Almost every house gets some cracks. Most are harmless. A few are early warnings of a foundation problem that gets more expensive every year you wait. As civil engineers, we use a simple set of checks to tell the difference: how wide the crack is, which direction it runs, whether it is growing, and what else is happening in the house. This guide gives you those same checks, an engineering crack-width scale you can use with a ruler, and clear advice on who to call first.
Quick answer: when should you worry?
Call a structural engineer if you see any of these:
- A crack wider than about 1/4 inch (6 mm).
- A horizontal crack in a basement or block wall, or a wall that bows inward.
- A crack where one side is higher or pushed out compared with the other.
- A crack that is getting longer or wider over weeks or months.
- New cracks together with doors or windows that suddenly stick, sloping floors, or gaps between walls and ceiling.
Thin hairline cracks (less than about 1/16 inch) that don’t change are usually shrinkage and normal settling.
The engineer’s crack-width scale
Engineers in the UK and elsewhere widely use a damage scale from the Building Research Establishment (BRE Digest 251). It ranks visible damage in walls mainly by crack width. Here it is in simple words, with inches added:
| Category | Crack width (approx.) | Description | What to do |
|---|---|---|---|
| 0 – Negligible | Up to 0.1 mm (hairline) | Barely visible | Nothing. Paint over it |
| 1 – Very slight | Up to 1 mm (about 1/32 in) | Fine cracks, easily filled | Fill during normal decorating |
| 2 – Slight | Up to 5 mm (about 3/16 in) | Easily filled; may show outside; doors may stick slightly | Monitor; repair finishes; check drainage |
| 3 – Moderate | 5 to 15 mm (about 3/16 to 5/8 in) | Needs opening up and patching; doors and windows sticking; pipes may be affected | Get a structural engineer’s assessment |
| 4 – Severe | 15 to 25 mm (about 5/8 to 1 in) | Extensive repair; walls leaning or bulging; floors sloping | Engineer and structural repair needed |
| 5 – Very severe | More than 25 mm (over 1 in) | Major repair or rebuilding; danger of instability | Urgent engineering advice |
Reading the direction of a crack
- Vertical, even width: usually shrinkage in poured concrete. Common and low risk.
- Vertical, wider at the top or bottom: one part of the foundation is moving more than the other (differential settlement or heave).
- Diagonal from window or door corners: openings are weak points, so movement shows there first. A crack that is wider at one end usually shows which part of the wall moved more.
- Stair-step in brick or block: the mortar joints are cracking because one end of the wall is settling.
- Horizontal in a basement wall: soil and water pressure from outside is bending the wall. This is a structural warning, not a cosmetic one.
Why houses settle
All houses settle a little in the first years as the soil takes the load. Problems start when the settlement is uneven. The usual causes are:
- Clay that swells and shrinks with seasons and moisture. See foundations on expansive soil.
- Fill soil that was not compacted properly before building. See what soil compaction is and why it matters.
- Soft clay slowly consolidating under the weight of the house. The engineering is explained in consolidation of soil (Terzaghi theory).
- Water problems: leaking pipes, poor gutters, and ground sloping toward the house.
For the different types (uniform, differential, tilt), read settlement in foundations: types and effects.
How to monitor a crack (free and easy)
- Mark the ends of the crack with a pencil line and write the date.
- Measure the width at the widest point with a crack-width card or a ruler. Write it down.
- Take a photo with a coin or ruler next to the crack, from the same spot each time.
- Check every month for 3 to 6 months. Cracks that open and close with the seasons point to clay moisture changes. Cracks that only grow point to ongoing settlement.
- For better accuracy, fix an inexpensive tell-tale crack monitor (a two-part plastic gauge) across the crack.
Your notes are very useful to the engineer and can save the cost of a second visit.
Who to call: structural engineer or contractor?
| Structural engineer | Foundation repair contractor | |
|---|---|---|
| Role | Finds the cause and specifies the fix | Carries out the repair |
| Sells repairs? | No (independent) | Yes |
| Typical cost | About $300 – $1,000 for a residential inspection and report (around $600 on average) | Free inspection, but tied to a quote |
| Report useful for | Buying/selling, insurance, comparing quotes, permits | The contractor’s own scope |
| Call first when | The cause is unclear, cracks are moderate or worse, or you are buying a house | You already have an engineer’s repair plan |
Our advice: for anything beyond a small hairline crack, pay for an independent engineer first. A few hundred dollars can save thousands in the wrong repair. Then get three contractor quotes based on the engineer’s plan. Typical repair prices are in our foundation repair cost guide.
What you can do today
- Clean gutters and extend downspouts so water lands several feet from the house.
- Check that the ground slopes away from the foundation.
- Look for plumbing leaks (a running water meter with all taps off is a sign).
- Keep soil moisture steady around the house in long droughts if you have clay soil.
- Start your crack log.
Frequently asked questions
Are cracks in a new house normal?
Small hairline cracks in drywall, plaster and concrete in the first 1–2 years are common as materials dry and the house settles. Wide, growing or horizontal cracks are not normal at any age.
Can a crack close again by itself?
Yes. In clay soils, cracks often open in dry months and close in wet months. That seasonal pattern is useful information for the engineer.
Will a home inspector tell me if a crack is serious?
A general home inspector can flag cracks, but usually recommends a structural engineer for a judgement on cause and repair.
Related guides
- Foundation Repair Cost 2026: Cracks, Methods & Prices
- Basement Waterproofing Cost: Interior vs Exterior
- Why Concrete Columns Crack After Casting
Sources: BRE Digest 251 “Assessment of damage in low-rise buildings” (damage categories); inspection cost ranges from This Old House. General information only. For any structural concern, get an on-site inspection by a licensed engineer.
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