By-law to Prohibit Construction of Reverse Slope Driveways

In the wake of a damaging and costly storm in 2005, the City of Markham, Ontario, enacted a series of legislative actions to combat the heightened risk of flooding caused by reverse-slope driveways. These are a specific form of driveway in which the main garage is located below grade and the driveway slopes downhill towards it. As a result, in the event of a rainfall the water landing on the driveway is conveyed directly into the home’s basement via the below-grade garage. Banning the use of reverse-slope driveways in all new home construction was one of several steps that helped reduce the risk of basement and sewer backup flooding during a rainstorm in Markham.

Understanding and Assessing Impacts

The threat of basement and sewer backup flooding is already high in Canada, causing some $2 billion in damages annually across the country. This danger, and the associated costs of the damage, is projected to only increase in the future as a result of global climate change. Reverse slope driveways serve to exacerbate the risk posed by flooding events as the driveway, sloping downward towards the house, ensures that any water on its surface will flow into the structure or directly into the sanitary sewer through basement floor drains. It is also often the case that these driveways have a catch basin connected to the weeping tile systems or the sanitary sewer lateral. When these basins overflow, or are clogged with debris, they can increase the likelihood of a sewer backup flood. In the particular case of Markham, Ontario, this problem as highlighted as a result of widespread basement flooding in the aftermath of a major rainstorm in 2005. Though now since surpassed, this event was at the time the costliest natural disaster in Ontario’s history, causing some $713 million in damages. It was in response to this specific event that Markham took measures to reduce the risk of flooding from major storm events.

Identifying Actions

The primary trigger for the creation of this new bylaw, and other associated measures, was the major rainstorm in the Toronto area in August of 2005. During this event, some 153mm of rain fell in the span of three hours, causing widespread flooding and roughly $718 million in damages across Southern Ontario. At the time, it was the costliest natural disaster in Ontario’s history. In Markham, the neighbourhoods of West Thornhill were most heavily affected, with the local sewer systems unable to handle the high levels of overland flow and sewer infiltration. Immediately after the storm, the public petitioned the City to take proactive measures to ensure that the residents and their homes were better protected from similar events in the future. The City’s first step was to conduct a Municipal Class Environmental Assessment study and also to hire a consultant to investigate potential alternatives to the existing stormwater system that might offer better protection from flooding. Additionally, the City of Markham undertook an internal review of their wastewater management system in an attempt to increase capacity and find a workable solution to the problem of excess infiltration (in which rainwater enters the sewer system through poorly sealed connections or cracks in the pipes). The collaboration between the external and internal review teams put forth a series of recommendations to increase Markham’s resilience to future storm events.


Following an extensive review and analysis, several propositions were put forward to help increase Markham’s resilience to future storm and flooding events. In 2012, a new by-law was enacted that banned the use of reverse-slope driveways in all newly-built homes in the City. Specifically, where a driveway leads to a private garage attached to a dwelling, that garage’s finished floor height must be higher than the elevation of the street or lane from which the garage is connected. This by-law is enforced during the planning and design phase of new construction when the developer submits their plan to the city for review. The city is also taking other, non-legislative, step to ensure a higher resilience to flooding. Markham has been working to reduce the depth of water pooling on the street by increasing the sewer and inlet capacity on public roads. With less water pooling on the street, there is less water available to flow into those homes that do have a reverse slope driveway. The city is also working to provide minor grading changes to reverse sloped driveways by creating a slight downslope away from the lip of the driveway where it begins its descent to the garage; this reduces the amount of surface area that is drained by the reverse slope of the driveway and, therefore, the volume of water. The City is also undertaking smoke-testing to find locations where a cross-connection results in a reverse slope driveway’s catch basin being connected to the sanitary sewer system. Homeowners will be required to remedy this problem.


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 flooding, 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. Maximum 1-day total precipitation
  3. Wet days (>1mm, >10mm, >20mm)