Much of Ottawa is built on Champlain Sea sediment — the sensitive marine clay commonly called Leda clay. It is a genuinely unusual material: deposited in a saltwater sea that flooded the region after the last glaciation, then leached of its salt by fresh groundwater, leaving a soil skeleton that holds a great deal of water and changes volume as that water content changes.
For homeowners this has one practical consequence. The ground under a lot of Ottawa houses moves, seasonally and predictably, and understanding that is the difference between panic and perspective.
What the clay actually does
Sensitive marine clay shrinks when it dries and swells when it wets. In a dry Ottawa summer the clay under and beside a foundation loses moisture and contracts; the foundation settles slightly. In a wet autumn or spring melt it takes water back and expands, and the foundation rises slightly. This is a seasonal cycle, and a house on this soil is very gently breathing up and down through the year.
Two things make it worse:
- Large trees. A mature tree draws an enormous volume of water from the soil in a growing season. A big silver maple close to a foundation on clay creates a localised dry zone, and the foundation settles more on that side. This is the single most common cause of differential movement in Ottawa's older, tree-lined neighbourhoods.
- Drought years followed by wet ones. The bigger the moisture swing, the bigger the movement.
Separately, the clay has a second and more dramatic property: when disturbed severely it can lose most of its strength. That behaviour is what drives the region's landslide history along river valleys. It is not what is happening to a house on a flat lot, and it should not be confused with routine seasonal movement.
How to read the cracks
| What you see | Usually means | Concern |
|---|---|---|
| Fine vertical cracks in poured concrete, often mid-wall | Concrete shrinkage during curing — nearly universal | Low; monitor for water entry |
| Hairline diagonal cracks at window and door corners that open in late summer and close in spring | Seasonal clay movement | Low to moderate; monitor |
| Interior doors sticking in August, fine in November, every year | Seasonal clay movement | Low; a signature, not a failure |
| Diagonal cracking that widens progressively at one end, over years | Differential settlement | High — get an engineer |
| Horizontal cracking in a block wall with inward bowing | Lateral soil pressure — a different failure entirely | High — get an engineer |
| Stair-step cracking through block or brick, widening | Differential settlement | High — get an engineer |
| Sudden new cracking after major nearby excavation | Disturbance of sensitive clay | High — document immediately |
What you can actually control
You cannot change the geology, but movement is driven by moisture and moisture is manageable.
- Keep it consistent. The enemy is the swing, not the level. Steady moisture around the foundation means less shrink-swell.
- Get roof water away. Downspouts discharging at the foundation soak one spot and starve another. This is the largest single controllable factor.
- Think hard about trees. Large, thirsty species close to a foundation on clay are a real risk. Removing a mature tree is also disruptive — the soil rehydrates and can heave. Get an arborist and, if there is already movement, an engineer, before deciding.
- Do not let it bake. In an extreme drought summer, light watering of the soil near the foundation can reduce shrinkage. This sounds counterintuitive in a city where everyone is trying to keep water away from foundations, but it is about consistency.
- Be careful with excavation. Landscaping, pools and additions near a foundation on sensitive clay deserve engineering input, not enthusiasm.
What this means when you are buying
Do not reject an Ottawa house because it has foundation cracks — on this soil, in this climate, most houses over thirty years old have some. Ask instead: is there evidence this is progressing? Look for previous repairs that have re-cracked, doors and windows that have been trimmed to fit, floors out of level in a consistent direction, and cracking that is wider at the top or bottom rather than uniform.
When the evidence points to active movement, the right next step is a structural engineer, not a waterproofing contractor and not a foundation repair company offering a free assessment. An engineer has no product to sell you.
Frequently asked
Is Leda clay dangerous for my house?
For a typical house on a flat lot, it causes gradual seasonal movement and cosmetic cracking rather than danger. The dramatic landslide behaviour associated with sensitive marine clay relates to slopes and major disturbance, not to ordinary residential settlement.
Are foundation cracks normal in Ottawa?
Fine vertical shrinkage cracks in poured foundations are nearly universal and usually harmless. What matters is whether a crack is moving, so mark it, date it and photograph it. Progressive widening, horizontal cracking with bowing, or stair-step cracking through block warrants an engineer.
Should I remove a large tree near my foundation?
Not automatically. Large trees on clay can cause settlement by drying the soil, but removing a mature one can also cause heave as the soil rehydrates. Get an arborist and, where there is existing movement, a structural engineer before deciding.