Why do my drains back up when it rains?

1. The city sewer is overloaded

In a major storm, the sewer mains under the street can fill beyond their capacity. With nowhere else to go, water pushes back up the pipes connected to them, including yours. The lowest drain in your house is where it shows up. A backwater valve is the usual protection against this.

2. Tree roots or damage in your sewer line

Many older Lower Mainland homes still have original clay or concrete sewer pipes. Roots find their way into the joints, and the pipe can crack or sag with age. A partly blocked line copes on a dry day but can't handle the extra flow in a storm.

3. Clogged perimeter drains

Perimeter drains, also called drain tile, run around your foundation and carry groundwater away. When they fill with silt and roots, rainwater builds up against the foundation and finds its way inside, often as damp walls or water at the base of the wall.

4. Roof water going to the wrong place

Downspouts that empty right beside the house, or into a blocked underground pipe, pour a roof's worth of water next to your foundation. In some older homes, roof water is still connected into the sewer line, which adds to the load in a storm.

Signs it's more than a simple clog

• It happens mostly, or only, when it rains.
• Several drains are slow at once, not just one sink.
• Toilets bubble or gurgle when another fixture drains.
• Water or a sewer smell comes from the basement floor drain.

What a plumber does about it

The first step is usually a camera inspection: a small camera is fed down the line to see exactly what's wrong and where. From there, the fix might be clearing roots, cleaning the perimeter drains, repairing a damaged section of pipe, or fitting a backwater valve. Knowing the real cause first saves you paying for the wrong fix.