Why Ottawa basements crack: Leda clay, freeze-thaw and what to watch for
Updated
How Ottawa's clay soils, frost and water put stress on foundation walls, how to tell a cosmetic crack from a structural one, and when to get it looked at.
The clay under much of Ottawa
A lot of Ottawa sits on Champlain Sea clay, often called Leda clay or sensitive marine clay. It was laid down when the sea covered the Ottawa Valley after the last ice age. It holds a lot of water.
That water content is the problem. The National Research Council's Canadian Building Digest 184 notes that these St. Lawrence and Ottawa valley clays have very high natural water content and can shrink a great deal when they dry out. When the soil under one corner of a house shrinks more than the rest, the footing moves unevenly. Walls crack to relieve the stress.
Trees make it worse. The same digest describes deep-rooted trees in dry summers pulling enough water from the clay to cause several inches of uneven movement in a single growing season. This is why the City of Ottawa has published tree planting guidelines for sensitive marine clay soils, most recently in 2017, with setback rules for trees near houses.
Frost, water and the other usual causes
Clay is not the only cause. Most cracks in poured basement walls are shrinkage cracks. Concrete shrinks as it cures, and a long wall relieves that stress by cracking, often near windows and corners. These usually show up in the first few years.
Water does the rest. Soil that stays wet against a wall pushes on it, and in an Ottawa winter that wet soil freezes and expands. Poor grading, short downspouts and clogged weeping tile all keep soil wet. Public Safety Canada's flood guidance says soil next to the foundation should sit about 8 cm higher than soil 1.5 m away, so water runs away from the house.
Older homes add their own issues. Block walls crack along mortar joints. Century-old stone foundations in the Glebe and Old Ottawa South lose mortar over time. Those walls need different repairs than a 1990s poured wall in Barrhaven.
Cosmetic cracks versus structural ones
Most cracks are not structural. A thin vertical or slightly diagonal crack in a poured wall, hairline to a few millimetres wide, with both sides flush, is usually a shrinkage crack. It matters mainly because it can leak.
Signs that point to movement are different. Look for cracks wider at one end than the other, horizontal cracks, stair-step cracks in block walls, or one side of the crack sitting proud of the other. A wall that bows inward is a warning, and so is a crack that keeps growing.
Clues upstairs help too. Doors that start to stick, new drywall cracks at the corners of openings and sloping floors can all point to the foundation moving rather than just shrinking.
When to call someone
Public Safety Canada suggests getting professional advice for cracks wider than about 5 mm (a quarter inch), cracks that are growing, networks of many cracks, and any crack with water coming through. It also flags finished basements, since the walls are hidden and harder to judge.
Add to that list any horizontal crack, any bowing and any crack where one side has shifted. Those need someone qualified to look at them. A structural engineer may be needed for the worst cases. Their fee is separate from the repair.
Leaking shrinkage cracks are not an emergency, but they are worth fixing before you finish a basement. Drywall over a wet crack hides the problem and grows mould.
What you can do this week
Mark both ends of each crack with a pencil and write the date beside it. Take a photo with a ruler or coin for scale. Check again after the spring thaw and after a dry summer. Growth tells you more than any single look.
Walk the outside of the house in a heavy rain. Look for water pooling near the wall, downspouts that empty right at the foundation and flower beds that slope toward the house. Fixing grading and extending downspouts costs little and helps every repair last longer.
Keep large, thirsty trees in mind if you are planting near the house. The City's marine clay guidelines exist for a reason. If you already have a big tree close to the wall and new cracks are appearing, mention it when you book an assessment.
When you do call, have your photos and dates ready. A contractor or engineer can tell a lot from how a crack changed over a year.