OBC On-site Sewage Systems common mistakes
OBC On-site Sewage Systems · Understand the traps and how to avoid them.
Treating the Ontario Building Code Act as if it were the technical code, and citing 'the Building Code Act' for soil or clearance requirements instead of Div. B Part 8.
The Act and the Code are studied together and candidates collapse them into one document, so they lose track of which instrument contains enforceable technical provisions versus administrative powers such as inspection, orders, and permits.
What to do instead
Build a two-column summary: Act = administration, permits, inspections, orders, offences, powers of entry; Div. B Part 8 = technical requirements for sewage systems. Before answering, ask 'is this a power/process question or a technical question?' and cite accordingly.
Applying Part 8 requirements to a system type or building that falls outside its scope, or assuming a permit is always required, without checking the scope and exemption provisions first.
Candidates memorize technical numbers but skim the application, scope, and exemption sections, so they never learn which systems, capacities, or situations Part 8 governs and when a building permit is not needed.
What to do instead
Write out the scope and exemption provisions in your own words and pair each with one worked scenario (e.g., a replacement system, an increase in capacity, a system serving a seasonal dwelling). Test every practice question against that scope list before doing any calculations.
Sizing systems from bedroom count or floor area alone while ignoring the actual daily design flow, fixture and occupancy basis, and reductions or allowances the Code permits.
Rules of thumb are easier to remember than the flow basis, and candidates confuse minimum flows, per-person or per-bedroom allowances, and when a designer may use a lower figure.
What to do instead
Create a one-page flow table listing each occupancy type, its design flow basis, and any permitted reduction, with the condition that must be met to use it. Practise converting a described building into a design flow and state your assumptions explicitly in the answer.
Judging a site suitable from a single soil observation or a visual surface impression, without a proper test pit or borehole to the required depth and without recording soil texture, structure, and water table or bedrock contact.
Candidates treat site evaluation as a checklist item rather than a design driver, so they skip the depth of investigation and the logging detail that determine whether a leaching bed, filter bed, or mound is even permissible.
What to do instead
Memorize the required investigation depth relative to the proposed system and the items that must be recorded (horizon description, texture, restrictive layer, groundwater, bedrock). Practise writing a short site report from a described profile and then justify the chosen system type from that profile.
Confusing loading rates and required areas between leaching beds, filter beds, and mounds, or applying a single rate regardless of soil percolation or texture class.
The three system types look similar on paper but have different area and rate bases, and candidates memorize one number and reuse it everywhere.
What to do instead
Build a comparison grid: system type, soil suitability limits, area or rate basis, separation to limiting layer, and vertical separation requirement. For each row, work one numeric example so the formula, not just the number, is fixed in memory.
Treating tank requirements as generic, missing the specific provisions for capacity, compartments, access openings, risers to grade, and outlet configuration, and ignoring what must be inspected and maintained at the tank.
Tanks are assumed to be 'off-the-shelf' items, so candidates under-study the installation details and the maintenance obligations that the Code and manufacturer instructions impose.
What to do instead
Produce an annotated tank diagram from memory showing capacity, compartment arrangement, access and inspection openings, riser and lid requirements, and outlet device. Then list the maintenance items an owner must be told about, and check your diagram against the Code text.
Sizing pumps and dosing volumes from tank or field size intuition rather than from the design flow, dose volume, head, and pipe friction, and ignoring how distribution laterals must be balanced.
Hydraulics is the least familiar topic for many candidates, so they avoid the calculation and answer from memory of typical equipment sizes.
What to do instead
Practise a full dosing calculation chain: design flow to dose volume to dose frequency to pump discharge rate to total dynamic head to pipe sizing, then verify equal distribution across laterals. Learn the purpose of each control (float, timer, alarm) by asking what failure it protects against.
Measuring setbacks from the wrong reference point, such as from the tank instead of the system component or property line the provision names, or applying a single distance to all features.
Setback tables are dense, and candidates memorize a few distances without anchoring each to its measured-from and measured-to points.
What to do instead
Rebuild the setback table as a matrix: feature (well, property line, building, watercourse, easement) against component (tank, bed, distribution box, treatment unit). For each cell, note the distance and the exact point it is measured from, then sketch a site plan and mark each measurement.
Describing installation or inspection in general terms ('install per manufacturer', 'inspect regularly') instead of naming the specific hold points, tests, and records required before backfill and at final inspection.
Candidates study design heavily and treat installation and maintenance as common sense, so answers lack the concrete verification steps an inspector would actually perform.
What to do instead
List the hold points in sequence (excavation and bearing verification, tank set and water test, pipe slope and alignment check, distribution lateral verification, backfill, final grade and access at grade). For each, state what is checked, who checks it, and what record proves it was done.
Submitting or reviewing a package that omits required drawings, the site evaluation information, design calculations, or the maintenance and operating information, and treating the permit as the end of the process rather than the start of an inspection sequence.
Documentation is studied last and briefly, so candidates cannot list what a complete submission contains or explain how permit conditions, inspections, and final approval connect.
What to do instead
Assemble a model submission checklist from memory and compare it to the Code's requirements: plans and specifications, site and soil information, design basis and calculations, and owner information. Then map the permit lifecycle from application to occupancy or final inspection, noting what triggers each inspection.