors to end-users and operations teams. This meticulous approach is a foundational standard at Kingsway Builders.