Quick answer
Diagonal foundation cracks are cracks that slant across a foundation wall rather than running level or plumb. They often taper, wider at one end and thinner at the other, and they are a clue to uneven movement, not a diagnosis.
The same slanted look can come from settlement or from the wall drying and shrinking. Direction of movement, history and soil all help tell them apart.
Quick facts
The key facts about Diagonal Foundation Cracks, with sources:
- Direction
- Slanted across the wall, often tapering 1
- Typical locations
- At corners and next to openings, along a rough diagonal through mortar joints 1
- Settlement link
- More rapid settlement produces cracks that taper, largest at one end 1
- Look-alike
- Shrinkage cracks in concrete block can taper and widen diagonally upward 1
- When settlement happens
- Most often early in a building’s life or after a dramatic change in ground conditions 1
- Who reads it
- A licensed structural engineer, with a soils engineer where settlement is involved 1
Key takeaways
- A diagonal crack slants across a wall. It often tapers, wide at one end and a hairline at the other 1.
- In masonry, settlement cracks tend to run along a rough diagonal through mortar joints at corners and beside openings 1.
- Shrinkage cracks in block walls can look the same. HUD says they are often mistaken for settlement cracks 1.
- Telling them apart takes history, location, measurements and soil information, not a photo.
- A pattern is a clue, not a diagnosis. Record it and ask a licensed structural engineer if it changes or you doubt it.
What a diagonal foundation crack is
Picture a wall as a flat panel. If one end sinks a little more than the other, the panel tries to rotate, and the stress shows up as a slanted crack. That is the simple picture of the diagonal crack. Many cracks look like this, and not all of them come from sinking ground.
HUD’s inspection guide describes how settlement shows in masonry. Cracking is most likely at corners and adjacent to openings, and usually follows a rough diagonal along mortar joints, though individual masonry units may split.
More rapid settlements produce cracks that taper, being largest at one end and diminishing to a hairline at the other 1.
The same guide warns that gradual settlement over a long time may produce no masonry cracking at all, especially in older walls with softer bricks and high-lime mortars, because the wall deforms elastically instead 1.
So a lack of diagonal cracks does not prove a wall has not moved. This page defines the pattern. For a compare-all view, read the guide to foundation cracks.
General information, not a diagnosis of your house
General information, not a diagnosis of your house. A diagonal crack can reflect settlement, shrinkage, or something else. Only a licensed professional who sees the wall can say which.
Diagonal cracks: pattern, possible contributing factors and what to record
| Pattern | Possible contributing factors | What to record |
|---|---|---|
| A slanted crack from a window or door corner | Settlement beside an opening; cracking is most likely at corners and next to openings 1 | Which corner, which direction, which end is wider |
| A tapering crack, wide at one end | Rapid settlement; HUD says such cracks taper to a hairline 1 | Width at each end, as a photo with a scale |
| A diagonal crack widening upward in a block wall, mid-wall | Drying shrinkage of block, which can look like settlement 1 | Where along the wall; whether the footer looks intact |
| A slanted crack at a porch, bay or addition | Differential settlement under the projecting part 1 | Which side sits lower; any change over time |
| A crack that opens and closes with the seasons | Thermal or moisture movement 1 | Season and weather when it looks widest |
Four wall sketches showing a horizontal crack, a vertical crack, a diagonal crack and a stair-step crack in block, each with a short note on what the pattern can be consistent with.
Illustrative, not to scale, not a design. It sketches horizontal, vertical, diagonal and stair-step cracks. A pattern is a clue, not a diagnosis.
How a diagonal crack forms
- Support changes unevenly. One part of the footing sinks more than the next, or a projecting part has less support 1.
- The wall is stretched along a line. Masonry resists squeezing well and pulling poorly, so it cracks where it is pulled.
- The crack follows the weak path. In masonry, that is often the mortar joints, so the crack runs on a rough diagonal 1.
- The crack tapers. One end opens more where movement is greatest, and the other end fades to a hairline 1.
- Drying can do something similar. Block shrinkage can make tapering cracks that widen diagonally upward 1.
Example: telling a settlement clue from a shrinkage clue
The table below lists the clues HUD gives for separating look-alike cracks 1. The walls are not real. It shows which facts a professional checks.
| Clue | Points toward settlement | Points toward shrinkage |
|---|---|---|
| Where on the wall | Corners and beside openings | The middle third of the wall |
| The footing below | May have moved or lost support | Footer remains intact |
| When it formed | Often early in life or after a dramatic ground change | Usually the building’s first year |
| History | May still be moving | In an older building, often old and not moving |
None of these clues proves the cause alone. The professional weighs them together with measurements. Cracks can also be repaired in the past and then reappear.
Where diagonal cracks show up
In foundations, they show up in block and brick walls most often, at corners and next to window wells and doors. In poured concrete, cracks of this kind may run from a corner of an opening or a change in section.
In the rest of the house, a settling foundation can show up as diagonal cracks above doors and windows, in drywall corners, and in brick veneer. HUD lists indications of differential settlement as vertical distortion or cracking of masonry walls, warped interior and exterior openings, sloped floors, and sticking doors and windows 1.
HUD also says that many signs of masonry distress are wrongly diagnosed as settlement when in fact they are due to moisture and thermal movements 1. That is a reason for care before you assume.
Why naming the pattern helps
What a clear description gets you
- You can say which corner, which slant and which end is wider. An engineer can use that.
- You know to check for sticking doors and sloped floors nearby.
- You know the look-alike exists, so you do not jump to a conclusion.
What the name cannot do
- The name does not separate settlement from shrinkage.
- It does not say whether movement is ongoing.
- It does not choose a repair.
What this pattern does not tell you
- A diagonal crack does not prove settlement. Look-alike shrinkage cracks are common, and HUD says they are often mistaken for settlement cracks 1.
- It does not prove the ground moved down. Frost or swell can also stress walls. See settlement vs. heave.
- It does not give a safe width. No width is offered here, and none should be trusted from a blog.
- It does not tell you the amount of movement. Only repeat measurements do.
- It does not rule out heat and moisture. The National Park Service notes a crack that follows the brick in a stair-step pattern can come from thermal or moisture expansion 4.
What drives the cost of dealing with a diagonal crack
We print no prices. If the crack is old and stable, cost may be small and tied to sealing or finish repair. If settlement is ongoing, cost follows the soil, the loads and the engineer’s design, which can include supporting or strengthening the foundation.
HUD notes that large differential settlements are more serious, and that older buildings with severe settlement problems may be very costly to repair 1. Drivers include how far the structure has moved, the number of locations, access and soil depth. Investigation and monitoring add to the early cost.
See the cost guide and the cost calculator for factors tied to your inputs.
What to do if you find one
- Photograph the whole crack, with a ruler or coin for scale and the date. Take one wide shot and one close shot.
- Note which end is wider. Mark both ends with a pencil and date the marks if the surface allows.
- Check the house. Do any doors or windows stick? Are floors sloped? Write it down 1.
- Monitor it. See foundation movement monitoring for gauges and logs.
- Check the water. Look at gutters, downspouts and grade near that part of the house.
- Ask a licensed structural engineer if the crack changes or other signs appear. A soils engineer may also be needed 1.
A pattern is not a diagnosis
Do not dig beside the foundation, jack the house, or fill a crack in a way that hides change before it has been recorded. Photographing, dating, clearing gutters and installing a crack monitor are safe homeowner tasks.
Habits that help
- Keep gutters and downspouts working, with discharge well away from the wall. See downspout drainage.
- Keep soil next to the wall sloped away, and avoid ponding.
- Watch trees and large shrubs close to the wall, since soil that loses moisture to plants can shrink 1.
- Repair plumbing leaks quickly.
- Keep your dated photos and notes together, and add notes after storms or dry spells.
Risks if a diagonal crack is ignored
If the crack comes from settlement that is still going, it can grow. HUD notes that large differential settlements cause movements in beams, joists, floors and roofs that must be evaluated for loss of bearing and, occasionally, fracture 1. Water can then enter through the crack.
There is also the opposite risk of assuming settlement and spending money on the wrong thing. HUD says many signs are blamed on settlement when moisture or thermal movement is the cause 1. A professional’s assessment helps avoid both errors.
Never assume a specific crack is harmless, and never assume every crack needs underpinning.
When to call a licensed structural engineer
Call when movement is significant, widening or changing; when walls are displaced or bowing; when new cracks come with sticking doors or windows; when walls separate at corners; or if you doubt the safety of the house. See do I need a structural engineer.
Codes, standards and who decides
No residential code gives a rule for what a diagonal crack means. Codes set design minimums for new work.
HUD’s masonry study notes that cracking in concrete masonry can occur from temperature and moisture change or differential settlement of foundation soils, and that the model building codes of that time did not include prescriptive requirements for crack control 3.
For an existing wall, a licensed engineer’s assessment is the real test. This page did not load ICC code text, so it quotes no section numbers. Ask your local building department which code applies and whether a soils report was required when the house was built.
Edition and adoption vary. Check the code your jurisdiction adopted.
Scope of the source
HUD’s masonry study reviewed case-study homes and describes practice at that time. It is background, not a rule for your wall. Your jurisdiction may adopt other editions and amendments.
Diagonal cracks compared with other patterns
| Pattern | What HUD’s guide associates with it | Where to read more |
|---|---|---|
| Diagonal | Tapering cracks at corners and openings in settlement; shrinkage look-alikes 1 | This page |
| Stair-step | Diagonal path that follows mortar joints in brick and block 1 | Stair-step cracks |
| Vertical | Shrinkage, thermal movement, or settlement at projections 1 | Vertical foundation cracks |
| Horizontal | Backfill, vibration, swelling or freezing soil; potentially serious 1 | Horizontal foundation cracks |
Myths about diagonal cracks
- Myth A diagonal crack always means settlement.
- Reality Shrinkage cracks can look the same, and HUD says they are often mistaken for settlement cracks 1.
- Myth No diagonal cracks means no settlement.
- Reality Gradual settlement may produce no masonry cracking at all in some walls 1.
- Myth Settlement is always a dramatic event.
- Reality HUD says foundation settlement usually happens over time and is not a dramatic event 2.
- Myth A tapering crack is always the worst kind.
- Reality Taper is a clue to where movement is greatest. It is not a severity scale.
- Myth Every diagonal crack needs underpinning.
- Reality Underpinning is a footing repair. An engineer decides if it fits at all.
Where soil and site raise the odds
HUD lists causes of settlement that depend on the site: soil shrinkage from loss of moisture to nearby trees, swelling from blocked drainage, frost or root heaving, clay soils drifting downhill on slopes, changes in water table, and erosion from leaking mains 1.
Colorado SP-11 adds that differential heave, not total movement, is generally responsible for major structural damage on swelling soils 5. Which causes apply depends on your region and lot.
The NRCS Web Soil Survey and your local building department can show soil conditions near you.
Choosing your next step
Match what you see with a first move.
- If A thin diagonal crack that photos show unchanged, no other signs
- Then Record and monitor; ask an engineer when convenient
- If A tapering crack with a nearby sloped floor or sticking door
- Then Ask a licensed structural engineer; a soils engineer may be needed
- If A crack that keeps widening
- Then Call a licensed structural engineer
- If Walls displaced or separating at corners
- Then Call a licensed structural engineer promptly
Engineer first, contractor second
A licensed structural engineer with no repair to sell is the usual first call. Try the engineer-or-contractor tool.
Questions about Diagonal Foundation Cracks
What do diagonal cracks in a foundation mean?
They can point to uneven movement, such as one part of a wall settling more than another. HUD describes settlement cracks as running on a rough diagonal at corners and beside openings 1. They can also come from shrinkage, so the pattern alone does not diagnose.
Are diagonal foundation cracks serious?
It depends. Some are stable and cosmetic, and some mark ongoing movement. HUD says large differential settlements are more serious because they move beams, joists, floors and roofs 1. Whether yours is serious needs history, measurement and an on-site review.
Why does a diagonal crack get thinner at one end?
Taper shows where movement is greatest. HUD says more rapid settlements produce cracks that taper, largest at one end and diminishing to a hairline at the other 1. The wider end is where the opening is greatest. It is a clue, not a severity grade.
Can shrinkage cause diagonal cracks?
Yes. HUD says drying shrinkage of block walls can produce tapering cracks that widen diagonally upward, often mistaken for settlement cracks. They usually occur in the middle third of the wall with the footer intact, and form in the first year 1.
How do I know if my diagonal crack is still moving?
Measure it over time. Photograph it from the same spot, mark its ends, or add a crack gauge, then compare. HUD notes cyclical movements may take six months or more to measure 1. Read through the seasons before you judge.
Do diagonal cracks mean my foundation is sinking?
Not necessarily. The pattern fits settlement but also fits shrinkage and other movement. HUD says many signs are wrongly blamed on settlement 1. A professional uses measurements, history and soil data to say what is happening.
What is the difference between a diagonal and a stair-step crack?
A stair-step crack is a diagonal crack that follows mortar joints in brick or block, so it steps. A diagonal crack can also cut through poured concrete. In masonry the two terms overlap, which is why HUD describes settlement cracks as a rough diagonal along mortar joints 1.
Should I fill a diagonal crack?
Record it first. Filling hides change. If the crack is cyclical, HUD says rigid mortar can simply cause the masonry to crack somewhere else, and flexible sealants are preferred after the cause is known 1. Ask a professional which approach fits.
Does insurance cover diagonal foundation cracks?
It depends on policy wording, exclusions, endorsements, cause and state law. Many policies exclude settlement and earth movement. Do not assume coverage either way. Read your policy and ask your insurer or state insurance department.
Who should evaluate a diagonal crack?
A licensed structural engineer is the usual first call, ideally one who does not sell repairs. HUD advises a structural engineer for masonry problems, and a soils engineer where differential settlement is involved 1.
Sources
- HUD User, Residential Inspection (rehabilitation inspection guide), chapters 2 and 4 (file rehabinspect_2.pdf), retrieved .
- U.S. Department of Housing and Urban Development, The Rehab Guide, Volume I: Foundations (prepared by Steven Winter Associates), retrieved .
- U.S. Department of Housing and Urban Development, Office of Policy Development and Research, Building Concrete Masonry Homes: Design and Construction Issues, retrieved .
- National Park Service, Common Problems with Brick Masonry (updated June 29, 2018), retrieved .
- Colorado Geological Survey, Special Publication 11: Home Construction on Shrinking and Swelling Soils (Holtz and Hart for the American Society of Civil Engineers, 1978), retrieved .
Expert review
Written by the HyreFoundation Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Crack-pattern statements come from HUD’s rehabilitation inspection, foundation and masonry guides, loaded on 2026-10-14. Patterns are presented as clues.
No crack-width threshold, soil value or price is given. ICC code text could not be loaded, so no code section is quoted. This page has not been reviewed by a named licensed engineer.
Suggest a correction. We fix errors and say what changed.