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.
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.