Step cracks in a concrete block wall need repair and remediation (C) Daniel FriedmanDiagnose & Evaluate Step Cracks in Concrete Block Walls / Foundations
Angular cracks / step cracking in concrete block, brick or other CMU walls

  • DIAGONAL CRACKS in BLOCK FOUNDATIONS, WALLS - CONTENTS: How to Evaluate Step Cracking, Stair-Step or Diagonal Foundation Cracks in concete block, masonry block, "cinder block", brick or other concrete masonry unit (CMU) walls & foundations. What are the typical causes of diagonal step cracking in concrete block foundations? What is the impact of diagonal foundation cracks on a building's stability? Are repairs needed? Photographs of types of diagonal foundation cracks
  • POST a QUESTION or READ FAQs about how to diagnose & repair diagonal or step cracking in building foundations
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Diagonal foundation or wall cracks in concrete block or other CMU walls & foundations, diagnosis & repair:

This chapter of the Foundation Crack Bible discusses in detail the process of evaluating stair-stepped or diagonal cracking and related signs of foundation movement or damage in concrete block walls. Similar terms to "concrete block" used by some include "cinder block" or masonry block or concrete masonry or CMU walls and foundations.

Diagonal foundation cracks and movement are discussed by type and location of the cracks and their common causes. Foundation cracks, which are signs of foundation damage, can mean very different things depending on the material from which a foundation is made, the location, size, and shape of the foundation crack, and other site observations.

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Diagonal Step Cracking in Concrete block or Brick Walls Caused by Vertical Movement - Structural Damage vs Minor Damage

Step cracking in a block wall due to frost (C)

[Click to enlarge any image]

The stair-stepped block wall crack shown above has been "repaired" using mortar but without more information we don't know if the cause of this cracking has been properly identified and cured. We will discuss the crack pattern and location and what they mean in this photo later in this articvle. A more thorough inspection of the entire foundation, site, building age, construction methods and other details are needed.

Article Contents

Hairline Step Cracking in Block Foundations or Walls

Below is hairline step cracking observed in a concrete block foundation near the corner of a home just a few years old.

Hairline step cracking in cinder block wall (C) Daniel Friedman

The hairline (less than 1/16" wide) stair-step cracking following the mortar joints in this foundation wall and located near a building corner will often be found at corners on buildings in a climate subject to freezing or where there expansive clay soils. Frost heave or expanding soil heave and subsidence may cycle seasonally to produce this damage.

A structural engineer or masonry engineer will usually call any crack in his foundation wall a "failure". But not all "failures" are equally scary. At the time of inspection of the foundation above the amount of movement suggested by a hairline crack like this is very unlikely to (alone) present much risk of a catastrophic collapse, but conditions may worsen if the cause of cracking is not found and corrected. Start by controlling roof runoff and getting it away from the building. Further inspection inside would be useful too.

Concret block wall diagonal step cracking shows thorugh foundation coating (C) Daniel Friedman

I'd need to look inside the foundation wall shown just above to better assess the amount of movement that is going on, but I figure that by the time I see diagonal stair step cracking appearing through the foundation coating or parging on a foundation wall, if I pick away some of the loose coating around the crack I'm going to see a wider crack (and thus more movement) than was evident before.

A typical cause of diagonal or "step" cracking in a concrete block wall (and some brick or other CMU walls) is frost heave or settlement, but as you can see in the photo below, a collapsing masonry block foundation may also show diagonal cracking near wall corners. That's because the intersecting wall is resisting movement in the collapsing wall while more movement and damage occurs towards the wall center.

Moderate Step Cracking in Block Foundations or Walls

Step cracks in a concrete block wall need repair and remediation (C) Daniel Friedman

The step cracks in this block foundation wall are more than 1/8" wide and there have been (bungled) prior repair attempts, probably to try to stop water entry through the crack. When I see a repair that has re-cracked I've got pretty good evidence that there is ongoing or cyclic damage to the wall, perhaps from frost or seasonally expanding soils. I'll say more below about why step cracks in block walls usually occur near the building corners.

I don't think this wall threatens imminent collapse but leaks and damage will increase until we find and fix the cause of this step cracking.

Step cracking in a block wall due to frost (C)

Above we see a different type of step cracking or diagonal cracks in a concrete block wall as well as a horizontal crack a bit above mid-wall height (right side of the photo). This cracking is caused by earth pressure on the outside of the wall, most likely due to a combination of wet soils and frost-push if this building is in a freezing climate. We might see similar block foundation wall damage from heavy vehicle traffic passing close-by a wall too.

Damaged foundation in step crack or stair step pattern (C) Dan iel Friedman

Shown above is fairly-typical stair-step cracking near the corner of a building's block foundation wall. Click to enlarge this photo and you'll see that the top of the foundation footing is showing - at grade level. If I see that the step crack extends down through the footing then the footing, too, has been damaged. This New York home's foundation will continue to do its's annual frosty weather dance but it might quiet down if the owner extends that downspout about six feet further away from the building and into an area where the water keeps going away.

Very Serious Step Cracking Damage in Block Foundations or Walls

Diagonal cracks in collapsing block foundation wall (C)

Vertical movement in a concrete block or brick wall might appear as either vertical cracks but more often as step cracks in which the crack pattern follows the mortar joints between the masonry units in a stair stepping pattern. Our next collapsing concrete block wall photo (below) was shared with us by Carson Dunlop Associates, a Toronto home inspection & Education firm.

Collapsing concrete block foundation (C) InspectApedia Carson Dunlop Associates Toronto

In both of these photographs, major vertical dislocation, foundation settlement, has caused large step-cracking in the concrete block foundation wall. In addition to diagnosing and correcting the reason for this settlement or foundation movement, this section of wall will have to be rebuilt.

Step Cracks Caused by Impact on a Block Wall

Diagonal cracks in a block wall due to vehicle impact (C) Daniel Friedman

The step cracked block wall above was damaged by impact by a large truck that was pulling into the area of a loading dock. Below I show a larger view of the same wall, making the point that you have to use some common sense when interpreting foundation cracks: look at what's going on.

Concrete wall damage at a loading dock (C) Daniel Friedfman

This wall needs to be re-built and the truck drivers who smash(ed) into the wall ought to get a life.

List of Typical Diagonal Crack Patterns in Building Foundations & Walls

These crack patterns form clues to help diagnose the probable cause of diagonal foundation cracks in buildings:

Note that often at these foundation failures cracks are visible both outside and inside, but outside they may be covered by backfill.

For detecting evidence of sink holes in an area by visual inspection see Sink Holes: Can X-Ray Vision [Advanced Building & Building Site Inspection Techniques] Warn of Sink Holes? in Florida or elsewhere

How to Trace the Direction of Foundation Wall Movement From a Diagonal Crack

Where step cracks are present, if you draw an imaginary line at right angles (orthogonal) to the diagonal formed by the stair stepped cracking, the downwards direction of the line will generally point to the center of the point of downwards (or up and down) movement in the structure.

But unfortunately even this "rule" has exceptions. In Florida we observed a concrete block home with step cracking high in some of its walls. The cracks were traced to settlement at the other end of the building which was responding to soil subsidence over a sinkhole.

Typical repairs for diagonal cracks in a concrete block foundation wall include the following steps

1. Assess and confirm the type of foundation cracking that has occurred in the block foundation so that we understand its cause - since knowing the cause of a crack helps understand the probability of future movement or damage. For the foundation damage shown in our photo above we suspect severe frost pressure on the wall combined with footing heaving or settlement, but we won't be confident about that analysis before inspecting the rest of the building and the building exterior and site.

2. Assess any impact of the diagonal foundation crack on the structure or its stability. For the concrete block foundation diagonal crack above there is no question that the crack involves significant structural damage, and it's likely that an expert on site will recommend reconstruction of the wall.

But before supporting the structure, removing the wall, and rebuilding this section of the foundation, it makes sense to form a complete picture of the sources of movement and damage. For example, the foundation footings may have been set on poorly prepared soil or on fill, there may be roof or surface runoff problems to correct, and we may also need to install a working foundation drainage system.

3. Repair (or rebuild) the foundation. A crack such as the block wall damage shown above should not simply be sealed with caulk or epoxy. Repairs are needed.

A complete guide to foundation repairs for all types of damage is found beginning

Watch out: even if a masonry block wall is rebuilt, as for the concrete foundation discussed at DIAGONAL CRACKS in CONCRETE FOUNDATIONS, WALLS, all foundation waterproofing solutions should begin with an identification of the source of water entry and steps to correct it outside if at all possible. The most common sources of foundation leaks are improper handling of roof runoff or surface runoff - problems that can often be corrected without digging up the foundation.


At WATER ENTRY in BUILDINGS we provide a series of articles detailing approaches to basement waterproofing, starting with the simple, inexpensive basics but also including the use of excavation, geotextiles, etc.

For evaluating the seriousness of foundation damage see

FOUNDATION MOVEMENT ACTIVE vs. STATIC which helps determine if the foundation movement is ongoing,

FOUNDATION DAMAGE SEVERITY for a discussion of just how much foundation movement is likely to be a concern.

Foundation Movement Articles


Continue reading at EARTHQUAKE DAMAGED FOUNDATIONS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.




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