Not every crack is a disaster. Some are.
The fastest way to know what category your crack falls into is to measure it with a feeler gauge or business card. A standard business card is roughly 1/64″ thick — if it slides into the crack with room to spare, you're past hairline. If you can fit a quarter (about 1/16″), you're into “watch carefully” territory. If you can fit your finger, call a structural engineer.
Hairline shrinkage cracks — usually fine.
When a concrete wall is poured, it begins curing immediately. As the water in the mix evaporates over the next 28 days, the concrete shrinks slightly — typically 0.04 to 0.08% of its dimension. On an 8-foot tall wall, that's about a quarter-inch of total shrinkage distributed across the entire structure. Concrete is strong in compression but weak in tension, so when shrinkage forces exceed the tensile strength of the cured concrete, you get hairline cracks.
These cracks are almost always:
- Vertical (top to bottom of the wall)
- Less than 1/8″ wide at any point
- Most visible at the top half of the wall (where tension was highest during cure)
- Not getting wider over time
- Often perfectly straight, sometimes following form-tie locations
They look concerning to a homeowner who's never seen them before. They're almost always cosmetic. The standard test: take a photo today, take a photo from the same angle in 6 months, and compare. If the crack hasn't widened, it's shrinkage and you can ignore it structurally. If it's leaking water, an epoxy or polyurethane crack injection seals it for a few hundred dollars and you move on with your life.
Vertical cracks wider than 1/8″.
Vertical cracks that are wider than 1/8″ might still be shrinkage that's been there for decades, or they might indicate:
- Differential settlement — one part of the foundation has sunk slightly more than another, opening the crack
- Frost heave damage — accumulated freeze-thaw cycles have widened an originally hairline crack
- Hydrostatic pressure effects — water pressure has worked open a previously stable crack
The first diagnostic question is whether the crack is moving. Set a marker — a piece of tape spanning the crack with a pen line drawn across — and check it monthly. If the tape splits or the line offsets within a year, the crack is active and needs attention. If the tape stays intact for 12 months, the crack is stable and probably won't move further.
For stable wider cracks that are leaking, polyurethane crack injection is the standard fix. The polyurethane is hydrophobic, expands on contact with water, and creates a watertight seal that flexes with future minor movement. Cost is typically $400-$800 per crack depending on length, and we can do it without disrupting your basement.
Horizontal cracks — structural concern.
A horizontal crack across a basement wall is almost never normal. It usually means soil pressure has exceeded what the wall was designed to resist.
Horizontal cracks form when the wall is bending inward under lateral pressure from the soil. The mechanism is typically one of:
- Hydrostatic pressure — water in saturated soil pushing the wall in
- Frost pressure — soil expanding as it freezes, pushing inward
- Vehicle or surcharge loads — driveway adjacent to foundation transmitting weight to the wall
- Tree root pressure — large trees too close to the foundation
A horizontal crack is structural. Don't patch it from the inside and hope. The wall is moving and a patch won't stop the cause. The right response is to bring in a foundation engineer or structural specialist to assess the wall's condition, recommend a stabilization approach (carbon fiber straps, helical piers, wall braces, or in severe cases full wall replacement), and only after that's addressed do the waterproofing work.
Stair-step cracks in block walls.
Concrete block (CMU) walls show their structural distress differently than poured concrete walls. Because the mortar joints are weaker than the blocks themselves, cracks tend to follow the mortar in a stair-step pattern — horizontally across the top of one block, vertically up one side, horizontally again across the next.
| Cosmetic stair-step | Structural stair-step | |
|---|---|---|
| Width | Under 1/8″ throughout | Wider than 1/8″, especially at corners |
| Location | Upper portion of wall, near corners | Lower wall, multiple courses, spreading |
| Movement over time | Stable across multiple seasons | Widening within months |
| Accompanying signs | None — just the crack | Inward bowing, displaced blocks, water entry |
| Fix | Crack injection if leaking | Foundation engineer assessment first |
Old Minneapolis bungalows with cinder-block foundations frequently show some stair-step cracking — most of it is decades old, stable, and cosmetic. The trick is identifying which cracks are the new active ones vs the old stable ones. If you've been in the house for years and you're sure the crack wasn't there last spring, that's active. If it's been there since you bought the place and hasn't changed, that's probably stable.
When to call a waterproofer vs a foundation engineer.
- •Call us (waterproofer) for: hairline cracks that leak, stable vertical cracks under 1/2″, cove-joint failure, sump pump issues, drainage problems
- •Call a foundation engineer for: horizontal cracks, inward-bowing walls, active widening cracks, displaced blocks, settlement that's affecting upper floors
- •Sometimes both: after structural work is complete, drainage system is usually still needed to keep water away from the repaired wall
- •Take photos with a ruler in frame — quarterly or after big rain events, builds a record of stable vs active
- •Ignore horizontal cracks because “they're not leaking yet” — they will
- •Patch a horizontal crack from the inside expecting it to stop the bowing
- •Sign with the first waterproofer who tells you it's a structural emergency without showing you a foundation engineer's report
- •Spend $20K on drain tile to fix a crack that's actually a structural failure
When we walk through your basement at the inspection, we'll look at every visible crack, photograph the ones that warrant tracking, and tell you honestly which ones are in our lane (waterproofing) and which ones need a structural engineer first. We don't make money on structural work. We do make money on doing the right work in the right order — and if a structural issue is feeding the leak, drain tile alone won't solve it.