Drainage systems: connections, traps, arrangement and cleanouts (Plumbing

Last updated October 22, 2026

Values this chapter turns on

no connections this close to a busy stack base: 1.5 m; no connections this close to a busy stack base
max two-way cleanout spacing, NPS 3 and 4: 30 m; max two-way cleanout spacing, NPS 3 and 4
storage needed in a sump with a manual pump: 24 h; storage needed in a sump with a manual pump
max distance of a stack cleanout above its base: 3 m; max distance of a stack cleanout above its base

Subsections 7.4.2. to 7.4.7. decide how fixtures connect to the drainage system, how they are trapped, how the piping is arranged, and where cleanouts go. Sizing comes later in 7.4.8. to 7.4.10. The syllabus leaves out grease and oil interceptors, neutralizing tanks, manholes and several commercial cleanout sentences, so focus on house fixtures, sumps, backwater valves and cleanouts.

Connections to sanitary and storm systems

Fixtures connect directly to the sanitary drainage system, with listed exceptions (OBC Div. B 7.4.2.1.(1)):

Item	May go to
Drinking fountain	indirect to sanitary, or to storm (backwater valve if the system can back up)
Drain pan on a heating or cooling unit	storm system (backwater valve if subject to backflow)
Floor drain receiving only clear-water waste or storm water	storm system
Fixtures discharging only clear-water waste	storm system, or onto a roof
Food or drink equipment, sterilizers, devices using water for heating or cooling, water-operated devices, water treatment devices, drains or overflows from water or heating systems	must be indirectly connected

So a water softener discharge is an indirect connection, typically over a floor drain or laundry tub with an air break.

Stack base rule (7.4.2.1.(2)): where a stack receives 30 or more fixture units, or fixtures on 2 or more storeys, nothing may connect to the horizontal drain within 1.5 m horizontally of the stack base (or base of the upper section above an offset). This keeps branches out of the turbulent zone.

No other fixture may connect to a lead bend or stub serving a water closet (7.4.2.1.(3)). A rainwater tank overflow to the storm system needs an air break or a backwater valve (7.4.2.2.).

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Fixture location and treatment of wastes

Sanitary units, bathtubs and showers must not be placed against wall and floor surfaces that are pervious to water (7.4.3.1.).
Indirect connections and traps that may overflow must not be in a crawl space or other unfrequented area (7.4.3.2.).
A macerating toilet system is only allowed where no gravity sanitary connection is available, and per the manufacturer's instructions (OBC Div. B 7.4.3.5.).
Wastes that could damage the system or the sewage works must be treated first (7.4.4.1.), and the drainage system must suit the temperature of heated wastes (7.4.4.2.).

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Traps

Every fixture gets its own trap, except (OBC Div. B 7.4.5.1.):

one trap for all compartments of a 2- or 3-compartment sink or laundry tray,
one trap for 2 similar single-compartment fixtures in the same room (for example two lavatories),
one trap for a group of floor drains, shower drains or washing machines in the same room, if they cannot receive food or organic matter.

A domestic dishwasher with a pump discharging directly into the sink or disposal piping must loop its discharge line as high as possible, just under the counter, and connect by a Y fitting on the inlet side of the sink trap or to the disposal unit (7.4.5.1.(6)).

Other trap rules:

A subsoil drainage pipe connected to the sanitary system connects upstream of a trap with a cleanout, or through a trapped sump. Foundation drains may connect to the sanitary system only where no storm system is available, or soil conditions prevent a culvert or dry well (OBC Div. B 7.4.5.3.).
A building trap, where installed, needs a cleanout directly over its upstream side and is located upstream of the building cleanout (7.4.5.4.).
Floor drain trap seals are maintained by a trap seal primer, an indirectly connected drinking fountain, or another equally effective means. Primer lines are at least 3/8 in. inside diameter (OBC Div. B 7.4.5.5.).
A storm floor drain needs a trap but no vent (7.4.5.2.(2)).

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Arrangement of drainage piping

No vertical pipe may carry both sanitary and storm sewage, and there must be no unused open ends (7.4.6.1.).
Sanitary drainage pipe must not run directly above food-handling equipment or non-pressure potable water tanks (7.4.6.2.).

Sumps (OBC Div. B 7.4.6.3.):

Rule	Detail
What may go to a sump	only piping too low to drain by gravity
Sanitary sump	water- and air-tight, and vented
Non-automatic pump	sump holds at least 24 hours of liquid
Sanitary sump discharge	union, backwater valve, shut-off valve, in that order
Storm sump discharge	union and check valve pumped above grade, or union, check valve and shut-off valve

Backwater valves (OBC Div. B 7.4.6.4.): a backwater valve that stops free air circulation is not allowed in a building drain or sewer. A "normally open" valve to the listed standards may go in a building drain if it serves no more than one dwelling unit. Where a drain may be subject to backflow, fixtures below the adjoining street level need protection, and a residential building on a public sewer needs protection for fixtures below the upstream manhole cover. Fixtures on one storey on the same branch may share one valve on the branch.

Where a building drain enters above the bottom of the wall, it may be taken to end where the pipe first turns from horizontal to vertical (7.4.6.6.).

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Cleanouts

Cleanouts must allow cleaning of the entire system (OBC Div. B 7.4.7.1.). House rules to know:

running trap: cleanout directly over its upstream side,
interior leader: cleanout at its bottom or within 1 m upstream,
building drain: NPS 4 or larger cleanout as close as practical to where the drain leaves the building,
stack: cleanout at its base, within 3 m upstream of the base, or on the Y joining the stack to the drain,
kitchen sink trap arm: cleanout close to the trap outlet, readily accessible,
building sewer: no change of direction or slope between cleanouts, except pipes up to NPS 6 may turn 5° every 3 m or use fittings totalling 45°.

Spacing on horizontal pipe comes from Table 7.4.7.2.:

Drain size	Cleanout size	One-way rodding	Two-way rodding
under NPS 3	same as pipe	7.5 m	15 m
NPS 3 and 4	NPS 3	15 m	30 m
over NPS 4	NPS 4	26 m	52 m

One-way cleanouts must face so you rod in the direction of flow (7.4.7.2.(5)). A building sewer joining another building sewer without a manhole is limited to 30 m developed length (7.4.7.2.(4)). Cleanouts must be accessible, not a floor hazard, and never used as a floor drain (7.4.7.4.).

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Worked example

A 25 m NPS 4 building drain runs under a slab with a two-way cleanout at the building wall. The table allows 30 m for two-way rodding on NPS 3 and 4, so no intermediate cleanout is needed for spacing. If the only cleanout allowed one-way rodding, the limit would be 15 m and another cleanout would be required.

Common traps

Putting a backwater valve on the sump discharge after the shut-off. The order is union, backwater valve, shut-off valve.
Allowing a normally open backwater valve to serve a whole semi-detached building. It may serve only one dwelling unit.
Using the 1.5 m stack base rule when the stack only serves one storey and fewer than 30 fixture units.

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
Connections to sanitary and storm systems	So a water softener discharge is an indirect connection, typically over a floor drain or laundry tub with an air break.
Traps	A domestic dishwasher with a pump discharging directly into the sink or disposal piping must loop its discharge line as high as possible, just under the counter, and connect by a…
Arrangement of drainage piping	Backwater valves: a backwater valve that stops free air circulation is not allowed in a building drain or sewer.
Cleanouts	Cleanouts must allow cleaning of the entire system.

Connections to sanitary and storm systems
So a water softener discharge is an indirect connection, typically over a floor drain or laundry tub with an air break.
Traps
A domestic dishwasher with a pump discharging directly into the sink or disposal piping must loop its discharge line as high as possible, just under the counter, and connect by a…
Arrangement of drainage piping
Backwater valves: a backwater valve that stops free air circulation is not allowed in a building drain or sewer.
Cleanouts
Cleanouts must allow cleaning of the entire system.

Usual exam traps

Putting a backwater valve on the sump discharge after the shut-off. The order is union, backwater valve, shut-off valve.
Allowing a normally open backwater valve to serve a whole semi-detached building. It may serve only one dwelling unit.
Using the 1.5 m stack base rule when the stack only serves one storey and fewer than 30 fixture units.

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Questions on Drainage systems: connections, traps, arrangement and cleanouts, in the exam format, every answer explained and cited to its article. Sign in to start; a free account gets the free questions.

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Code cited in this chapter

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

OBC Div. B 7.4.2.1. Connections to Sanitary Drainage Systems
(1)Fixtures shall be directly connected to a sanitary drainage system, except that
(a)drinking fountains are permitted to be
(i)indirectly connected to a sanitary drainage system, or
(ii)connected to a storm drainage system, provided that where the system is subject to backflow, a backwater valve is installed in the fountain fixture drain,
(b)drainage pans on heating/cooling units are permitted to be connected to a storm drainage system, provided that where the system is subject to backflow, a backwater valve is installed,
(c)a floor drain is permitted to be connected to a storm drainage system, provided it is located where it can receive only clear-water waste or storm water,
(d)fixtures or appliances that discharge only clear-water waste are permitted to be connected to a storm drainage system or be drained onto a roof, and,
(e)the following devices shall be indirectly connected to a drainage system:
(i)a device for the display, storage, preparation or processing of food or drink,
(ii)a sterilizer,
(iii)a device that uses water as a cooling or heating medium,
(iv)a water operated device,
(v)a water treatment device,
(vi)a drain or overflow from a water system or a heating system.
(2)The connection of a soil or waste pipe to a nominally horizontal soil or waste pipe or to a nominally horizontal offset in a soil or waste stack shall be respectively at least 1.5 m measured horizontally from the bottom of a soil or waste stack or from the bottom of the upper vertical section of the soil or waste stack that,
(a)receives a discharge of 30 or more fixture units, or
(b)receives a discharge from fixtures located on 2 or more storeys.
(3)No other fixture shall be connected to a lead bend or stub that serves a water closet.
(4)Where a change in direction of more than 45° occurs in a soil or waste pipe that serves more than one clothes washer, and in which pressure zones are created by detergent suds, no other soil or waste pipe shall be connected to it within a length less than
(a)40 times the nominal pipe size of the soil or waste pipe or 2.44 m maximum vertical, whichever is less, before changing direction, and
(b)10 times the nominal pipe size of the nominally horizontal soil or waste pipe after changing direction. (See Note A-7.4.2.1.(4))
(5)Where a vent pipe is connected into a suds pressure zone referred to in Sentence (4), no other vent pipe shall be connected to that vent pipe within the height of the suds pressure zone. (See Note A-7.4.2.1.(4))
Open OBC Div. B 7.4.2.1. in the code viewer
OBC Div. B 7.4.2.2. Connection of Overflows from Rainwater Tanks
(1)Where an overflow from a rainwater tank is connected to a storm drainage system, it shall be connected by
(a)an air break, or
(b)a backwater valve installed on the storm drainage pipe before the connection to the storm building drain.
Open OBC Div. B 7.4.2.2. in the code viewer
OBC Div. B 7.4.3.1. Plumbing Fixtures
(1)Sanitary units, bathtubs and shower baths shall not be installed adjacent to wall and floor surfaces that are pervious to water.
Open OBC Div. B 7.4.3.1. in the code viewer
OBC Div. B 7.4.3.2. Restricted Locations of Indirect Connections and Traps
(1)Indirect connections or any trap that may overflow shall not be located in a crawl space or any other unfrequented area.
Open OBC Div. B 7.4.3.2. in the code viewer
OBC Div. B 7.4.3.5. Macerating Toilet System
(1)A maceration toilet system shall only be installed
(a)where no connection to a gravity sanitary drainage system is available, and
(b)in accordance with the manufacturer's instructions.
Open OBC Div. B 7.4.3.5. in the code viewer
OBC Div. B 7.4.4.1. Sewage Treatment
(1)Where a fixture or equipment discharges sewage or waste that may damage or impair the sanitary drainage system or the functioning of a sanitary sewage works or sanitary sewage system, provision shall be made for treatment of the sewage or waste before it is discharged to the sanitary drainage system.
Open OBC Div. B 7.4.4.1. in the code viewer
OBC Div. B 7.4.4.2. Cooling of Hot Water or Sewage
(1)Where a fixture discharges sewage or clear-water waste that has been heated, the drainage system shall be suitable for the temperature of the sewage or clear-water waste being discharged. (See Note A-7.4.4.2.(1))
Open OBC Div. B 7.4.4.2. in the code viewer
OBC Div. B 7.4.5.1. Traps for Sanitary Drainage Systems
(1)Except as provided in Sentences (2) to (5) and Article 7.4.5.2., fixtures shall be protected by a separate trap.
(2)One trap is permitted to protect
(a)all the trays or compartments of a 2- or 3- compartment sink,
(b)a 2- or 3- compartment laundry tray, or
(c)2 similar type single compartment fixtures located in the same room.
(3)One trap is permitted to serve a group of floor drains and hub drains, a group of shower drains, a group of washing machines or a group of laboratory sinks if the fixtures
(a)are in the same room, and
(b)are not located where they can receive food or other organic matter.
(4)Reserved.
(5)An interceptor with an effective water seal of not less than 38 mm is permitted to serve as a trap. (See Note A-7.4.5.1.(5))
(6)Where a domestic dishwashing machine equipped with a drainage pump discharges through a direct connection into the fixture outlet pipe of an adjacent kitchen sink or disposal unit, the pump discharge line shall rise as high as possible to just under the counter and connect
(a)on the inlet side of the sink trap by means of a Y fitting, or
(b)to the disposal unit.
Open OBC Div. B 7.4.5.1. in the code viewer

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OBC Div. B 7.4.5.3. Connection of Subsoil Drainage Pipe to a Sanitary Drainage System
(1)Where a subsoil drainage pipe is connected to a sanitary drainage system, the connection shall be made on the upstream side of a trap with a cleanout or a trapped sump.
(2)Except as permitted in Sentence (3), no foundation drain or subsoil drainage pipe shall connect to a sanitary drainage system.
(3)Where a storm drainage system is not available or soil conditions prevent drainage to a culvert or dry well, a foundation drain or subsoil drainage pipe may connect to a sanitary drainage system.
Open OBC Div. B 7.4.5.3. in the code viewer
OBC Div. B 7.4.5.4. Location and Cleanout for Building Traps
(1)Where a building trap is installed, it shall
(a)be provided with a cleanout fitting on the upstream side of and directly over the trap,
(b)be located upstream of the building cleanout, and
(c)be located
(i)inside the building as close as practical to the place where the building drain leaves the building, or
(ii)outside the building in a manhole.
Open OBC Div. B 7.4.5.4. in the code viewer
OBC Div. B 7.4.5.5. Trap Seals
(1)Provision shall be made for maintaining the trap seal of a floor drain or a hub drain by
(a)the use of a trap seal primer,
(b)using the drain as a receptacle for an indirectly connected drinking fountain, or
(c)other equally effective means.
(2)Where a mechanical device is installed to furnish water to a trap, the pipe or tube conveying water from the device to the trap shall be at least 3/8 in. inside diameter.
Open OBC Div. B 7.4.5.5. in the code viewer
OBC Div. B 7.4.5.2. Traps for Storm Drainage Systems
(1)Where a storm drainage system is connected to a combined building sewer or a public combined sewer, a trap shall be installed between any opening in the system and the drain or sewer, except that no trap is required if the opening is the upper end of a leader that terminates
(a)at a roof that is used only for weather protection,
(b)not less than 1 m above or not less than 3.5 m in any other direction from any air inlet, openable window or door, and
(c)not less than 1.8 m from a property line. (See Note A-7.4.5.2.(1))
(2)A floor drain that drains to a storm drainage system shall be protected by a trap that
(a)is located between the floor drain and a leader, storm building drain or storm building sewer,
(b)may serve all floor drains located in the same room, and
(c)need not be protected by a vent pipe.
Open OBC Div. B 7.4.5.2. in the code viewer
OBC Div. B 7.4.6.1. Separate Systems
(1)No vertical sanitary drainage pipe shall conduct both sanitary sewage and storm sewage.
(2)Reserved.
(3)There shall be no unused open ends in a drainage system and dead ends shall be so graded that water will not collect in them.
Open OBC Div. B 7.4.6.1. in the code viewer
OBC Div. B 7.4.6.2. Location of Sanitary Drainage Pipes
(1)A sanitary drainage pipe shall not be located directly above,
(a)non-pressure potable water storage tanks,
(b)manholes in pressure potable water storage tanks, or
(c)food-handling or food-processing equipment.
Open OBC Div. B 7.4.6.2. in the code viewer
OBC Div. B 7.4.6.3. Sumps or Tanks
(1)Only piping that is too low to drain into a building sewer by gravity shall be drained to a sump or receiving tank.
(2)Where the sump or tank receives sanitary sewage, it shall be water- and air-tight and shall be vented.
(3)Where the sump or tank receives subsurface water from a subsoil drainage pipe, it shall be provided with a water-and air-tight cover.
(4)Equipment such as a pump or ejector that can lift the contents of the sump or tank and discharge it into the sanitary building drain or sanitary building sewer shall be installed.
(5)Where the equipment does not operate automatically, the capacity of the sump shall be sufficient to hold at least a 24 hour accumulation of liquid.
(6)Where there is a building trap, the discharge pipe from the equipment shall be connected to the sanitary building drain downstream of the trap.
(7)The discharge pipe from every pumped sump shall be equipped with a union, a backwater valve and a shut-off valve installed in that sequence in the direction of discharge.
(8)The discharge piping from a pump or ejector shall be sized for optimum flow velocities at pump design conditions.
(9)The discharge pipe from every pumped storm sewage sump shall be equipped with
(a)a union and a check valve installed in that sequence in the direction of discharge and pumped to above grade level, or
(b)a union, a check valve and a shut-off valve installed in that sequence in the direction of discharge.
Open OBC Div. B 7.4.6.3. in the code viewer
OBC Div. B 7.4.6.4. Protection from Backflow
(1)Except as permitted in Sentence (2), a backwater valve that would prevent free circulation of air shall not be installed in a building drain or in a building sewer.
(2)A backwater valve may be installed in a building drain provided that,
(a)it is a "normally open" design conforming to,
(i)CAN/CSA-B70, "Cast Iron Soil Pipe, Fittings, and Means of Joining",
(ii)CAN/CSA-B181.1, "Acrylonitrile-Butadiene-Styrene (ABS) Drain, Waste, and Vent Pipe and Pipe Fittings",
(iii)CAN/CSA-B181.2, "Polyvinylchloride (PVC) and Chlorinated Polyvinylchloride (CPVC) Drain, Waste, and Vent Pipe and Pipe Fittings", or
(iv)CAN/CSA-B182.1, "Plastic Drain and Sewer Pipe and Pipe Fittings", and
(b)it does not serve more than one dwelling unit.
(3)Except as provided in Sentences (4) and (5), where a building drain or a branch may be subject to backflow,
(a)a backwater valve shall be installed on every fixture drain connected to it when the fixture is located below the level of the adjoining street, or
(b)a backwater valve shall be installed to protect fixtures which are below the upstream sanitary manhole cover when a residential building is served by a public sanitary sewer.
(4)Where more than one fixture is located on a storey and all are connected to the same branch, the backwater valve may be installed on the branch.
(5)A subsoil drainage pipe that drains into a sanitary drainage system that is subject to surcharge shall be connected in such a manner that sewage cannot back up into the subsoil drainage pipe.
Open OBC Div. B 7.4.6.4. in the code viewer
OBC Div. B 7.4.6.6. Building Drain Ends
(1)Where a building drain enters a building above the elevation of the bottom of the wall of a building, the building drain may be deemed to terminate at the first point that the drainage pipe changes direction from the horizontal to the vertical.
Open OBC Div. B 7.4.6.6. in the code viewer
OBC Div. B 7.4.7.1. Cleanouts for Drainage Systems
(1)Sanitary drainage systems and storm drainage systems shall be provided with cleanouts that will permit cleaning of the entire system. (See Note A-7.4.7.1.(1))
(2)A cleanout fitting shall be provided on the upstream side and directly over every running trap.
(3)Interior leaders shall be provided with a cleanout fitting at the bottom of the leader or not more than 1 m upstream from the bottom of the leader.
(4)Where a cleanout is required on a building sewer of NPS 8 or larger, it shall be a manhole.
(5)A building sewer shall not change direction or slope between the building and public sewer or between cleanouts, except that pipes not more than NPS 6 may change direction
(a)by not more than 5° every 3 m, or
(b)by the use of fittings with a cumulative change in direction of not more than 45°.
(6)Building drains shall be provided with a cleanout fitting of NPS 4 or larger that is located as close as practical to the place where the building drain leaves the building. (See Note A-7.4.7.1.(6))
(7)Stacks shall be provided with a cleanout fitting
(a)at the bottom of the stack,
(b)not more than 3 m upstream of the bottom of the stack, or
(c)on a Y fitting connecting the stack to the building drain or branch.
(8)A cleanout shall be provided to permit the cleaning of the piping immediately downstream of an interceptor.
(9)Every indirect drainage pipe carrying waste from a food receptacle shall have a cleanout access at every change of direction of more than 45°.
(10)A cleanout shall be installed on a trap arm serving a kitchen sink as close as practical to the trap outlet and shall be readily accessible.
Open OBC Div. B 7.4.7.1. in the code viewer
OBC Div. B Table 7.4.7.2. Permitted Size and Spacing of Cleanouts
Table 7.4.7.2. — Permitted Size and Spacing of Cleanouts Forming Part of Sentence 7.4.7.2.(1) Nominal Pipe Size of Drainage Pipe, NPS | Minimum Nominal Pipe Size of Cleanout, NPS | Maximum Spacing, m for One-Way Rodding | Maximum Spacing, m for Two-Way Rodding less than 3 | Same NPS as drainage pipe | 7.5 | 15 3 and 4 | 3 | 15 | 30 over 4 | 4 | 26 | 52
Open OBC Div. B Table 7.4.7.2. in the code viewer
OBC Div. B 7.4.7.2. Size and Spacing of Cleanouts
(1)Except as provided in Sentences (2), (3) and 7.4.7.1.(6), the nominal pipe size and spacing of cleanouts in nominally horizontal pipes of a drainage system shall conform to Table 7.4.7.2.
(2)The spacing between manholes serving a building sewer
(a)of NPS 24 or less shall not exceed 90 m, and
(b)over NPS 24 shall not exceed 150 m.
(3)The developed length of a building sewer between the building and the first manhole to which the building sewer connects shall not exceed 75 m.
(4)Where a building sewer connects to another building sewer other than by a manhole, the developed length between the building and the building sewer to which it connects shall not exceed 30 m.
(5)Cleanouts that allow rodding in one direction only shall be installed to permit rodding in the direction of flow.
(6)Manholes shall be located at all junctions and all changes in grade, size or alignment (except for curvilinear alignment) on a sanitary building sewer that is NPS 8 or larger.
(7)Manholes shall be located at changes of grade, size or alignment (except for curvilinear alignment) on a storm building sewer or exterior storm drainage pipe that is NPS 8 or larger.
Open OBC Div. B 7.4.7.2. in the code viewer
OBC Div. B 7.4.7.4. Location of Cleanouts
(1)Cleanouts and access covers shall be located so that their openings are readily accessible for drain cleaning purposes.
(2)A cleanout shall not be
(a)located in a floor assembly in a manner that may constitute a hazard, and
(b)used as a floor drain.
(3)Reserved.
(4)Each change of direction of the piping between a cleanout fitting and the drainage piping or vent piping that it serves shall be accomplished by using 45° bends.
(4.1)A cleanout shall be provided to serve vertical drainage piping from a wall hung urinal and shall extend above the flood level rim of the fixture.
(5)Cleanouts serving fixture drains in healthcare facilities, mortuaries, laboratories and similar occupancies, where contamination by hazardous waste is likely, shall be located a minimum of 150 mm above the flood level rim of the fixture.
Open OBC Div. B 7.4.7.4. in the code viewer