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The Institute for Catastrophic Loss Reduction has issued a new report, Lot-side risk reduction through voluntary retrofit programs, code interpretation and by-laws. The report shows wide variations across Canada in how backflow prevention rules are interpreted, resulting in many new houses still being built without backflow preventers. Backflow preventers can be a key tool in preventing basement flooding as climate change increases the severity of storms. As flooding becomes more common and more expensive, more property owners are finding themselves unable to obtain insurance against it, with potentially catastrophic consequences for family/small business finances. Clear rules requiring backflow preventers, and having them installed in all new buildings, could help.


While it has been previously reported that resolution of code enforcement issues may result in reduced vulnerability to extreme natural events, issues surrounding code interpretation have not previously been studied. Among other topics, this study investigated interpretation of code wordings that relate to installation of backwater valves to protect homes from sewer backup—a significant cause of basement flooding associated with extreme precipitation events and urban flooding. Despite consistent application of code wordings related to backwater valves across the regions of Canada represented in this study, it was found that there are differing interpretations of code wordings, which resulted in differing reported frequencies of installation of backwater valves on both sanitary/combined and storm sewer service connections. Thus, the primary recommendation of this report is that sentences in the National Plumbing Code and provincial building and/or plumbing codes that relate to installation of backwater valves to protect against sewer backflow be reworded or clarified to ensure they are clearly and consistently interpreted and applied. There are many advantages of installing backwater valves in new homes. Due to the unpredictable nature of extreme rainfall events and the unpredictability of infiltration and inflow (I/I) in relatively new, separated sewer systems, it is often impossible to identify which regions of an urban municipality are exposed to sewer backup risk until wide spread or regional sewer backup events have occurred. It is also more economical to install backwater valves in new homes when compared to retrofitting valves into existing homes. For example, several Canadian municipalities provide partial retrofit subsidies of several thousand dollars for the retrofit of backwater valves, while installation of valves in new homes costs approximately $250. Requiring installation of valves in new homes would also help offset relatively low uptake frequencies for municipal subsidy programs aimed at encouraging homeowners to adopt urban flood risk reduction measures.

The study revealed that backwater valve code wording is interpreted differently across the country, though there is greater interpretation consistency in some regions than in others. Specifically, the survey revealed that 19% of British Columbia respondents, 81% of Alberta respondents, 86% of Saskatchewan respondents, 72% of Manitoba respondents, 26% of Ontario respondents and 58% of respondents from New Brunswick and Nova Scotia interpreted code wordings in a manner that required backwater valves to be installed in all or most new homes. The study further revealed that interpreting code wording in this manner was strongly correlated with a higher frequency of installation of backwater valves in new homes, indicating the importance of code interpretation for backwater valve installation.

This study recommends that code wordings related to protection from backflow through the use of backwater valves be clarified, or that provincial and national code authorities provide guidance to local authorities about how code wordings related to protection of homes from sewer backflow should be interpreted. This guidance should outline that code wordings be interpreted in a way that requires the mandatory installation of backwater valves in all or most new Canadian homes. The substantial costs associated with sewer backup insurance claims, the legal liability of municipalities generated by regional sewer backup events, health and home liveability risks posed to households created by sewage flooding and the fact that urban flooding and sewer backup occurrences are likely to increase as a result of increasing frequency of extreme rainfall caused by climate change justify a mandatory requirement for installation. The need for lot-side protection from backflow is exacerbated by the changing nature of stormwater runoff caused by increasing urban development and the unpredictable nature of I/I leading to surcharging in separated sewer systems.

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