What causes horizontal cracks in NB foundation walls?

Horizontal cracks in a foundation wall are one of the most serious types of foundation damage you can find in a New Brunswick home — they typically indicate lateral pressure pushing inward against the wall, and they require prompt professional assessment. Do not treat a horizontal crack as a routine repair or ignore it.

Foundation walls in NB are designed to handle vertical loads (the weight of the structure above) very well, and they have reasonable resistance to outward pressure from the soil on the exterior. What they resist poorly is inward lateral force — soil pressure pushing against the wall from outside. When that force exceeds what the wall was designed to handle, the wall begins to bow inward, and horizontal cracks develop, usually at the mid-height of the wall where bending stress is highest.

The primary causes of horizontal cracking in NB foundations include:

Frost heave and lateral soil pressure — when saturated soil freezes, it expands not just vertically but also laterally. In NB, where frost penetrates 4 to 5 feet, an unreinforced concrete block or even poured concrete wall can be subjected to enormous horizontal pressure each winter from freezing soil pressing inward. Over multiple seasons, this cyclic pressure fatigues the wall.

Hydrostatic pressure — waterlogged soil during NB's spring thaw exerts significant hydrostatic pressure against foundation walls. Homes with poor drainage, failed weeping tile, or low-lying lots in Fredericton's river valley flood plain, Riverview's flat terrain, or Dieppe's clay-soil areas are particularly vulnerable.

Surcharge loads — heavy vehicles parked near the foundation, stockpiled firewood or materials against the exterior wall, or heavy retaining features placed close to the wall add pressure that the wall was not designed to carry.

Wall deterioration — older block foundations can have deteriorated mortar joints that allow individual blocks to shift under normal soil pressure, producing cracks at mortar lines that appear horizontal.

The critical variable is whether the crack is active. A static horizontal crack (the wall has stopped moving) is concerning but may be stabilizable. A crack that is growing — getting wider, longer, or where the wall is visibly bowing inward — is an emergency that requires engineering assessment immediately. You can monitor crack movement by marking the crack ends with pencil or applying a small plaster patch — if the marks move or the plaster cracks, the wall is still moving.

Repair options for horizontal foundation cracks range from carbon fibre strap reinforcement (bands strapped to the interior of the wall to resist further inward movement) to wall anchors (steel plates on the exterior connected through the wall to interior anchors, gradually straightened over time) to full wall replacement in severe cases. None of these are DIY projects — all require structural assessment.

If you have found a horizontal crack in your NB foundation, contact a structural engineer or an experienced foundation repair contractor for assessment. New Brunswick Concrete can connect you with qualified professionals throughout NB.