Sewer Backup vs Overland Flooding

The short version

A sewer backup happens when water and waste flow backward out of the sewer system and into your home, usually through the basement floor drain, a basement toilet or a laundry tub. It can be caused by a blockage in your own lateral, or by a municipal sewer overwhelmed during heavy rain.

Overland flooding is surface water entering from outside: rain or meltwater running across the ground and in through window wells, doors, cracks in the foundation or low spots in the wall. It comes from the outside in, rather than from the drains up.

The two can happen in the same storm, and the damage can look alike at first glance. Telling them apart matters for safety, for cleanup and for insurance. Oakville's older south-end streets and its newer north-end subdivisions can both see either kind, so no neighbourhood is immune.

Step by step

1. Look at where the water appeared first. Water rising from the floor drain or toilet points to a sewer backup. Water at a window well, along a wall or under a door points to overland flooding or groundwater.
2. Note the smell and colour. Sewage has an unmistakable odour and is often dark. Treat any backed-up water as contaminated.
3. Stop using water. In a sewer backup, every flush and every load of laundry adds to the problem.
4. Document everything. Take photos and video before cleaning. Note the time and the weather.
5. Call your insurer. Sewer backup and overland flood coverage are frequently separate parts of a home policy. Ask what yours includes.

Mistakes to avoid

• Assuming it was the city. A backup during dry weather is more likely a blockage in your own line, such as roots or grease.
• Cleaning sewage with household methods alone. Contaminated drywall, carpet and insulation usually have to go.
• Ignoring downspouts and grading. Overland water often comes from roof runoff dumped right beside the foundation or soil sloping toward the house.
• Skipping prevention after the first event. The same storm pattern will come again.

When to call

Call a plumber when water rises through a drain, or as soon as it is safe after any flood. The plumber can clear a blocked lateral with main sewer line clearing, run a camera to find the cause, and recommend protection. A backwater valve guards against sewer backups. A reliable sump pump manages groundwater. Grading, downspouts and window well covers handle overland water.

For the hours after a flood, see basement flood cleanup plumbing. For year-round planning, read basement flooding prevention.

Keep reading

• Backwater Valve Installation in Oakville

  A summer storm overloads the sewer and it pushes back toward your basement.

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• Basement Flood Plumbing Response in Oakville

  Water across the basement floor.

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• Main Sewer Line Clearing in Oakville

  When every drain in the home slows at once, the trouble is not under the sink.

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• Preventing Basement Flooding in Oakville

  Most basement floods start outside the house or in the sewer.

  Read more →