Diagnosis
Which foundation cracks actually matter
Most cracks mean nothing. Width, change over time and whether the structure itself has moved tell you which ones do.
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Written by HyreFoundation Research Desk Primary-source research and fact checking
The short answer
Most foundation cracks are harmless. The damage scale engineers publish grades cracks by width, not direction: under 1 mm is very slight, up to 5 mm is slight, and about 5 mm is the important threshold.
Its authors warn that width alone is not a measure of severity. A crack that is widening, or doors, floors and walls that have moved, matter more.
If this is happening now
A wall that has moved suddenly, a door frame visibly out of square, a floor that has dropped, or masonry that is leaning or bulging rather than just cracked — those are not monitoring situations.
That is the point at which a structural engineer should look at it before anything else happens. You can also reach HyreFoundation on (866) 582-8523.
Key takeaways
We looked, and we say so below.
, and it is free. Two dated photographs months apart beat one alarming photograph.
What is actually diagnostic, in order
Roughly in order of how much it tells you, which is close to the reverse of the order it usually gets discussed in.
A crack the same width since 2019 is a historical event. One that opened 2 mm this summer is an active process. This is the most informative thing you can establish, and it costs nothing.
Doors and windows out of square, floors sloping, walls leaning or bulging, separation at wall-to-ceiling junctions. Burland’s categories 4 and 5 are defined largely by these, not by crack width.
A pattern of cracking concentrated at one corner of a house says something a single crack does not.
A crack card or a set of feeler gauges costs very little. "About a quarter inch" is usually 4 mm and sometimes 9 mm, and those are different categories.
A tree removed or planted, a drought or an unusually wet year, a new patio or driveway, irrigation running against the wall, a plumbing leak, or excavation next door. Soil moves because water does; see why foundations move.
Concrete shrinks as it cures and most slabs develop shrinkage cracking in the first year or two. That is expected, not settlement.
How to monitor a crack properly, for almost nothing
Monitoring is the highest-value thing on this page. Evidence of change is what turns an anxious guess into an engineering question with an answer.
Put a ruler or a coin against the crack and photograph it square-on, not at an angle. Write the date on paper in the shot; photo metadata gets stripped and argued about. Do this for every crack, inside and out.
Cracks are not a constant width. Mark a specific point with a pencil and always measure there. A crack comparator card costs a few dollars and removes the guesswork that makes the reading useless.
A pencil tick at each end of the crack, dated. A crack that lengthens is telling you something that a crack that merely looks bad is not.
Expansive clay cycles with moisture, so cracks that open in a dry late summer and close in a wet spring are consistent with seasonal soil movement rather than progressive failure. One reading cannot show you that. Two, six months apart, can.
Cracks are the symptom; the shape of the floor is closer to the cause. Our elevation log records two readings over time, which is what makes it evidence rather than a number.
An engineer arriving to a house with eight months of dated readings can answer a question that an engineer arriving cold can only estimate. It also makes you a considerably harder person to oversell to.
The damage classification engineers actually publish
Burland’s classification of visible damage to walls is published by the International Society for Soil Mechanics and Geotechnical Engineering and was adopted with slight modifications by BRE, the Institution of Structural Engineers and the Institution of Civil Engineers.
It describes damage to masonry and plaster by what it takes to repair it, a more honest framing than "serious or not serious".
| Option | What it means for you |
|---|---|
| 0 — Negligible less than about 0.1 mm | Hairline cracks. |
| 1 — Very slight up to 1 mm | Fine cracks easily treated during normal decoration. Damage generally restricted to internal wall finishes; close inspection may reveal some cracks in external masonry. |
| 2 — Slight up to 5 mm | Cracks easily filled; redecoration probably required. Cracks may be visible externally and some repointing may be needed for weathertightness. Doors and windows may stick slightly. |
| 3 — Moderate 5 to 15 mm, or several greater than 3 mm | Cracks require opening up and patching by a mason. Repointing and possibly some brickwork replaced. Doors and windows sticking. Service pipes may fracture. Weathertightness often impaired. |
| 4 — Severe 15 to 25 mm, and depends on the number of cracks | Extensive repair involving breaking out and replacing sections of wall, especially over doors and windows. Frames distorted, floor sloping noticeably, walls leaning or bulging noticeably, some loss of bearing in beams, service pipes disrupted. |
| 5 — Very severe greater than 25 mm, and depends on the number of cracks | Major repair involving partial or complete rebuilding. Beams lose bearing, walls lean badly and require shoring, windows broken with distortion, danger of instability. |
Source: Mair, Taylor & Burland (1996), Prediction of ground movements and assessment of risk of building damage due to bored tunnelling, ISSMGE. Categories 0–2 are aesthetic, 3–4 affect serviceability, and 5 affects stability.
The paper calls "the division between damage categories 2 and 3" an important threshold: about 5 mm. The scale grades damage to walls and finishes, not foundation failure.
The caveat printed on the scale itself matters as much as the numbers.
The table carries this note in its own header: "Crack width is only one factor in assessing category of damage and should not be used on its own as a direct measure of it."
One 6 mm crack in a render coat is a different finding from twenty 3 mm cracks across a load-bearing wall with a door frame out of square.
What to ask before you sign
The full checklist is on choosing a foundation contractor.
Where this goes wrong
We could not find a single code, standard or engineering society document that assigns diagnostic meaning to crack orientation. See the FAQ below. It is the most repeated claim in the field and the least supported.
A photograph shows a crack. It cannot show movement, and movement is the question. Nobody — us, an engineer, or a contractor — can tell from an image whether something is progressing.
Sealing it destroys the evidence and does nothing about the cause. If it is being filled for weathertightness, photograph and measure first.
Concrete shrinks as it cures. Fine cracking in a slab or a foundation wall in the first couple of years is common and usually unrelated to settlement.
A free inspection can be entirely honest and is still a sales visit. It is also, in practice, where a category-2 crack most often becomes a piering proposal.
The classification describes damage today. A category-1 crack in an actively moving structure is a category-1 crack so far. That is exactly why the monitoring matters more than the measurement.
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A straight answer first. Matching is still being built, so this is not an instant quote. If a wall is moving or a door frame has racked out of square this week, call a structural engineer first.
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Questions
Is a horizontal crack in a basement wall always serious?
How wide does a foundation crack have to be to worry?
Are stair-step cracks in brick a foundation problem?
How much floor slope is too much?
Can I just monitor a foundation crack instead of paying someone?
Is there an American version of the crack damage scale?
What about the angular distortion limits quoted online?
Written and audited by
HyreFoundation Research Desk
Primary-source research and fact checking
We read the model building code, the federal soil and moisture guidance and the insurer position ourselves, and we publish each figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we say so instead of printing it. HyreFoundation does not inspect, engineer or repair foundations. Authorship is organizational: this desk, not a named persona.
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How this desk works
- Primary sources only. Code statements come from the International Residential Code, Chapter 4, as the model code. Soil statements come from USDA and USGS. Insurance statements come from the policy language and the Insurance Information Institute. We open the source rather than cite a blog that cites it.
- The code is a model, and adoption is local. Your jurisdiction adopts an edition and amends it, and foundation chapters are amended more than most because frost depth and soil are local facts. Every code figure says so.
- We hold no job data. We have no dataset of bids, pier counts or invoices, so no page claims an average from one. Calculators show the arithmetic and the inputs they assumed, and label the result as an estimate.
- We do not diagnose your house. A crack guide can tell you which pattern to measure and when to call a structural engineer. It cannot tell you whether your foundation is failing. An engineer on site does that work.
- We do not repair foundations, and we take no payment for placement, ranking or a favorable mention. Nobody buys a position on this site.
Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
Information on this page is general and varies with soil, climate, foundation type, building code, insurance policy and circumstance.
It is not engineering, insurance or legal advice, and no page can tell you whether your house has a structural problem: that needs a licensed professional on site.
HyreFoundation is an independent foundation resource, not a repair contractor or an engineering firm, and does not perform, supervise or warrant foundation work.