Basement flooding

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Causes of basement flooding

The sanitary sewer system collects wastewater from your home through a pipe called a sanitary lateral. This lateral carries wastewater from the following:

  • Toilets
  • Washing machines
  • Dishwashers
  • Showers
  • Sinks

From here, the water runs to the sewer main, which carries wastewater from other homes and businesses to the City's wastewater treatment facility.

Foundation drains collect groundwater from around the foundation of your home and are usually connected to the storm sewer by a storm lateral. In some older homes, these drains may be connected to the sanitary or combined sewer by a sanitary lateral, or may not be present. Water from rainfall events and snow melts are also collected by storm sewers and directed to nearby watercourses or stormwater management ponds.

While the City regularly cleans, inspects and repairs the sanitary sewer systems, unanticipated problems can occasionally occur, causing increased water levels in the sewer (or surcharging).

Flood prevention diagram

Backups and flooding can occur as a result of:

Increased water level of the City’s sewer main

If the sewer main, generally located under the street, is blocked or damaged, sewage may enter your home due to increased water level or surcharging in the City’s sewer system if you don’t have a backwater valve or if your backwater valve has not been maintained. Sewer surcharging may be due to:

  • Collapse or other structural defects.
  • Blockage by waste and debris.
  • Heavy rainstorms.
  • Construction activities in the area.

Blockage of the lateral

If the lateral from your home becomes blocked, sewage from inside your home may back up into the basement. The blockage may be due to:

  • Accumulation of grease, paper, kitchen waste or other foreign objects
  • Presence of tree roots (private or City-owned trees)
  • Collapse, misalignment or other structural defects of the lateral
  • Clogged up backwater valve

Spring runoff or Heavy rainfall event

Melting snow and ice from spring runoff or a heavy rainfall event can leak through cracks or joints of your foundation in your basement walls or floor.

Overland flooding

Overland flooding is where water flows overland and seeps into buildings through the following:

  • Windows
  • Doors
  • Cracks

The two most common causes of overland flooding is snowmelt and rainstorms.

River flooding

River flooding sometimes occurs in some areas of the city. The City clears ice each spring from the Rideau River to prevent serious flooding.  

To find out if you live in Ottawa's 1-in-100 year flood plain, consult the City's interactive map.

Alternate types of sewer systems

In some older homes, foundation drains are connected to the sanitary or combined sewer by a sanitary lateral, or are not present. Basement flooding caused by sewer backups can originate from different sources depending on the type of sewer system in your area.

Please note: New buildings (or “infill”) in older areas of the city may have more modern sewer connections than described below.

Partially separated area

  • Older buildings with foundation drains connected to the sanitary sewer.
  • Storm sewers only have a street's catch basins connected.
  • Basement flooding risk is generally from the sanitary sewer.

Partially separated area (former combined sewer area)

  • Older buildings with no foundation drains.
  • Catch basins are connected to the storm sewer.
  • Basement flooding risk is generally from the sanitary sewer.

Combined sewer area

  • Older buildings with no foundation drains present.
  • No storm sewer is present.
  • All sanitary laterals, foundation drains and catch basins connected to one combined sewer.
  • Basement flooding risk is generally from the combined sewer.

Responsibility for sewer laterals

  • The City is responsible for the portion of the sanitary and storm sewer laterals from the property line to the street.
  • The homeowner is responsible for the portion of the sanitary and storm sewer laterals from the property line to the home.
  • There may also be shared responsibility if a lateral requiring repair or replacement crosses the property line.

How to prevent basement flooding

You can follow the following steps to reduce the risk of basement flooding: 

  • Seal window wells and cracks in floors, walls and the foundation.
  • Slope ground away from the foundation to allow rainwater to flow away from the home.
  • Direct downspouts from eaves troughs away from the foundation (minimum of 1.2 metres) or to a rain barrel.
  • Disconnect downspouts from the sewer system or foundation drains.
  • Don't put grass clipping, leaves, or other debris on the streets as they can plug the drains and prevent proper drainage, particularly during heavy rainfalls. Plugged drains cause water to build up on the street, increasing your risk of basement flooding.
  • Don't throw garbage, grease or hazardous waste down your drains. Garbage that gets into your sewer through your drains can clog your sewer and cause sewer backup and grease hardens as it cools and sticks to the inner lining of sewer pipes, eventually causing a blockage.
  • Ensure foundation drains direct water to the storm sewer or sump pump. Foundation drains should not be connected to the sanitary sewer.
  • Clear snow away from your foundation to prevent snowmelt from seeping in through the walls.
  • Ensure the sump pump is connected to the storm sewer or discharges to the ground at least 1.2 metres from the foundation.
  • Install protective plumbing devices, such as backwater valves which protect against surcharging in City sewers. The City’s Residential Protective Plumbing Program offer rebates to qualified homeowners.
  • Maintain existing protective plumbing devices according to the manufacturer’s instructions. A protective plumbing device should be maintained periodically or before a forecasted heavy rainfall to ensure it is free of debris, functioning properly and that cleanout caps and access covers are firmly secured.
    • In older homes, especially those with cast iron pipes, additional maintenance may be required, as rust can accumulate at the hinge and prevent proper closure of the backwater valve during a surcharge event.

Additional resources on preventing basement flooding

What to do if your basement has flooded

Recommended homeowner response to basement flooding:

  1. Can you determine where the water Is coming from? You can check if your toilets and sinks are draining properly by running your toilet or sink and observing if that causes water to come up through your drain; if no water comes up through your drain when running water, the problem is likely ground water.
    1. Avoid using fixtures and appliances which require water, until after the issue has been resolved, as any water sent down the drain may end up in your basement.
  2. Wearing appropriate protective clothing, clear any blockages to ensure proper flow.
  3. Call 3-1-1 to report your flood. The City has expert staff on duty at all times who will respond as soon as possible to these situations. If requesting City assistance, ensure someone 18 years or older is at home to meet City staff.
  4. Contact your insurance company
  5. Locate the sewer clean-out caps. Do NOT attempt to open. Clean-out caps are usually located in the basement floor at the front of the home (near the water meter). Ensure an area at least one meter on all sides of the clean-out is free from obstructions (boxes, furniture) to provide access to City staff.


Do NOT enter the flooded area until a qualified professional has determined it is safe to enter.

Natural Gas

If the flood water level is threatening any gas-fired equipment such as a furnace, hot water heater or stove, contact Enbridge Gas.


If the flood water level has reached any plug, electrical outlet, extension cord or electrical appliance such as a baseboard heater, contact Hydro Ottawa.

More information on flood safety can be found from the Electrical Safety Authority.

Bacteria or viruses

Basement flood water may contain sewage from the sanitary sewer. Flood water contaminated with sewage may contain bacteria and viruses which can affect your health and the health of your family. Be sure to wear protective clothing such as rubber boots, gloves, safety glasses and a facemask, and wash thoroughly after any contact with flood water or items which may have been in contact with flood water.

Flooding and your health: What you need to know

The following links provide information on preparing for a potential flood; protecting yourself and your family during a flood; and restoring your home after a flood. 

For more information on cleaning, disinfecting, mould and re-entering your home, refer to the Government of Canada – Get Prepared or Centers for Disease Control and Prevention.

Contacting your insurance company

Your insurance company will be able to advise you regarding standard clean-up and claims procedures, along with contractors to contact. Generally,

  • Understand your coverage and what type(s) of flooding damage are covered.
  • Photograph and document damaged items or items which will need to be replaced.
  • Retain receipts for any emergency and repair work, along with replacement items.

For more information on home insurance, visit the Insurance Bureau of Canada website. 

City response to basement flooding

  1. City staff will inspect the sewer main near your home to determine whether or not it is functioning normally. If there is a problem, it will be fixed as soon as possible.
  2. If the problem is not related to the sewer main, City staff can help you identify the problem, if resources are available.
  3. If the problem is in the home’s drain system, the homeowner will be advised to contact a plumber.
  4. If the problem is related to either the sanitary or storm lateral, repairs may be the homeowner's responsibility, City's responsibility, or a shared responsibility. Responsibility can only be determined after City staff have reviewed all of the available information.

Preventative Maintenance Program

The City maintains a list of properties confirmed to have experienced a sewer backup caused by City-owned tree roots or structural defects on the City-owned portion of the sewer that cannot readily be repaired.

As part of its Preventative Maintenance Program, the City offers a sewer maintenance service annually (or as otherwise modified) to these impacted properties to ensure the proper function of the sewer and help prevent sewer backups.

For further information, please contact 3-1-1.

Responsibility for the repair costs

The cause and location of the blockage generally determines who is responsible for the cost of removing the blockage or repairing the lateral. Generally, lateral blockages are the responsibility of the homeowner, unless the blockage is due to City-owned tree roots or the damaged portion is on City property. For more information on repair costs and responsibility for those costs, refer to the Sewer Connection By-law, as amended.

If the flooding is due to a blockage on the private portion, a contractor must be contacted. As a homeowner, there is the option to select a contractor of your choice, or the City's contractor. Work not related to removal of a blockage such as repair, relining and replacement are billed directly by the contractor to the homeowner.

If, during the course of the repair work, it is determined that the blockage is located on City property, the homeowner will not be charged for the repair work. If payment has already been made for any work, the homeowner will be reimbursed. The reimbursement amount will be up to the amount typically paid by the City to its contractor for the same service. Requests for reimbursement should be directed to the City's Claims Unit.

In some cases, repair costs may be shared between the City and the homeowner.

Making a claim

Homeowners who have incurred damage to their home or its contents should contact their insurance company for assistance. Typically, your insurance company will submit a claim to the City for investigation/response on your behalf. Claims can be submitted to the City’s Claims Unit.

For more information, please contact our general claims line at 613-580-2655 or by email at

Residential grants for sewer backups

Residents affected by flooding three or more times could be eligible for a grant of up to $1,000 under the Residential Compassionate Grant policy for sewer backups.

Apply for a Residential Compassionate Grant

To qualify, the resident (property owner or tenant) must satisfy the following conditions:

  • Proof of three or more sewer backups during a 15-year period that are related to sewer surcharging as a result of significant rainfall
  • Owned or rented the home during all three backups
  • Experienced at least $1,000 of damage during the most recent backup
  • Property owners must have their property taxes paid in full
  • Sewer backups must be reported to the City within four months of taking place

All backups that occur after September 1, 2009 must be reported to the City within four months of the event. You must submit your application within four months of the third backup.

For additional information regarding the Residential Compassionate Grant Program, contact:

Residential Compassionate Grant Program
City of Ottawa
Infrastructure and Water Services Department
951 Clyde Avenue
Ottawa ON K1Z 5A6
Tel: 3-1-1
Fax: 613-728-6928
TTY: 613-580-2401