Concrete footings are the foundational element for all forms of construction, serving as the principal means to transfer loads from above-grade structures into the underlying soil. In Alberta, these elements must not only comply with expected load transfer requirements but must also be constructed to account for soil variability, harsh climatic cycles, and durability requirements unique to the region. At the heart of footing size parameters lies NBC 9.15.3.8.(1), which stipulates that the minimum allowable thickness of concrete footings supporting exterior walls must be the greater of 100 mm (4 inches)-or the width of the projection of the footing extending beyond the element being supported.

The Code Requirement: Clause-by-Clause Analysis

Per NBC 9.15.3.8.(1):

  • Concrete footings supporting exterior walls must be no less than 100 mm thick, or
  • The thickness must at least match the width of the projection beyond the supported element.

This appears straightforward-yet the implications for design, construction, inspection, and long-term building performance are far-reaching, particularly as they relate to accommodating variable structural demands and site conditions frequently encountered in Alberta multifamily construction.

Illustrating the Projection Rule

Where a 200 mm thick exterior wall sits on a 500 mm wide footing, a 150 mm projection remains on each side. NBC requires that the thickness of the footing in this case be at least 150 mm, even though the base minimum is 100 mm. If the projection increases to, say, 200 mm-perhaps because soil bearing capacity is unusually low-the thickness requirement immediately jumps to match that projection. Failure to do so could result in undermined load distribution and potential premature failures, especially in soils prone to settlement or lateral movement.

Structural Rationale: Shear Failure Planes and Concrete Footings

The logic behind linking footing thickness to its projection above the supported wall is rooted in the behavior of unreinforced concrete under load.

Shear planes in concrete commonly form at approximately 45°, emanating from the wall’s edge downward and outward in the plane of the footing. When the projection is excessive in relation to thickness, there exists an inherent risk that a diagonal shear crack could propagate through insufficient concrete cover, leading to potential punching shear failure.

By mandating a minimum thickness equal to the overhanging width, the code attempts to ensure the concrete mass has adequate depth to develop proper shear resistance along those load-transfer planes. This is a conservative approach, especially in regions with active soils and frost, both formidable factors in Alberta. The risk of footing failure-due either to differential settlement or shear cracking-can be significantly mitigated by adhering strictly to the projection-based thickness criterion.

The Minimum: 100 mm (4 in.)

Where projections are modest (100 mm or less), the 100 mm default thickness prevails. This threshold, tested by generations of builders, has proven adequate for single- and multifamily construction typical of the region, provided other factors (like soil bearing and reinforcement-where permitted) conform to code minimums. The logic is clear: 100 mm provides a workable cross section for safe load transfer and constructibility, supporting rebars if engineered, and facilitating proper concrete placement in even the most adverse weather.

Practical Footing Design: When Projections Exceed the Minimum

Cases regularly arise where established (or field-measured) soil bearing pressures demand that the footing be notably wider than the wall. For instance, on a challenging site with low plastic clay requiring a 600 mm wide spread footing beneath a 200 mm concrete foundation wall-resulting in a projection of 200 mm on each side-the required minimum thickness is now 200 mm, no exceptions. Adhering to the minimum would result in under-reinforcement against shear, cracking, and possibly overturning under eccentric or lateral loads.

This relationship compels structurally informed design, prompting advanced general contractors and engineers to assess the interplay between soils, loading, and geometry instead of defaulting to one-size-fits-all solutions. It also accommodates the reality that larger multifamily loads or tall townhouse end walls may necessitate both wider and thicker footings to ensure long-term stability, even if construction costs rise moderately as a result.

Example Scenarios

  • Scenario A: Standard Bearing Soil
    • Typical residential structure, 200 mm wall, 400 mm footing.
    • Projection = (400 mm - 200 mm) / 2 = 100 mm.
    • Required thickness: 100 mm (applies, as 100 mm = projection).
  • Scenario B: Poor Bearing Soil
    • Three-storey multifamily, 200 mm wall, 600 mm footing for reduced bearing (<1.5X factor for clay).
    • Projection = (600 mm - 200 mm) / 2 = 200 mm.
    • Required thickness: 200 mm (matches projection).
  • Scenario C: Narrow Projection
    • Structural upgrade, using narrow 250 mm footing under 190 mm ICF form.
    • Projection = (250 mm - 190 mm) / 2 ≈ 30 mm.
    • Thickness: 100 mm minimum still governs-it exceeds the 30 mm projection.

From Code to Mud: Construction Realities in Alberta

Constructing compliant footings in Alberta’s active climate and soils is not a theoretical exercise. Thermal cycling, deep winter frost lines, and dramatically variable soils-from high-plasticity plains clay to sandy glacial tills and high water table riverine deposits-all demand adjustments far beyond simple code adherence.

Soil Bearing Capacity: The Real Driver Behind Footing Width (and Thus, Thickness)

Effective spread footing performance depends on selecting a width that aligns to the allowable soil bearing pressure. Granular soils, particularly when water tables fluctuate, rapidly lose bearing-an issue highlighted not just in the code, but also in practical field experience throughout Southern Alberta during spring thaw or after heavy rainfall. Narrow, undersized footings are prone to rotation and differential settlement leading to pronounced wall cracking above grade.

  • Low-Bearing Soils: Increased width is non-negotiable; as width grows, so does required thickness under NBC 9.15.3.8.(1).
  • Urban infill sites: Reworked fills or poorly compacted subsoils often require engineered solutions, oversize footings, or even mat systems-again, with thickness dictated by the largest projection.

Bear in mind that code minimums are precisely that. Alberta builders regularly exceed the 100 mm default where practical experience or geotechnical advice indicates a risk, and municipal inspectors in some jurisdictions have been known to insist on 150 mm as a builder standard to reduce callback risk, particularly on larger projects or problematic soils.

Footing Sizing for Heavier Loads: Multi-Storey and Masonry Veneer Assemblies

The NBC is explicit: masonry veneer or multi-storey masonry assemblies must sit on proportionately wider-and sometimes thicker-footings. The reason is not only the increased dead and live loads, but also the continuous vertical load imposed by these materials with far lower allowable compressive deformations compared to lightweight wood or steel frame assemblies. Overstressed footings beneath such walls risk lengthwise cracking, corner rotation, and unacceptable settlement rates.

Every additional storey compounds this effect. While thickness remains pegged to projection per the code, practical engineering and good construction economics often see thickness specified above code minimum-to ensure sufficient mass for cold weather curing, resistance to frost heave, and to accommodate rebar cages where specified. For dense multifamily or podium projects, engineering sign-off will frequently prescribe even more robust detailing to mitigate risk of long-term differential movement.

Steel Reinforcement: A Qualified Exception

The NBC recognizes the role of steel reinforcement in shaping footing design. In some circumstances-especially for engineered, reinforced footings-reductions in thickness may be permissible if the reinforcement is sufficient to compensate for potential shear failures (provided all loads, soil conditions, and code requirements are satisfied). This strategy, however, must bear the stamp and calculation package of a licensed structural engineer familiar with Alberta soils and loading conventions.

  • Slender Footings with Rebar: Possible for interior partitions or light structural scenarios.
  • Exterior Walls: For critical loads or on suspect soils, engineers will mandate minimum thicknesses aligning to projection (or more), typically with robust rebar placed on proper chairs to ensure depth of cover and effective crack control.

Consistent best practice remains: never reduce thickness below projection for exterior wall elements, except by sealed engineering letter. Any value engineering initiative cutting corners on this point risks future structural claims and expensive callbacks, especially given Alberta’s aggressive residential warranty regime.

Formwork and Quality Control: Achieving Precision on Site

Inconsistent field installations are a common source of code violations and premature failures, especially as construction schedules compress due to development or weather delays. The use of adjustable, well-anchored forms is crucial to ensuring both compliance and quality.

  • Form-based Construction: Delivers predictable, code-compliant thickness at every point along the wall, regardless of soil undulations or site super elevation.
  • Integrated Benchmarks: Setting string lines and laser station benchmarks allows easy spot-checking of thickness, minimizing the risk of underpouring in sloped or frost-heaved subgrade.
  • Inspection Access: Inspections and photographic documentation prior to pour provide essential records, especially on high-value multifamily projects or where risk insurance requirements demand third-party verification.

Floating footings-those simply cast in a trench-may deliver the minimum in a perfect world. However, subtle irregularities in trenching, overdig, or freezing weather can shrink footing depth at the worst time. Forms mitigate these risks and streamline verification, helping GCs and site supers demonstrate compliance in the event of a performance audit.

Weather Effects in Alberta: Curing and Exposure Risks

Alberta’s climate-marked by wide temperature swings and recurring freeze-thaw cycles-directly influences how concrete footings should be constructed and cured to achieve both code-mandated thickness and required material integrity. Shallow (100 mm) footings are at especial risk from rapid surface freezing, which can halt proper hydration and compromise performance at both the compression and tension face. Heavier, code-compliant (or better) thicknesses offer additional thermal mass, slowing temperature change and minimizing the risk of cold seams or early-age cracking.

  • Cold Weather Concreting: Where pours occur near freezing, both forms and blankets provide additional energy savings, and thicker footings better retain exothermic heat, enabling more controlled strength gain and performance longevity.
  • Spring Thaw Considerations: On early-season projects, thicker footings are far more tolerant to subgrade heave or settlement as frost leaves the ground, reducing the risk that initial shrinkage cracks become propagation sites for long-term movement or moisture infiltration.

In Alberta's climate, the margin for error with thin footings is minuscule. Experienced concrete subs routinely increase thickness 10-25% above minimum, particularly where weather, schedule, or site access conspire against perfect working conditions.

Deviations From NBC Minima: When and How They’re Justified

Situations abound where site realities or programmatic requirements necessitate either exceeding the minimum requirements-or, in rare edge cases, proposing a justified reduction below them. The NBC specifically contemplates this flexibility under the authority of a licensed structural engineer, who must evaluate all loads (vertical, lateral, seismic where appropriate), soils data, and the overall performance history of similar builds before agreeing to any departure from 9.15.3.8.(1).

Cases Justifying Increased Thickness

  • Poor or Expansive Soils: Swelling clays or poorly drained fill demand more robust footings to guard against settlement or heave, with matching thickness along increased projections.
  • Point Loads or Concentrated Columns: Even if supporting an exterior wall, presence of a significant column adjacent to or integrated with a wall may drive up minimum footing thickness both for local shear and punching resistance.
  • Thermal Performance and Durability: High-performance buildings may specify thicker footings for better insulation and reduced risk of frost penetration along the thermal envelope, beyond pure structural necessity.

Code-Justified Reductions

  • Engineered Reinforcement: Provided full calculations and inspection, footings for non-critical elements (sometimes interior partitions) may go below projection-based minimums, but never for exterior walls without specialty engineering.
  • Slab-on-Grade Systems with Integrated Grade Beams: These hybrid assemblies, engineered for their unique load path, may permit optimized thickness and rebar based on detailed finite element analysis-again, only under engineering seal.

It remains rare in Alberta practice for engineered reductions to be carried out for exterior walls of residential or multifamily buildings, since the cost savings are minor compared to the risk of callbacks or building envelope warranty claims-especially given the sector’s risk profile and competitive insurance climate.

Direct Construction Cost Implications: Value Engineering Versus Risk Management

The pressure to value engineer foundation elements is unrelenting in today’s market. Concrete costs, formwork labor, and accelerated construction schedules press GCs and site supers to squeeze efficiency out of every pour. Yet pushing below the proven minimum thicknesses quickly transitions from cost management into red-flag territory:

  • Material Cost Curve: Jumping from 100 mm to 200 mm in thickness-when driven by projection-increases concrete volume (and cost) by 100%, but the actual dollar delta on most projects is modest relative to total foundation or superstructure cost.
  • Remediation Premium: The cost of repairing failed or substandard footings can easily wipe out any savings-a reality made more pressing by Alberta’s legal framework for design/build liability in multifamily work.
  • Inspection and Insurance: Modern risk-mitigation contracts demand evidence of full code compliance; premature reduction in thickness can result in failed inspections, uninsurable projects, or denied warranty claims-costs which dwarf even significant material increases.

Skilled project managers therefore take a strategic view: exceed code minimums where warranted by site conditions, err on the side of overbuilding where bearing pressures are uncertain, and always document every engineering and construction decision with quality assurance photos and reports for post-occupancy defense.

Inspection, Testing, and Quality Assurance: Best Practices

Alignment with NBC 9.15.3.8.(1) is not simply a matter of following drawings. It is a continuous, iterative process involving design, field measurement, inspection, and documentation. On Alberta sites-especially those subjected to inspection regimes of large municipalities or third-party warranty providers-the following procedures ensure best practice compliance:

  • Pre-Pour Inspection: Always have forms, projection dimensions, and reinforcement placement signed off by superintendent and QC rep before concrete is ordered.
  • Post-Pour Verification: Laser level, tape, and core samples (if necessary) taken at multiple points validate that thickness is maintained along the full wall length, not just key sections. Thickness records provide defense against any post-occupancy claims or disputes.
  • Compaction Reports: Well-compacted subgrade is as critical as footing itself; these records, in conjunction with soil bearing tests, provide documentary assurance in the event of settlement or heave.
  • Photo Documentation: High-resolution photos, timestamped and archived, of footing thickness (at form corners and ends) are now standard on larger sites, both for general contractors and insurer review.

The Alberta construction climate is robustly competitive and standards-driven. Builders who consistently exceed code minimums and fully document compliance find themselves better positioned for successful inspections, lower operational risks, and smoother turnover to owners and end-users.

Urban Trends and Market Evolution: Larger Projections in Infill and Podium Projects

Recent urbanization trends in Alberta are seeing larger, more complex foundation systems-especially with densified multifamily and infill townhouse blocks. Designs now routinely call for footings that exceed the 100 mm projection threshold on both sides due to cumulative load or variable site conditions. Larger projections increase the minimum thickness threshold accordingly, shifting what was once an exception into the norm for many urban projects.

At the same time, energy and sustainability requirements are leading to heavier envelope assemblies and the potential for increased dead load, emphasizing the importance of careful, code-compliant (and often overbuilt) footing design. Overbuilt footings act not only as structural elements but as long-term insurers of building performance amid the inevitable cycles of freeze, thaw, and subsurface movement.

Future-Proofing: Footing Thickness and Sustainability

Beyond compliance and risk management, attention is turning to the sustainability of foundation design. Footings sized only to the strictest code minimum may perform adequately but fail to provide the necessary resilience for future ground movement, increased stormwater, or potential adjacent development that changes local surface drainage or bearing profiles.

Alberta’s variable climate and the ongoing advance of denser, heavier, high-performance builds are pointing to a steady upward drift in average installed footing thickness. Proactive developers and GCs increasingly balance immediate construction cost against lifecycle durability and reputation-a calculation that always favors conservative adherence to (or modest excess of) the NBC projection-plus-minimum standard.

Conclusion

For multifamily and urban residential builds in Alberta, the concrete footing-subject to the governing logic of NBC 9.15.3.8.(1)-remains the primary determinant of long-term structural health and resilience. The code requirement, that the minimum thickness be the greater of 100 mm or the width of the projection, is built on decades of structural engineering evidence and shaped by the tough lessons of Alberta’s field-tested climate and soils. Practical, code-compliant construction-leveraging robust formwork, proper site assessment, thoughtful value engineering, and detailed documentation-ensures that every project stands not just for completion, but for durability and value in use.

Kingsway Builders leads Alberta in foundational quality, delivering multifamily projects built to the highest standards of safety, durability, and code compliance.