Mandatory Backwater Valves on Storm and Sanitary Laterals

In 2009, the City of Ottawa, Ontario, was subjected to a major rainstorm and flood and, in the aftermath, realized that the effective use of backwater valves was a critical, but secondary, line of defence against urban flooding. An analysis conducted after this flooding event found that out of some 1500 homes that received flood damage, about eight percent already had a backwater valve installed. The end result of a more detailed analysis of this problem indicated that backwater valves are indeed a useful tool in preventing flood damage but need support and maintenance in order to continue functioning properly. As they are located in homes, it is therefore paramount that the city educate homeowners on what backwater valves are, how they work, what protection they offer, and how to maintain that protection over time. This education was a key component of the 2009 by-law requiring the installation of backwater valves in all new homes.

Understanding and Assessing Impacts

Backwater valves are, effectively, a one-way gate that is placed inside the sanitary sewer lateral of a home. They allow for wastewater to flow out from the home into the sewer system, but prevent sewage from flowing into the home via the sewers during a flooding event. Research has consistently shown that these are an effective tool for the prevention residential flooding, which is a phenomenon that now consistently causes more than $2 billion in damages annually in Canada. However, a backflow valves does require maintenance in order to work properly. An analysis of Ottawa’s 2009 flood indicated that the most common point of failure was the valve cover, which was often not screwed into place adequately, causing eh system to fail. Other notable possible causes of backwater valve failure is a degradation of the O-rings in the system, preventing a complete seal, or the collection of debris under the gate itself, which can cause the gate the stick in the open position and thus rendering in unable to stop sewer backflow from entering a home. As the climate continues to change, it is likely that extreme rainfall events will become more common, necessitating a greater reliance on preventative devices like backwater flow valves.

Identifying Actions

In the wake of the flooding event in 2009, the City of Ottawa undertook a comprehensive review to understand what caused water to enter homes protected by a backwater valve, to investigate City standards with regard to current industry technology and practices, and to see what other municipalities were doing to prevent sewer backups. Upon the completion of this review, the City of Ottawa created a five-step plan to optimize the potential of backwater valves and reduce the occurrence of basement flooding in the future. One of the foremost aspects of this plan was the recognition that while backwater valves prove to be an effective means of protecting against sewer backflow events, they should not be the only tool for doing so; the sewer system itself, alongside aboveground stormwater management tools, must be continuous maintained and upgraded to act as the most important mechanisms in protecting homes from flooding. The review also recommended that backflow valves be made mandatory on all new homes. This was at least partially based off of a cost-benefit analysis noting that installing a backflow valve as a retrofit in an existing home would cost roughly $1400 while installing one in a new home costs approximately $250. Recognizing the role that homeowners must play in maintaining these devices to ensure their continuous and effective operation, the report also recommended an education and outreach program to ensure that people have the skills necessary to keep the valves operational over time.


Acting on the recommendations proposed in the wake of the 2009 flood, in that same year the City of Ottawa implemented a bylaw requiring all newly built structures to have a backwater flow valve installed. There was an already an existing law that required the installation of a sewage backflow prevention device on any new foundation drain systems connected to a City storm or combined sewer system. This new law updates that and makes such devices explicitly mandatory on all new buildings, both residential and commercial.

Outcomes and Monitoring Progress

The rebate program is not mandatory but has still resulted in the installation of more than 900 backwater protection valves between the program implementation in 2005 and the writing of this case study in 2014.


Link to Full Case Study

Using climate change projections enables better adaptation decisions, as it allows you to better understand how the climate may change. To learn how to choose, access, and understand climate data, visit’s Learning Zone.
For more information on variables that may be useful in work related to stormwater and wastewater management, visit and click “Explore by Variable”. Here you will find pertinent future climate projections related to:

  1. Historic and climate change scaled IDF data
  2. Total precipitation
  3. Maximum 1-day total precipitation
  4. Wet days (>1mm, >10mm, >20mm)