Why Corners Are Crack Magnets
Rick explained this with his usual bluntness: "Corners are where stress goes to die." He's colorful like that.
The Stress Concentration Thing
When concrete shrinks, the forces have to go somewhere. They flow through the wall until they hit a corner. Then they concentrate there. Too much stress in one spot, and the concrete cracks to relieve it.
It's like when you tear paper. You don't tear from the middle of a solid sheet. You start at an edge or corner. Concrete works the same way.
Settlement Magnification
If any part of your foundation is going to settle differently than the rest, it's often a corner. The soil conditions can vary. The loads can vary. Drainage often concentrates at corners.
When one corner settles a little more than the others, a crack forms to accommodate that movement. It's actually the wall being flexible instead of rigid. Better to crack than to push the whole wall out of position.
The Corners in My Basement
Let me describe exactly what I found, because it might match what you're looking at.
Inside Corner Where Two Walls Meet
There's a vertical crack running from floor to about waist height right where my two longest walls meet. Classic inside corner crack.
The engineer I eventually hired barely looked at it. "Shrinkage," he said. "The walls want to shrink inward, the corner resists, something cracks." He said these are in basically every poured foundation ever.
I marked it. Four years later, no change. He was right.
Window Corner Crack
A diagonal crack running from the upper corner of a basement window toward the top of the wall. Maybe 2 feet long. This one worried me most because diagonal = settlement, right?
Yes and no. The engineer explained that window openings are major stress concentrators. The crack probably formed during the initial settling in the first few years after construction. It's been painted over at least once, so it was there before the previous owner painted.
Same thing, marked it. Four years of no change.
Exterior Corner Crack
There's a crack visible on the outside of the foundation at one corner, running down from grade level. This one's in direct contact with weather, freeze-thaw cycles, all that.
The engineer said exterior corner cracks are often thermal. The exposed part of the foundation expands and contracts with temperature. The buried part doesn't. That differential movement causes cracking.
I sealed this one to prevent water getting in and making it worse. The crack itself is cosmetic but the water infiltration isn't.
When Corner Cracks Actually Indicate Problems
Not every corner crack is fine. Here's what I learned to watch for.
Displacement
If one side of the crack has shifted forward or back relative to the other side, that's real movement. Run your finger across the crack. If both sides are flush, it's just separation. If one side is sticking out, the wall sections have actually moved relative to each other.
My corners? All flush. No displacement. Just simple separation from shrinkage.
Width That Keeps Growing
My corner cracks are all hairline to about 1/8 inch. Been that width since I started tracking them. A corner crack that's getting wider over time is telling you something is still moving.
Helen down the street has an outside corner crack that she says is "definitely wider than last year." That's worth investigating. Could be drainage issues, could be ongoing settlement.
Related Symptoms
A corner crack plus a sticking door above that corner plus a sloping floor toward that corner? Now you're seeing a pattern. The crack is part of larger movement.
An isolated corner crack with no other symptoms? Probably just the universal concrete phenomenon.
The Downspout Connection
This part surprised me. One of my corner cracks is at the corner where a downspout used to discharge.
Water + Corner = Settlement
When I bought the house, the downspout at that corner ended right at the foundation. All the roof water from that section was dumping into the soil next to that corner.
Over years, that much water can wash away soil, cause erosion, saturate clay and make it soft. The corner settles more than the rest because the soil under it is compromised.
Was my corner crack caused by the downspout? Hard to say. But I extended that downspout out into the yard by 6 feet the first summer I lived here. The $30 extension might have saved me thousands in future problems.
Check Your Drainage
If you have corner cracks, look at where your water goes. Downspouts discharging at corners. Grade sloping toward corners instead of away. Puddles forming at corners after rain.
Fix the drainage first. The crack might be old history, but without fixing the water, it could become new history too.
My Monitoring Approach
I check my corner cracks once a year, same time every year.
The Marks
Pencil lines across each crack, dated. Takes 5 minutes to check if the lines still align and if the crack ends are still at my marks.
The Photos
Same angle, same lighting, ruler for scale. Looking back through four years of identical photos is boring in the best way possible.
The Peace of Mind
I used to worry about every corner. Now I have data. Four years of data that says nothing is moving. That's worth more than any repair.
If anything ever does change, I'll catch it early. That's the point.