Foundations and Cracks: Reading, Sealing and When to Call

Not every crack is a crisis, and not every crack is harmless. The difference is rarely width alone: direction and movement matter more, and a hairline horizontal crack can be far more serious than a wider vertical one. This hub explains how to read what you are looking at, and is deliberately conservative about where the homeowner’s job ends.

Read the crack before you seal it

Three things describe a crack. Width, measured at its widest point with a gauge rather than estimated. Direction, which is vertical, diagonal, horizontal, or stair-step following mortar joints in block. And movement, which you can only know by marking both ends in pencil with the date and re-measuring weeks later. The Foundation Crack Triage Tool sorts yours and tells you who should look at it.

Usually cosmetic

Narrow, stable, vertical cracks in poured concrete are common and are often shrinkage from the original cure. They may still let water through, which is a drainage question rather than a structural one.

Usually structural

Horizontal cracking often indicates lateral soil pressure against the wall. Stair-step cracking in block follows the mortar joints and commonly indicates differential movement. A wall that is bowing or leaning is a structural condition regardless of crack width. All three belong to a licensed structural engineer.

Get the engineer before the contractor

This is the single most useful piece of advice on this page. An engineer is not selling you the repair, so the specification you get is independent. A repair contractor arriving first will quote for the product they install. Where there is any sign of movement, pay for the inspection first.

Water through a crack

A crack that leaks is both a crack problem and a drainage problem. Sealing the inside face without addressing why water is against the wall tends to move the leak rather than stop it. Start outside with the Yard Drainage Grading Calculator.

Sealing, and what it cannot do

Sealing has a real role. EPA’s radon-resistant construction guidance lists sealing openings, cracks and crevices in the foundation floor and walls with polyurethane caulk as one feature of a radon-resistant home, alongside the gravel layer and the vent pipe. It is a supporting measure, not a structural repair, and sealing a moving crack destroys the evidence an engineer needs.

Start with these guides

• Foundation Crack Repair: Which Cracks Matter and Why How to read a foundation crack by direction, width and movement, which ones an engineer should see first, and what each repair method actually does.

Tools for this topic

• Foundation Crack Triage Tool – cosmetic, water path or structural
• Yard Drainage Grading Calculator – reduce the water load on the wall
• Basement Dry-Up Plan Builder – where crack work fits in your plan

Latest Foundations guides

New guides are added as they pass our fact-check. Browse everything in Foundations, or get new guides by email.

Sources we rely on

• EPA: A brief guide to mold, moisture and your home
• EPA: Radon-resistant construction basics and techniques
• Ready.gov: Floods

Frequently asked questions

When should I worry about a foundation crack?

When it is horizontal, when it is stair-step through block mortar joints, when the wall is bowing or leaning, or when it is measurably widening over time. Any of those warrants a licensed structural engineer.

Are vertical cracks in a basement wall serious?

Narrow, stable vertical cracks in poured concrete are common and often shrinkage. They can still let water in, which is a drainage issue rather than a structural one.

How do I tell if a crack is moving?

Mark both ends in pencil, write the date beside it, and measure the width with a crack gauge. Re-measure after a few weeks and again after a wet season.

Should I call an engineer or a waterproofing company?

An engineer first where there is any sign of movement, because they are not selling you the repair. The specification is then independent of who installs it.