OBC Small Buildings common mistakes
OBC Small Buildings · Understand the traps and how to avoid them.
Treating the Part 9 scope thresholds as a single number (e.g., assuming any building under 600 m² automatically falls under Part 9) and applying Part 9 construction rules to a building that actually triggers Part 3 because it exceeds an area, height, or occupant-load limit.
Candidates memorize one familiar figure and overlook that Part 9 applicability is a combination of building area, building height, and occupant load, plus specific exclusions for major occupancies and special uses. A building can be small in area but excluded by height or occupancy.
What to do instead
Before answering any scope question, write out all three tests (area, height, occupant load) and check each independently against the described building, then check the occupancy-based exclusions. Practice with borderline scenarios where only one criterion fails.
Classifying a building by its current use or by the occupant's business name rather than by the defined major occupancy of the space, leading to the wrong construction type and fire-protection conclusions.
Real-world descriptions often use informal labels ('shop', 'office', 'warehouse') that do not map cleanly onto the defined occupancy groups. Candidates anchor on the label instead of the activity and hazard.
What to do instead
For each scenario, restate the actual activity and hazard, then match it to the definition wording. Where a building has multiple occupancies, identify each major occupancy separately and note which is the principal one before selecting construction requirements.
Calculating occupant load from the actual number of people expected or from the number of chairs provided, instead of applying the prescribed occupant-load factor to the floor area of each space.
Candidates reason from common sense ('it's a small shop, maybe five people') and forget that code occupant load is a prescribed design value that drives egress width, exit count, and travel distance.
What to do instead
Always compute occupant load as floor area divided by the applicable factor for that use, per space, then sum. Show the arithmetic in your working so you can catch a wrong factor before it propagates into egress answers.
Counting exits by counting doors, including doors that do not qualify as exits (doors through another suite, sliding doors, doors serving only a corridor, or doors too close together to be considered separate).
Candidates equate 'opening in the wall' with 'exit' and skip the qualifying conditions for exit doors, including remoteness and the requirement that each exit be independent.
What to do instead
For every exit-count question, verify each candidate door against the exit criteria: leads directly to a public way or to a permitted path, is not through another occupancy, and is separated from the other exit by the required distance. State why each door does or does not count.
Applying the travel-distance limit measured in a straight line on plan rather than along the actual path of travel, and forgetting that the limit changes with occupancy and with whether the building is sprinklered.
Straight-line measurement is faster and feels adequate, and candidates often use a single remembered distance for all occupancies.
What to do instead
Trace the path of travel from the most remote point, following corridors and around obstructions, and use the limit specific to that occupancy and sprinkler condition. Keep a small table of the limits you are responsible for and consult it every time.
Confusing fire-resistance rating with fire separation distance, or applying a fire-resistance rating where only a separation is required (or vice versa), when evaluating exterior walls and openings near property lines.
Both concepts use time and distance language and both appear in tables, so candidates swap the units and the intent: one limits spread within a building over time, the other limits exposure to adjacent property by geometry.
What to do instead
For each exterior wall question, first determine the limiting distance to the property line, then look up the permitted cladding and opening limits for that distance, and keep fire-resistance ratings for the assemblies that specifically require them. Label your working 'distance' or 'rating' explicitly.
Designing barrier-free paths that satisfy width and door clearances but fail at level changes, thresholds, ramps, and required accessible facilities such as an accessible washroom or turning space.
Candidates focus on the dimensions they have memorized (corridor width, door width) and neglect the sequence of connected elements that make an accessible route continuous, plus the grab bar and clearance details inside small washrooms.
What to do instead
Walk the route element by element from parking or street to the farthest point, checking level changes, ramps and their landings, thresholds, door approach clearances, and turning space at each destination. For small washrooms, verify fixture clearances and grab bar locations against the required layout.
Mixing up limit states and load combinations in structural questions, such as comparing an ultimate limit state load to a serviceability deflection limit, or omitting load factors and importance factors.
Candidates memorize load values without the accompanying factors and combinations, and the two limit states use different factored and unfactored values.
What to do instead
For every structural item, first name the limit state being checked, then state which load combination and factors apply before doing arithmetic. Separate your notes into 'strength' and 'serviceability' columns so the correct values are used.
Applying energy-efficiency and environmental separation requirements (thermal resistance, airtightness, vapour and moisture control, window performance) as generic good practice without tying them to the specific assembly, climate, and prescriptive or trade-off path being used.
These requirements are numerous and interdependent, and candidates answer from general construction intuition rather than from the specific compliance path and the assembly being described.
What to do instead
Identify the compliance path first (prescriptive or performance/trade-off), then check the specific assembly: insulation level, continuous barrier alignment, vapour retarder location relative to the warm side, and window ratings. Answer with the value required by that path, not a typical field value.
Ignoring the administrative and general-provisions layer, such as permits, inspections, professional design and field review triggers, alternative solutions, and the distinction between the building code and referenced standards, and answering a procedural question as if it were a design question.
Candidates concentrate study time on technical numbers and skim the administrative sections, so they miss questions about who must design, what must be inspected, and when a referenced standard governs.
What to do instead
Build a one-page summary of administrative triggers and review it before the exam: when permits and inspections are required, when design must be by a qualified professional, when field review applies, and how referenced standards are incorporated. Read each question to decide whether it is asking about process or design before choosing an answer.