Width — the first cut.
- Under 1/16 inch (hairline): Almost always cosmetic. Caulk and forget.
- 1/16 to 1/4 inch: Worth investigating. Could be settling, could be early structural movement.
- Over 1/4 inch: Treat as structural until proven otherwise. Engineer time.
Measure with a credit card edge (about 1/16") or a quarter (about 1/16" thick). Width gives you 60% of the answer.
Direction — what the angle tells you.
- Vertical cracks in a poured wall — usually settlement. Common in homes 5+ years old. Not urgent.
- Diagonal cracks, especially running across a corner — structural movement. The wall is being pushed or pulled out of plane.
- Horizontal cracks — hydrostatic pressure from soil pushing the wall inward. Most serious. Call an engineer immediately.
- Stair-step cracks in brick or block — foundation movement underneath. Engineer required.
Location — where it shows up matters.
- Cracks AT a control joint or expansion joint: by design. Healthy.
- Cracks BETWEEN joints: joints were spaced too far apart, or the slab is moving.
- Cracks at door and window corners (inside the house): foundation rotation. Diagonals here are a tell.
- Cracks above a window or door (interior): could be a header issue, not foundation.
The Central Valley clay tell.
Expansive clay (montmorillonite) swells 30% when wet, contracts when dry. Symptoms:
- Doors that stick in summer, gaps in winter
- Slab cracks at fairly consistent intervals throughout the house
- Drywall cracks at the same locations every year
- Patio or porch slab tilting away from the house
This is soil movement, not foundation failure. The fix is moisture management (proper drainage, foundation watering in summer if your jurisdiction allows it), not foundation repair. Lots of contractors will sell you piers and underpinning for what's actually a drainage problem.
The 90-day rule.
Tape a piece of masking tape across any crack you're unsure about. Write the date on it. If the crack widens past the tape within 90 days, the foundation is actively moving and you need an engineer now. If it stays put, it's likely a settled-and-done historical crack.
This $0 test is the same one structural engineers run as their first diagnostic.
When to call an engineer (not a contractor).
- Cracks over 1/4 inch wide
- Diagonal cracks across corners
- Any horizontal crack in a basement wall
- Doors and windows that have started sticking within the last year
- Visible bowing or leaning of any foundation wall
An independent structural engineer's report runs $400–$800 and gives you an honest answer. A foundation-repair company's "free inspection" gives you a sales pitch.
The single biggest mistake: Hiring the foundation-repair company that did the inspection. Their financial incentive isn't aligned with telling you "you don't need this work." Get an independent structural engineer's report first — THEN shop the corrective work.
What success looks like.
You correctly identify a hairline as cosmetic and caulk it for $50 instead of being sold $25,000 in underpinning. Or you correctly identify a diagonal across a corner as structural and address it for $8,000 instead of waiting until the wall shifts and the fix is $45,000.
The right diagnosis up front is the difference between a $50 fix and a $45,000 disaster.
Skip the DIY?
FXR diagnoses foundation issues, coordinates engineering review when needed, and handles the corrective work — including the slab cuts and plumbing repairs that often come with it. No upsell pressure.
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