Foundation Crack Types: Vertical, Diagonal, Horizontal and Stair-Step
Cracks are the most common thing homeowners find, and the pattern says more than the width. Shrinkage cracks are normal, settlement cracks taper and follow joints, and horizontal cracks in a wall deserve prompt professional attention.
Start with the pattern, then the width, then whether it moves
Three questions sort most cracks: what direction does it run, how wide is it and does it vary along its length, and has it changed since you first saw it? Sources used here describe cracks mainly by cause rather than offering a fixed chart, so the table below summarizes what each source says rather than a universal rule.
| Pattern | What the sources say it often indicates | How concerned to be |
|---|---|---|
| Fine cracks in new concrete (shrinkage) | Concrete shrinks as it dries and cools, and cracking can result where the wall is restrained. The concrete industry's technical sheet lists temperature and drying shrinkage cracks first among basement wall cracks.[1] | Usually normal. The sheet states cracking is not necessarily a sign of poor materials, workmanship or a structural problem.[1] |
| Diagonal cracks running from corners of windows and doors | Called re-entrant corner cracks; stress concentrates at the corners of openings.[1] | Often minor when stable. Monitor, and look for matching movement elsewhere. |
| Tapering cracks, wide at one end and a hairline at the other, often at corners and following mortar joints | Settlement cracks, per InterNACHI.[2] | Depends on whether they are active. Mark and date them. |
| Block wall cracks that widen along a diagonal, usually forming in the first year and concentrated in the middle third of the wall | Shrinkage in concrete block, per InterNACHI, which describes them as rarely serious.[2] | Usually low concern if they are not growing. |
| Nearly vertical cracks at joints of bay windows, porches and additions | Differential settlement between the main structure and the added part, per InterNACHI.[2] | Worth an evaluation if widening or if the part is visibly sinking or tilting. |
| Horizontal or sweeping cracks in a foundation wall | InterNACHI calls these potentially serious and says they indicate the wall is bent or broken, with possible causes including improper backfilling, vibration, and swelling or frost-heaved soil.[2] | Have a structural engineer look at it promptly. See bowed basement walls. |
| Large "Z" shaped cracks | InterNACHI links these to a footing in disrepair or a missing footing.[2] | Evaluate. These indicate support problems. |
| Cracks over openings that meet in an inverted V above the middle of the opening | Failed lintels or arches, per InterNACHI. This is a wall-over-opening issue rather than a foundation wall pattern.[2] | Evaluate if the wall above is moving. |
Width: what the sources give and what they do not
Sources do not agree on a single width that separates harmless from serious, and InterNACHI's material states it gives no general width threshold for foundation cracks.[2] One data point: the concrete industry notes typical builder and third-party warranties require repair only when basement wall cracks leak, show measurable vertical displacement, or exceed 1/8 inch (3 mm) in width.[1] That is a warranty trigger, not an engineering safety limit.
InterNACHI also gives a useful way to read age: a 1/2-inch crack in a new building may indicate rapid settlement, while in a 50-year-old building it may reflect movement of only about 1/100 inch per year.[2] Width alone tells you little without the age of the crack and the building.
Is it active?
- A clean crack with sharp, fresh edges suggests recent movement; a dirty or previously filled crack may be inactive.[2]
- Measure and date it, then recheck over at least a season. Cyclical (seasonal) movement can take six months or more to measure.[2]
- A crack that leaks needs its water source addressed. The concrete industry advises checking and correcting drainage before repairing leaking cracks.[1]
What cracks in a slab floor mean
Concrete slabs also develop surface cracks while the concrete is fresh. The concrete industry describes plastic shrinkage cracks as shallow, roughly parallel and unsightly but rarely impairing the strength of floors and pavements.[3] A crack in a slab that is wide, offset (one side higher than the other), or accompanied by sloping floors and cracked finishes elsewhere is a different matter and is what an engineer will assess. See sloping floors and sticking doors.
How cracks get repaired
The concrete industry names epoxy injection, dry-packing, and routing and sealing as methods for basement wall cracks, advises professional advice for active cracks that keep widening, and points to the American Concrete Institute repair guide (ACI 224.1R) for detail.[1] Sealing a crack does not stop whatever moved the wall. If the crack is active, the cause comes first. See foundation repair methods compared.
Common questions
Are vertical cracks in a foundation serious?
Are horizontal foundation cracks always serious?
InterNACHI describes horizontal or sweeping cracks as potentially serious because they can mean the wall is bent or broken.[2] They warrant a structural engineer's evaluation rather than a cosmetic patch.
What is a stair-step crack?
A crack that follows mortar joints in a stepped pattern in brick or block. It is a settlement pattern when it tapers and is active; InterNACHI notes settlement cracks often form near corners and openings and follow a rough diagonal path along mortar joints.[2]
How wide is too wide?
Can I just seal the crack myself?
Sealing can stop water, but it does not address why the wall moved. If the crack is growing, get it evaluated first.
Related guides
Sources
Figures and claims marked with a number come from these sources. Dates are the publication or last-update dates shown by the publisher, where one was shown. Older sources are flagged in the text.
- CIP 7: Cracks in Residential Basement Walls. National Ready Mixed Concrete Association (NRMCA); copyright years 1979, 2000 and 2014 shown, with 2014 the most recent.Industry trade association for concrete producers. Focuses on poured-concrete basement walls.
- Inspecting Foundation Walls and Piers. InterNACHI (International Association of Certified Home Inspectors), revised January 2013.Inspector-training material. It describes cracks by cause rather than by a fixed vertical/diagonal/horizontal chart.
- CIP 5: Plastic Shrinkage Cracking. National Ready Mixed Concrete Association (NRMCA); copyright years 1978 to 2014 shown.