Why London, Ontario Basements Flood: Clay Soil, Storms and the Thames

Basement flooding in London, Ontario usually comes from one of four sources: groundwater held against the foundation by clay-heavy soil, surface water from heavy rain or snowmelt running toward the house, sewer backup through a floor drain during a storm, or a failed sump pump. River flooding from the Thames affects low-lying areas, but most wet basements in the city are nowhere near the river. Knowing which source you have tells you which fix to pay for.

How clay soil pushes water into basements

Much of the London area sits on clay-heavy soil. Clay drains slowly. When it rains for days or the snow melts, water soaks in and stays there, pressed against the foundation wall. That pressure finds every crack, every tie-rod hole and the joint where the wall meets the floor.

Clay also moves. It swells when wet and shrinks when dry. After a dry summer stretch, the soil can pull back from the foundation and leave a gap. The next downpour runs straight down that gap to the footing. Over years, the same swelling and shrinking can crack block walls and tilt window wells.

The soil used to backfill around a new foundation is looser than the undisturbed ground beside it, and it settles. Many houses that started with a proper slope away from the wall end up with a shallow trough along the foundation, which collects water exactly where you do not want it.

Spring thaw and summer storms

There are two high-risk seasons. In late winter and spring, snow melts while the ground is still frozen or already saturated. The water cannot soak in, so it runs across the surface toward the lowest point, which is often a window well or a basement door. Rain falling on snow makes it worse.

In summer, the risk is the short, intense thunderstorm. A large amount of rain falls in under an hour, eavestroughs overflow, and downspouts that discharge beside the wall dump a roof's worth of water into the backfill. These are the storms that also overload sewers.

Sewer backup and sump pump failure

During a heavy storm, municipal sewers can fill beyond capacity. When that happens, water and sewage can push back up the lateral and come out at the lowest opening in the house, usually the basement floor drain, a shower or a toilet. A backwater valve on the lateral closes when flow reverses and is the main defence.

Older neighbourhoods have another weak point. Mature trees send roots into old clay sewer laterals, and a partly blocked lateral backs up with far less rain.

A sump pump protects the basement from groundwater collected by the weeping tile. Its weak points are simple. The storms that raise the water table also knock out hydro, and a pump without battery backup stops exactly when it is needed. Floats stick, pumps wear out, and discharge lines freeze or get buried in winter.

What about the Thames River?

The Thames and its branches meet in the middle of London, and the land beside them is floodplain. Development in those areas is regulated by the Upper Thames River Conservation Authority, which also issues flood warnings. If your home is in or beside a mapped flood area, river levels are a real factor, and you should follow those warnings in spring and after major rain.

Higher river levels also slow the drainage of storm sewers and raise groundwater near the river. For most London homes though, the cause of basement flooding is on the property itself or in the sewer at the street.

Basement flooding in London, Ontario: signs of each source

• Water at the wall and floor joint or seeping through cracks after long wet spells points to groundwater and foundation drainage.
• Water below a window or coming in at one corner during a downpour points to grading, window wells or downspouts.
• Dirty water or a sewer smell coming up through the floor drain points to sewer backup.
• A full sump pit with a silent pump points to pump or power failure.
• Water that shows up with no rain at all points to plumbing, a water heater or an appliance.

Fixes that address the cause

1. Extend downspouts well away from the foundation, a couple of metres if you can, and keep eavestroughs clean.
2. Regrade so the soil slopes away from the wall, and top up settled backfill.
3. Add covers and proper drains to window wells.
4. Test the sump pump each spring and fall by pouring water into the pit, and add a battery backup and a high-water alarm.
5. Have a licensed plumber install a backwater valve if you do not have one.
6. Have the sewer lateral inspected by camera if the house is older or surrounded by large trees.

The City of London has offered subsidies toward items like sump pumps and backwater valves. Check with the City for what is currently available before you book the work. Also ask your insurance broker whether your policy includes sewer backup and overland water, since both are often optional. If your basement is wet now and you are not sure of the source, a moisture inspection with a thermal camera and meter can trace it, and those run $249 to $549.

Common questions