Skip to content
BCIN ExamBCIN Exam

Code guide

Backwater Valve Requirements in Ontario

Backwater valves in the 2024 Ontario Building Code: when fixtures below the street or the upstream manhole need one, normally open valves, sump discharges.
fixtures that need backflow protection
Below street
fixtures that need backflow protection
the only type allowed on a building drain
Normally open
the only type allowed on a building drain
most a building drain valve may serve
1 unit
most a building drain valve may serve
union, backwater valve, shut-off on sump discharge
3 parts
union, backwater valve, shut-off on sump discharge

Last updated

Under the 2024 Ontario Building Code, where a building drain or branch may be subject to backflow, a backwater valve must protect the fixtures below the level of the adjoining street, or, for a residential building on a public sanitary sewer, the fixtures below the upstream sanitary manhole cover (OBC Div. B 7.4.6.4.(3)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…). A valve on the main building drain must be a normally open design and serve not more than one dwelling unit (7.4.6.4.(2)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…).

This page explains the plumbing rules for backwater valves in Part 7 of Division B. Municipal sewer use by-laws and basement flooding programs add their own requirements and subsidies, and your building department decides how the code applies.

Backwater valve rules at a glance

Item Rule in short Where to look
What it is a check valve for a gravity drainage system Div. A 1.4.1.2.
Valves that block air not allowed in a building drain or sewer 7.4.6.4.(1)
Valve on the building drain normally open, CSA listed, one dwelling unit only 7.4.6.4.(2)
Fixtures below the street backwater valve on every fixture drain 7.4.6.4.(3)(a)
Residential on a public sewer protect fixtures below the upstream manhole cover 7.4.6.4.(3)(b)
Several fixtures on one branch one valve on the branch 7.4.6.4.(4)
Subsoil drain to a surcharging sewer no backup into the subsoil drain 7.4.6.4.(5)
Pumped sump discharge union, backwater valve, shut-off valve 7.4.6.3.(7)

This is our summary of the articles, not a copy of the code. Read the full text before you plan the plumbing.

What a backwater valve does

The code defines a backwater valve as a check valve designed for use in a gravity drainage system (Div. A 1.4.1.2.OBC Div. A 1.4.1.2.(1) Backwater valveBackwater valve means a check valve designed for use in a gravity drainage system.). Sewage flows out through it, and a flap closes if the municipal sewer backs up, so sewage cannot rise through basement floor drains, toilets and showers. Sewer backup during heavy rain is one of the most common causes of basement flooding in Ontario.

When the code requires one

Backflow protection is required where a building drain or a branch may be subject to backflow (7.4.6.4.(3)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…). There are two ways to provide it:

  1. A valve on every fixture drain where the fixture sits below the level of the adjoining street (7.4.6.4.(3)(a)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…).
  2. For a residential building served by a public sanitary sewer, a backwater valve that protects the fixtures located below the upstream sanitary manhole cover (7.4.6.4.(3)(b)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…).

Where several fixtures on one storey connect to the same branch, a single valve on the branch can serve them (7.4.6.4.(4)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…). A subsoil drainage pipe that drains to a sanitary system subject to surcharge must be connected so sewage cannot back up into it (7.4.6.4.(5)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…).

Main drain valves: normally open only

A backwater valve that would prevent the free circulation of air cannot go in a building drain or building sewer, because the drainage system relies on air moving through it (7.4.6.4.(1)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…). The exception is a normally open valve, which stays open until backflow closes it. It can be installed in a building drain where (7.4.6.4.(2)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…):

  • it conforms to one of the CSA pipe and fitting standards listed in the article (cast iron, ABS, PVC or plastic drain pipe), and
  • it serves not more than one dwelling unit.

So a single house can have a normally open mainline valve, but a building with several units protects fixtures on their own drains or branches.

Sumps and other places valves appear

The discharge pipe from every pumped sump must have a union, a backwater valve and a shut-off valve, installed in that order in the direction of discharge (7.4.6.3.(7)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…). Backwater valves also appear where a drinking fountain or a heating and cooling drain pan connects to a storm system that is subject to backflow (7.4.2.1.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…), and on a rainwater tank overflow connected to a storm drain (7.4.2.2.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.). See the sump pump guide for the sump rules.

Maintenance and permits

Install a backwater valve where it can be reached for inspection and cleaning, because debris can keep the flap from closing. Installing one in an existing drain is plumbing work, which the Building Code Act treats as a building, so a permit is normally needed. Many Ontario municipalities run basement flooding protection programs that subsidize backwater valves, often with conditions such as a permit and a licensed plumber.

Common mistakes

  1. A normally closed valve on the main drain. It blocks air circulation and is not allowed.
  2. One mainline valve for a multi-unit building. The building drain valve can serve only one dwelling unit.
  3. No access. A valve buried under a finished floor cannot be maintained.
  4. Valves out of order on a sump discharge. Union, backwater valve, then shut-off valve, in the direction of flow.
  5. Skipping the permit and subsidy paperwork. Many subsidies require both.

How to look it up yourself

Open Division B, Part 7 of the 2024 Compendium. Search terms that appear in the code text:

  • "Protection from Backflow" (7.4.6.4.)
  • "backwater valve" (Div. A 1.4.1.2. and Part 7)
  • "upstream sanitary manhole cover" (7.4.6.4.(3)(b))
  • "Sumps or Tanks" (7.4.6.3.)

Read the article in the code viewer.

BCIN exam angle

When a backwater valve is required and which type may go on a building drain are classic Plumbing House exam questions. Try the free quiz on the Plumbing House exam page, and see the Ontario plumbing code guide for the rest of Part 7.

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

© King's Printer for Ontario, 2024. Reproduced with permission.

Sources

Free accountNo card needed

Start practising for your BCIN exam, free

  • A free practice quiz for each exam
  • Every answer cites its 2024 OBC article
  • Full 70 question timed simulations with a paid plan
  • Works on phone, tablet and computer
Create a free account

Frequently asked questions

Is a backwater valve required in Ontario?
Where a building drain or branch may be subject to backflow, the code requires protection: a backwater valve on every fixture drain for fixtures below the level of the adjoining street, or, for a residential building on a public sanitary sewer, a backwater valve protecting the fixtures below the upstream sanitary manhole cover (Division B, Sentence 7.4.6.4.(3)).
What is a backwater valve?
The code defines it as a check valve designed for use in a gravity drainage system. It lets sewage flow out of the building and closes if sewage tries to flow back in, which protects basements when the municipal sewer surcharges in heavy rain.
Can a backwater valve go on the main building drain?
Only a normally open design meeting one of the listed CSA standards, and only where the drain serves not more than one dwelling unit. Otherwise a valve that would block free circulation of air cannot be installed in a building drain or sewer (Division B, Sentences 7.4.6.4.(1) and (2)).
Does a sump pump need a backwater valve?
The discharge pipe from every pumped sump needs a union, a backwater valve and a shut-off valve in that order in the direction of discharge. A pumped storm sump can instead use a union and a check valve pumped above grade, or a union, check valve and shut-off valve (Division B, Sentences 7.4.6.3.(7) and (9)).
Do I need a permit to install a backwater valve?
Plumbing counts as a building under the Building Code Act, so installing a backwater valve in a drain is normally plumbing work that needs a permit. Many municipalities also offer subsidies for backwater valves. Check with your building department.