When a Foundation Crack Needs a Pro vs a Caulk Gun (2024)

TL;DR: Hairline vertical cracks under 1/8 inch wide, especially in poured concrete, are almost always shrinkage and safe to seal yourself with polyurethane caulk or hydraulic cement. Cracks wider than 1/4 inch, horizontal cracks, stair-step cracks in block walls, or any crack where one side sits higher than the other need a structural engineer, usually $400 to $800 for an inspection, before you patch anything.

_Last reviewed: July 2026 · 7 min read_

You found a crack in your foundation wall and now you're standing in the basement with a flashlight, trying to decide if this is a $12 tube of caulk or a $10,000 problem. Most homeowners guess wrong in both directions, either panicking over a cosmetic crack or ignoring one that's actively getting worse. Here's how to tell which one you're looking at.

Okoniq Property Hub logs the width, location, and photos of any crack you find so you can track whether it's growing over months, which is the single best way to know if it's serious.

What separates a cosmetic crack from a structural one?

Width, direction, and pattern tell you almost everything. A vertical hairline crack under 1/8 inch, the width of a credit card edge, in a poured concrete wall is typically shrinkage from the concrete curing decades ago and isn't moving.

Structural cracks behave differently. They're often horizontal, running parallel to the floor, which signals soil pressure pushing the wall inward. They can also be diagonal at 45 degrees from a corner, or "stair-step" cracks that follow the mortar joints in block or brick foundations. Any crack wider than 1/4 inch, or one where you can fit a nickel into the gap, has moved past cosmetic territory. If a door frame or window near the crack has started sticking, that's a second signal the foundation is shifting, not just aging.

How do you measure a crack correctly before deciding?

Use a crack gauge or a simple ruler, and measure at the widest point, not the average. A $6 plastic crack comparator card from a hardware store gives you a precise reading in fractions of an inch, and it's worth the five minutes.

More important than the single measurement is tracking change over time. Mark both ends of the crack with a pencil and the date, then check back in 60 and 90 days. A crack that hasn't moved in three months is far less urgent than one that grew 1/16 inch in a month. If you're already logging other water issues in the house, like the kind covered in 5 signs of a slab leak under your floor, add crack tracking to the same routine so you're watching your foundation holistically instead of reacting to one symptom at a time.

When does a crack call for a structural engineer instead of a contractor?

Call an engineer, not a foundation repair company, whenever you see horizontal cracking, bowing walls, or displacement where one side of the crack sits higher than the other by more than 1/8 inch. Foundation repair companies often sell solutions before diagnosing the cause, so an independent structural engineer, who typically charges $400 to $800 for a written assessment, gives you an unbiased read before anyone quotes you a repair.

Bowing block walls are a specific red flag. If you can lay a level against the wall and see a curve of more than 1 inch over 8 feet, that's active soil pressure and needs engineered bracing, not sealant. Water is usually the root cause of that pressure, and problems like the ones described in 6 downspout mistakes that send water into your basement or 5 signs your downspouts are dumping water in the wrong place are often what's loading the soil against your foundation in the first place. Fixing drainage first can stop a crack from getting worse even before repair work starts.

| Cosmetic Crack | Structural Crack | |---|---| | Vertical, under 1/8 inch | Horizontal or diagonal, over 1/4 inch | | No offset between sides | One side higher or bowed | | Not growing over 90 days | Growing month over month | | DIY caulk or hydraulic cement | Engineer inspection required |

What's the right DIY fix for a cosmetic crack?

For a true hairline crack, hydraulic cement or a polyurethane concrete caulk applied with a caulk gun seals it against water in under an hour. Clean the crack with a wire brush first, then apply hydraulic cement if there's active seepage, since it expands and sets even against moisture. For dry cracks, a polyurethane sealant rated for concrete, costing around $8 to $15 a tube, flexes slightly with seasonal movement and won't crack again the way rigid patching compounds do.

Don't use standard silicone caulk. It doesn't bond well to concrete and won't hold up under hydrostatic pressure if water ever pushes against it from outside. Skip epoxy injections unless a professional recommends them for a specific structural crack, since epoxy is rigid and can mask movement instead of accommodating it.

FAQ

How wide does a foundation crack have to be before it's serious?

A crack wider than 1/4 inch, or one where you can fit a nickel into the gap, generally warrants a professional evaluation rather than a DIY patch.

Can a foundation crack let water into the basement even if it's small?

Yes, hairline cracks under 1/8 inch can still seep water during heavy rain, which is why sealing them with hydraulic cement or polyurethane caulk is worth doing even when they're not structurally concerning.

Does every new house get foundation cracks?

Most poured concrete foundations develop small shrinkage cracks within the first 1 to 2 years as the concrete cures, and this is considered normal, not a defect.

How much does a structural engineer inspection cost?

Expect to pay $400 to $800 for a written structural assessment, which is far cheaper than committing to a $5,000 to $15,000 foundation repair based on a contractor's sales pitch alone.

Should I fix drainage before or after repairing a foundation crack?

Fix drainage first whenever possible, since correcting downspouts and grading can relieve the soil pressure causing the crack and prevent it from reopening after repair.

This is educational information, not structural or engineering advice. Call a licensed structural engineer if a crack is wider than 1/4 inch, horizontal, or actively growing.