Why Cracks Go Diagonal
My buddy Rick explained this to me using a napkin at a barbecue. Probably the most useful napkin drawing I've ever seen.
The Stress Point Thing
When part of a foundation settles or moves, the stress has to go somewhere. Cracks form along the lines of least resistance, which in concrete often means a diagonal path. Straight lines are for shrinkage. Diagonal lines often mean something's moving.
Rick drew a rectangle with one corner pushed down. The cracks radiated out at angles from that corner. "Your house is basically trying to tear itself apart in slow motion," he said. Thanks Rick. Really helpful for my anxiety.
Windows Make It Worse
The crack in my basement starts at the corner of a window. Windows are basically holes in your foundation wall. Holes create stress concentrations. When forces act on the wall, cracks tend to start or end at these openings.
So finding a diagonal crack near a window corner doesn't necessarily mean disaster. It might just mean that's where the stress found its outlet.
The Good Kind vs. The Bad Kind
Not all diagonal cracks are created equal. The engineer I eventually hired walked me through the differences.
Settlement Cracks That Stopped
Most houses settle a little bit in the first few years. Soil compacts, concrete cures, things shift slightly. This can cause diagonal cracks that form once and then stay put forever.
Mine fits this category. The house is 45 years old. The crack has been painted over at least twice based on the layers I can see at the edges. The engineer said it probably formed in the first 5-10 years and has been stable ever since.
He measured the width at multiple points. All the same. Checked if either side had moved forward or back relative to the other. Nope. Stable as a rock. A rock with a crack in it, but still.
Active Settlement Cracks
The bad scenario is when the settling is ongoing. Helen down the street has this problem. Her house is on a slope, and over the past 10 years, the downhill corner has been slowly dropping.
She's got diagonal cracks that are visibly wider at the top than the bottom. The wall sections on either side of the crack are at different levels now. Doors upstairs don't close right anymore. That's active settlement, and it's expensive to fix.
She's looking at $18,000-24,000 for helical piers to stabilize her foundation. Should have dealt with it earlier, but that's easy to say from the outside.
The Width Matters
Rule of thumb from the engineer: hairline diagonal cracks near windows or corners are usually not concerning. Anything you can stick a pencil into deserves attention. Anything you can stick your finger into is a problem.
My crack? Barely fits a piece of paper. Helen's cracks? She can fit a nickel in the worst spots. That difference tells you everything.
My Paranoid Monitoring System
Even after the engineer said my crack was fine, I couldn't completely let it go. So I set up a monitoring system. Overkill? Probably. But it lets me sleep at night.
The Pencil Method
I drew small lines across the crack at three points with a fine-tip marker. If the crack widens, the lines would separate. If the two sides move relative to each other, the lines wouldn't align anymore.
I dated each mark. That was four years ago. They still align perfectly. The crack hasn't grown, widened, or shifted. My paranoia was unjustified, but at least I know for sure now.
Photo Documentation
I also take a photo of the crack once a year, same time each year, with a ruler held against it for scale. Keep them in a folder on my phone.
Looking back at four years of photos, they're identical. If anything ever changes, I'll see it immediately and can show the photos to a professional.
The Seasonal Check
Rick mentioned that problem cracks often change with seasons. Worse in winter when the soil freezes, or worse in dry summers when clay soil shrinks. So I check my crack once per season.
No seasonal variation. Same in January, same in July. Another sign that it's just a static remnant from decades ago.
When to Actually Call Someone
Based on my research binge and conversations with the engineer, here's when diagonal cracks warrant a professional look.
The Red Flags
Call someone if the crack appeared suddenly in an older house. Call someone if it's wider at one end than the other (indicates ongoing movement). Call someone if the two sides of the crack aren't level with each other (offset). Call someone if you have multiple diagonal cracks pointing toward the same corner.
Call someone if doors or windows near the crack are sticking when they didn't before. That means movement is happening now, not 30 years ago.
The "Probably Fine" Signs
If the crack is narrow, consistent width along its length, both sides level, starts at a window or door corner, and the house is older than 10 years? Probably fine. Monitor it if you want peace of mind, but don't lose sleep over it.
That described my situation perfectly. Wish I'd known that before those two weeks of anxiety.
The $350 Solution
When in doubt, hire a structural engineer. Not a foundation repair company. An engineer who gets paid for their opinion, not for selling you repairs.
I paid $350. The engineer spent an hour, looked at everything, took measurements, and told me to relax. Best $350 I ever spent on this house.