Occupant load and sanitary facilities (Part 3) (On-site Sewage Systems)

Last updated October 22, 2026

Values this chapter turns on

per sleeping room in a dwelling unit: 2 persons; per sleeping room in a dwelling unit
floor area per person for offices: 9.30 m²; floor area per person for offices
daily flow per person, apartments and condos: 275 L; daily flow per person, apartments and condos
needed on a self-service storage property: 2 washrooms; needed on a self-service storage property

Why does a septic exam test Part 3? Because sewage flows for many buildings are worked out per person, and the number of persons comes from the occupant load rules in Part 3. The syllabus also lists a handful of Part 3 articles that say when a building needs toilets at all, and when a privy or chemical closet can stand in for them. This chapter covers both.

Occupant load determination

The occupant load of a floor area is based on one of three things (OBC Div. B 3.1.17.1.(1)):

Situation	How to count persons
Assembly occupancy with fixed seats	the number of seats
Dwelling unit	2 persons per sleeping room
Anything else	the number the area is designed for, but not less than Table 3.1.17.1. gives, unless it can be shown fewer people will use it

Table 3.1.17.1. (Table) gives an area per person in square metres for each type of use. Divide the floor area by that number to get the occupant load. For example, the table lists 9.30 m² per person for offices.

A few more rules from the same article:

If an area is designed for a different load from the table, a permanent sign showing that load must be posted where it is easy to see (Sentence (2)).
Mezzanines, tiers and balconies count as part of the floor area (Sentence (3)).
If a room is used for different occupancies at different times, use the table value that gives the most people (Sentence (4)).

Why it matters for septic design

Notes to Tables 8.2.1.3.-A and 8.2.1.3.-B both say the occupant load is calculated using Subsection 3.1.17. (Table 8.2.1.3.-A, Table 8.2.1.3.-B). So any "per person" flow in those tables uses this count.

Worked example: apartment building flow

A small apartment building has 6 two-bedroom units. Each unit has 2 sleeping rooms, so 2 x 2 = 4 persons per unit, and 24 persons in total. Table 8.2.1.3.-A gives a per-person flow for apartments and condominiums (275 L). Daily design flow = 24 x 275 = 6,600 L.

The trap is to count one person per bedroom (12 persons, 3,300 L). The code says 2 persons per sleeping room.

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Drainage and grades

A building must be located and its site graded so that water does not collect at or near the building and does not harm adjacent properties (OBC Div. B 3.1.19.1.). Part 9 has a matching rule, covered in Chapter 3, plus a specific rule about wells and leaching beds.

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Plumbing and drainage systems

Under OBC Div. B 3.7.4.1.:

A building on a property that abuts a street with a public or municipal water main must have, or give its occupants access to, a plumbing system with potable water, a sanitary drainage system and plumbing fixtures (Sentence (1)).
Where a sanitary drainage system cannot be installed because there is no water supply, sanitary privies, chemical closets or other means of disposing of human waste must be provided (Sentence (2)). This is the link to Class 1 systems.
Fixtures are not needed in a building that is not normally occupied, where fixtures are impractical and other fixtures are available nearby while it is in use (Sentence (3)).

A dwelling unit with a piped water supply needs a kitchen sink, a lavatory, a water closet (or a drainless composting water closet) and a bathtub or shower stall (OBC Div. B 3.7.4.5.). Note the composting option: it is a Class 1 system under Part 8.

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Sanitary facilities in special buildings

Building	Rule
Portable classroom	washrooms not needed if the main school building's facilities meet Subsection 3.7.4. for the combined occupant load (OBC Div. B 3.9.3.8.)
Self-service storage building	Subsection 3.7.4. applies, and two washrooms, each with a water closet and lavatory, are needed in one of the buildings on the property, unless 3.7.4.1.(2) or (3) applies (OBC Div. B 3.10.2.7.)
Public pool	dressing rooms, showers, toilets and other facilities are required, with relief for a Class A pool at a recreational camp or a Class B pool where facilities are conveniently available elsewhere on the premises and foot sprays are provided (OBC Div. B 3.11.9.1.)
Tents	the number of water closets comes from Table 3.7.4.3.-E, but sanitary privies, chemical closets or other means may be used instead of toilet fixtures (OBC Div. B 3.14.1.8.)

For public pools, the syllabus lists only Sentences (1) to (3) of 3.11.9.1. Learn the two exemptions: they depend on the pool class and on facilities "conveniently available" elsewhere on the premises, plus foot sprays.

Worked example: storage units on a rural lot

A self-storage complex of three buildings is planned on a lot with no water supply. Sentence 3.10.2.7.(2) normally requires two washrooms in one building, but it allows the exceptions in 3.7.4.1.(2). With no water supply, sanitary privies or chemical closets may be provided instead, which would be a Class 1 sewage system under Part 8.

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Common traps

2 persons per sleeping room, not per bedroom pair or per dwelling.
Fixed seats: count the seats, not the floor area.
A different design load needs a permanent sign.
Tents and buildings without water may use privies or chemical closets.
Portable classrooms rely on the main school only if the main school meets Subsection 3.7.4. for the total load.

Chapter review

Chapter at a glance

One line per section: the idea to carry into the exam. Then the answers that most often catch candidates out.

Chapter at a glance: the key idea in each section
Section	Key idea
Occupant load determination	Table 3.1.17.1. gives an area per person in square metres for each type of use.
Drainage and grades	A building must be located and its site graded so that water does not collect at or near the building and does not harm adjacent properties.
Plumbing and drainage systems	A dwelling unit with a piped water supply needs a kitchen sink, a lavatory, a water closet (or a drainless composting water closet) and a bathtub or shower stall.
Sanitary facilities in special buildings	For public pools, the syllabus lists only Sentences (1) to (3) of 3.11.9.1. Learn the two exemptions: they depend on the pool class and on facilities "conveniently available"…

Occupant load determination
Table 3.1.17.1. gives an area per person in square metres for each type of use.
Drainage and grades
A building must be located and its site graded so that water does not collect at or near the building and does not harm adjacent properties.
Plumbing and drainage systems
A dwelling unit with a piped water supply needs a kitchen sink, a lavatory, a water closet (or a drainless composting water closet) and a bathtub or shower stall.
Sanitary facilities in special buildings
For public pools, the syllabus lists only Sentences (1) to (3) of 3.11.9.1. Learn the two exemptions: they depend on the pool class and on facilities "conveniently available"…

Usual exam traps

2 persons per sleeping room, not per bedroom pair or per dwelling.
Fixed seats: count the seats, not the floor area.
A different design load needs a permanent sign.
Tents and buildings without water may use privies or chemical closets.
Portable classrooms rely on the main school only if the main school meets Subsection 3.7.4. for the total load.

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Questions on Occupant load and sanitary facilities (Part 3), in the exam format, every answer explained and cited to its article. Sign in to start; a free account gets the free questions.

Practice this chapter On-site Sewage Systems calculations

Code cited in this chapter

Based on the official 2024 Building Code Compendium on ontario.ca.

OBC Div. B 3.1.17.1. Occupant Load Determination
(1)The occupant load of a floor area or part of a floor area, shall be based on
(a)the number of seats in an assembly occupancy having fixed seats,
(b)2 persons per sleeping room in a dwelling unit, or
(c)the number of persons for which the area is designed, but not less than that determined from Table 3.1.17.1. for occupancies other than those described in Clauses (a) and (b), unless it can be shown that the area will be occupied by fewer persons.
(2)If a floor area or part thereof it has been designed for an occupant load other than that determined from Table 3.1.17.1., a permanent sign indicating that occupant load shall be posted in a conspicuous location.
(3)For the purposes of this Article, mezzanines, tiers and balconies shall be regarded as part of the floor area.
(4)If a room or group of rooms is intended for different occupancies at different times, the value to be used from Table 3.1.17.1. shall be the value that gives the greatest number of persons for the occupancies concerned.
Open OBC Div. B 3.1.17.1. in the code viewer
OBC Div. B Table 3.1.17.1. Occupant Load
Table 3.1.17.1. Occupant Load Forming Part of Article 3.1.17.1. Type of Use of Floor Area or Part Thereof | Area per Person, m[2] Assembly uses | N/A space with fixed seats | [1] space with non-fixed seats | 0.75 stages for theatrical performances | 0.75 space with non-fixed seats and tables | 0.95 standing space | 0.40 stadia and grandstands | 0.60 bowling alleys, pool and billiard rooms | 9.30 classrooms | 1.85 school shops and vocational rooms | 9.30 reading or writing rooms or lounges | 1.85 dining, beverage and cafeteria space | 1.20 laboratories in schools | 4.60 Care, care and treatment or detention uses | N/A B-1 : detention quarters | 11.60 B-2 : treatment and sleeping areas | 10.00 B-3 : sleeping room areas | 10.00 (See also Article 3.7.1.3.) | (See also Article 3.7.1.3.) Residential uses | N/A dwelling units | [2] dormitories | 4.60 Business and personal services uses | N/A personal service shops | 4.60 offices | 9.30 Mercantile uses | N/A basements and first storeys | 3.70 second storeys having a principal entrance from a pedestrian thoroughfare or a parking area | 3.70 other storeys | 5.60 Industrial uses | N/A manufacturing or process rooms | 4.60 storage garages | 46.00 storage spaces (warehouse) | 28.00 aircraft hangars | 46.00 Other uses | N/A cleaning and repair goods | 4.60 kitchens | 9.30 storage | 46.00 public corridors intended for occupancies in addition to pedestrian travel | 3.70[3] Notes to Table 3.1.17.1.: (1) See Clause 3.1.17.1.(1)(a). (2) See Clause 3.1.17.1.(1)(b) (3) See Note A-3.3.
Open OBC Div. B Table 3.1.17.1. in the code viewer
OBC Div. B Table 8.2.1.3-A. Residential Occupancy — Forming Part of Sentence 8.2.1.3.(1)
Table 8.2.1.3.-A Residential Occupancy — Forming Part of Sentence 8.2.1.3.(1) Residential Occupancy | Volume, litres Apartments, Condominiums, Other Multi-family Dwellings - per person[1] | 275 Boarding Houses (a) Per person, (i) with meals and laundry facilities, or, | 200 Boarding Houses (a) Per person, (ii) without meal or laundry facilities, and | 150 Boarding Houses (b) Per non-resident staff per 8 hour shift | 40 Boarding School - per person | 300 Dwellings (a) 1 bedroom dwelling | 750 Dwellings (b) 2 bedroom dwelling | 1,100 Dwellings (c) 3 bedroom dwelling | 1,600 Dwellings (d) 4 bedroom dwelling | 2,000 Dwellings (e) 5 bedroom dwelling | 2,500 Dwellings (f) Additional flow for[2] (i) each bedroom over 5, | 500 Dwellings (f) Additional flow for[2] (f) (ii) (A) each 10 m² (or part of it) over 200 m² up to 400 m²[3], | 100 Dwellings (f) Additional flow for[2] (f) (ii) (B) each 10 m² (or part of it) over 400 m² up to 600 m²[3], and | 75 Dwellings (f) Additional flow for[2] (f) (ii) (C) each 10 m² (or part of it) over 600 m²[3], or | 50 Dwellings (f) Additional flow for[2] (iii) each fixture unit over 20 fixture units | 50 Hotels and Motels (excluding bars and restaurants) (a) Regular, per room | 250 Hotels and Motels (excluding bars and restaurants) (b) Resort hotel, cottage, per person | 500 Hotels and Motels (excluding bars and restaurants) (c) Self service laundry, add per machine | 2,500 Work Camp/Construction Camp, semi-permanent per worker | 250 Notes to Table 8.2.1.3.-A: (1) The occupant load shall be calculated using Subsection 3.1.17. (2) Where multiple calculations of sanitary sewage volume is permitted, the calculation resulting in the highest flow shall be used in determining the design daily sanitary sewage flow. (3) Total finished area, excluding the area of the finished basement.
Open OBC Div. B Table 8.2.1.3-A. in the code viewer
OBC Div. B Table 8.2.1.3-B. Airports, Bus Terminals, Train Stations, Dock/Port Facilities (Food Services excluded)
Airports, Bus Terminals, Train Stations, Dock/Port Facilities (Food Services excluded) Establishments[1] | Volume, litres (a) Per passenger, and | 20 (b) Per employee per 8 hour shift | 40 Assembly Hall — per seat Establishments[1] | Volume, litres (a) No food service, or | 8 (b) Food service provided | 36 Barber Shop/Beauty Salon Establishments[1] | Volume, litres per service chair | 650 Bowling Alleys (Food Service not included) Establishments[1] | Volume, litres per lane | 400 Churches and Similar Places of Worship — per seat Establishments[1] | Volume, litres (a) No kitchen facilities, or | 8 (b) Kitchen facilities provided | 36 Country Club (excluding Food Service) Establishments[1] | Volume, litres (a) Per resident, | 375 (b) Per employee per 8 hour shift, and | 50 (c) Per member or patron | 40 Day Care Facility Establishments[1] | Volume, litres per person (staff and children) | 75 Dentist Office Establishments[1] | Volume, litres (a) Per wet service chair, and | 275 (b) Per dry service chair | 190 Doctors Office Establishments[1] | Volume, litres (a) Per practitioner, and | 275 (b) Per employee per 8 hour shift | 75 Factory (excluding process or cleaning waters) — per employee per 8 hour shift Establishments[1] | Volume, litres (a) No showers, or | 75 (b) Including showers | 125 Flea Markets[2] (open not more than 3 days per week) Establishments[1] | Volume, litres (a) Per non-food service vendor space, | 60 (b) Per food service establishment / 9.25 m² of floor space, and | 190 (c) Per limited food service outlet | 95 Food Service Operations Establishments[1] | Volume, litres (a) Restaurant (not 24 hour), per seat | 125 (b) Restaurant (24 hour), per seat | 200 (c) Restaurant on controlled-access highway, per seat | 400 (d) Paper service restaurant, per seat | 60 (e) Donut shop, per seat | 400 (f) Bar and cocktail lounge, per seat | 125 (g) Drive-in restaurant per parking space | 60 (i) Cafeteria — per meal | 12 Take-out restaurant (no seating area) Establishments[1] | Volume, litres (h) (i) per 9.25 m² of floor area, and | 190 (h) (ii) per employee per 8 hour shift | 75 Food outlet Establishments[1] | Volume, litres (j) (i) excluding delicatessen, bakery and meat department, per 9.25 m² of floor space, | 40 (j) (ii) per 9.25 m² of delicatessen floor space, | 190 (j) (iii) per 9.25 m² of bakery floor space, | 190 (j) (iv) per 9.25 m² of meat department floor space, and | 380 (j) (v) per water closet | 950 Hospitals — per bed Establishments[1] | Volume, litres (a) Including laundry facilities, or | 750 (b) Excluding laundry facilities | 550 Long-Term Care Homes, etc. Establishments[1] | Volume, litres per bed | 450 Office Building[3] Establishments[1] | Volume, litres (a) Per employee per 8 hour shift, or | 75 (b) Per each 9.3 m² of floor space | 75 Public Parks Establishments[1] | Volume, litres (a) With toilets only per person, or | 20 (b) With bathhouse, showers, and toilets per person | 50 Recreational Vehicle or Campground Park Establishments[1] | Volume, litres (a) Per site without water or sewer hook-up, or | 275 (b) Per site with water and sewer hook-up | 425 Schools — per student Establishments[1] | Volume, litres (a) Day school, | 30 (b) With showers, | 30 (c) With cafeteria, and | 30 (d) Per non-teaching employee per 8 hour shift | 50 Service Stations (no vehicle washing)[3] Establishments[1] | Volume, litres (a) Per water closet, and | 950 (a) Per water closet, and (i) per fuel outlet[4], or | 560 (a) Per water closet, and (ii) per vehicle served | 20 Shopping Centre (excluding food and laundry) Establishments[1] | Volume, litres per 1.0 m² of floor space | 5 Stadiums, Race Tracks, Ball Parks Establishments[1] | Volume, litres per seat | 20 Stores[3] Establishments[1] | Volume, litres (a) Per 1.0 m² of floor area, or | 5 (b) Per water closet | 1,230 Swimming and Bathing Facilities (Public) Establishments[1] | Volume, litres per person | 40 Theatres Establishments[1] | Volume, litres (a) Indoor, auditoriums per seat, | 20 (b) Outdoor, drive-ins per space, or | 40 (c) Movie theatres per seat | 15 Veterinary Clinics Establishments[1] | Volume, litres (a) Per practitioner, | 275 (b) Per employee per 8 hour shift, and | 75 (c) Per stall, kennel or cage if floor drain connected | 75 Warehouse Establishments[1] | Volume, litres (a) Per water closet, and | 950 (b) Per loading bay | 150 Notes to Table 8.2.1.3.-B: (1) The occupant load shall be calculated using Subsection 3.1.17. (2) Flea markets open more than 3 days per week shall be assessed using the volumes stated under the heading "Stores". (3) Where multiple calculations of sanitary sewage volume is permitted, the calculation resulting in the highest flow shall be used in determining the design daily sanitary sewage flow. (4) The number of fuel outlets is considered the maximum number of fuel nozzles that could be in use at the same time
Open OBC Div. B Table 8.2.1.3-B. in the code viewer
OBC Div. B 3.1.19.1. Drainage
(1)The building shall be located and the building site graded so that water will not accumulate at or near the building and will not adversely affect any adjacent properties.
Open OBC Div. B 3.1.19.1. in the code viewer
OBC Div. B 3.7.4.1. Plumbing and Drainage Systems
(1)Except as provided in Sentence (3), each building situated on property that abuts on a street in which a public or municipal water main is located shall be provided with or have accessible to its occupants a plumbing system including a potable water supply, a sanitary drainage system and plumbing fixtures.
(2)When the installation of a sanitary drainage system is not possible because of the absence of a water supply, sanitary privies, chemical closets or other means for the disposal of human waste shall be provided.
(3)Plumbing fixtures need not be provided in a building that is not normally occupied by persons where such installations are impractical and other fixtures are available in nearby buildings when the subject building is in use.
Open OBC Div. B 3.7.4.1. in the code viewer
OBC Div. B 3.7.4.5. Plumbing Facilities for Dwelling Units
(1)A dwelling unit where a piped water supply is available shall be provided with a
(a)kitchen sink,
(b)lavatory,
(c)water closet or drainless composting water closet, and
(d)bathtub or shower stall.
Open OBC Div. B 3.7.4.5. in the code viewer
OBC Div. B 3.9.3.8. Washroom Facilities
(1)Washroom facilities need not be provided in a portable classroom where the facilities in the main school building comply with the requirements of Subsection 3.7.4. for the total occupant load of the main school building and the portable classrooms.
Open OBC Div. B 3.9.3.8. in the code viewer

Show all 11 articles

OBC Div. B 3.10.2.7. Sanitary Facilities
(1)Except as provided in Sentence (2), the requirements in Subsection 3.7.4. shall apply.
(2)Except as permitted in Sentences 3.7.4.1.(2) and (3), two washrooms, each containing a water closet and a lavatory, shall be provided within one of the buildings on the property.
Open OBC Div. B 3.10.2.7. in the code viewer
OBC Div. B 3.11.9.1. Dressing Rooms and Sanitary Facilities
(1)Except as otherwise permitted in Sentences (2) and (3), every public pool shall be equipped with dressing rooms, locker rooms, shower heads, water closets, urinals, lavatories and drinking fountains that shall be designed, constructed and equipped to comply with Sentences (4) to (14).
(2)Where a Class A pool is installed on the premises of a recreational camp, dressing rooms, locker rooms, shower heads, water closets, urinals, lavatories and drinking fountains are not required if
(a)dressing, water closet and shower facilities are conveniently available for bathers elsewhere on the premises, and
(b)foot sprays are provided in accordance with Sentence 3.11.3.1.(18).
(3)Where a Class B pool is installed, dressing rooms, locker rooms, shower heads, lavatories, water closets, drinking fountains and urinals are not required if
(a)dressing, water closet and shower facilities are conveniently available elsewhere on the premises for bathers when the pool is open for use, and
(b)foot sprays are provided in accordance with Sentence 3.11.3.1.(18).
(4)The minimum number of water closets, urinals and lavatories shall be determined from Article 3.7.4.3. and Table 3.7.4.3.-C for an occupant load based on
(a)the formula in Sentence 3.1.17.3.(1) for all public pools, except a wave action pool, or
(b)the formula in Sentence 3.1.17.3.(2) for a wave action pool.
(5)A minimum of one shower head shall be provided for every 40 bathers.
(6)Where dressing and locker rooms, water closets and urinals are provided in conjunction with a public pool, they shall be located in such a manner that bathers, after using them, shall pass through or by a shower area to reach the pool deck.
(7)All shower heads shall be supplied with potable water at a pressure of at least 140 kPa.
(8)The shower water system shall have one or more tempering devices capable of being adjusted to ensure that water supplied to shower heads does not exceed 40°C.
(9)Floors in washrooms, shower areas and passageways used by bathers shall slope to waste drains at not less than 1% and shall be of hard surfaced materials that do not become slippery when wet.
(10)Joints between floors and walls shall be coved in areas described in Sentence (9) and in dressing and locker rooms.
(11)Hose bibbs shall be provided in safe locations convenient for flushing down the walls and floors in washrooms, shower areas and passageways used by bathers.
(12)Partitions or walls shall be provided to ensure privacy of dressing rooms, washrooms and shower areas.
(13)The bottom of interior partitions in dressing rooms and washrooms shall be between 250 mm and 350 mm above the floor.
(14)Dressing and locker room floors shall have non-slip surfaces that permit convenient and thorough cleaning and disinfecting.
Open OBC Div. B 3.11.9.1. in the code viewer
OBC Div. B 3.14.1.8. Sanitary Facilities
(1)Except as provided in Sentence (3), the minimum number of water closets for tents shall be determined in accordance with Table 3.7.4.3.-E.
(2)Article 3.7.4.16. applies to sanitary facilities in Sentence (1).
(3)Sanitary privies, chemical closets or other means for the disposal of human waste may be provided in lieu of toilet fixtures.
Open OBC Div. B 3.14.1.8. in the code viewer