Photograph of a substantial settlement crack in poured concrete.How to Diagnose & Evaluate Diagonal Foundation Cracks
     

  • DIAGONAL FOUNDATION CRACKS - CONTENTS: How to Evaluate Diagonal Foundation Cracks. What are the typical causes of diagonal cracks in concrete 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
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Diagonal foundation cracks: diagnosis & repair: this chapter of the Foundation Crack Bible discusses in detail the process of evaluating foundation diagonal foundation cracks, step cracks, and related signs of foundation movement or damage. 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 FOUNDATION CRACKS - Diagonal & Step Crack Patterns in Building Foundations

Photograph of a substantial settlement crack in poured concrete. (C) Daniel FriedmanThe diagonal crack shown at left is interesting because of its width (more than trivial or hairline) and discontinuity, suggesting that it occurred during foundation curing, perhaps involving both shrinkage and footing settlement.

A more thorough inspection of the entire foundation, site, building age, construction methods and other details are needed.

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:

  • From corner towards adjacent opening, wider at top than bottom - often due to foundation settlement, expansive clay soil, frost damage, or damage from a shrub/tree close to the foundation wall.
  • Under a ground floor window, from sill to ground, sill bowed up - often due to foundation heave, clay soil, frost, shallow or absent footings
  • In the foundation wall anywhere, wider at bottom than top - settlement under building
  • At building corners in cold climates - frost heave, frost lensing, shallow footings, water problem, or insufficient backfill. In a typical raised ranch with a garage located in part of the basement, and with the garage entering at one end of a home, we often find step cracks in the front and rear foundation walls only on the garage-end of the home.

    These cracks may correspond to some related observations:

    (1) there may be less backfill against the front and rear foundation walls where a garage entry is located between them;

    (2) the reduction in backfill combined with an un-heated garage may expose these building corners to more frost damage;

    (3) if a building downspout or gutter defect spills roof drainage against the building wall, these forces will often combine to make more severe frost cracks appear on the garage-entry end of the home.
  • Vertical or diagonal crack which over a short time - settlement over sink holes- serious, open suddenly after rain; or ravines, mulch, fill, organic debris (later rots and settles).
  • Over window/door, straight or diagonal - loading/header defect - may appear as horizontal along top or bottom of header, vertical at ends of header (possibly due to differences in thermal expansion of different materials of header vs. wall) or vertical/diagonal at center of header (loading failure) or at corners (possible point-load failure)
  • Cracks in a poured concrete foundation which are diagonal or vertical and which are generally uniform in width, or which taper to an irregular hairline form, usually in fact a discontinuous crack in the hairline area, are usually shrinkage cracks and should not be ongoing nor of structural significance, though they may invite water entry through the wall.

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

Diagonal Cracks in Poured Concrete Foundations

Photograph of diagonal cracking in poured concrete (C) Daniel Friedman

Concrete walls tend to display vertical cracks but settlement or frost heaving at a corner of a concrete wall can produce diagonal cracks or breaks in that location.

Steep diagonal cracks may also appear in concrete foundations due to unusual point loads that exceed the compressive strength of the concrete (maybe it was weak concrete not high loading), and we've seen steep diagonal cracks in poured concrete and other high-rise masonry buildings exposed to frost damage.

Steep diagonal shrinkage cracks: But in this photograph of a diagonal crack in a poured concrete foundation, we are almost certainly looking at a large shrinkage crack. Notice that discontinuity in the crack pattern?

A typical cause of diagonal shrinkage cracks in a concrete foundation wall is an improper mix or improper curing conditions at the time that the foundation wall was set in place or "poured".

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

1. Assess and confirm the type of foundation cracking that has occurred so that we understand its cause - since knowing the cause of a crack helps understand the probability of future movement or damage - that is, confirm that we're looking at a shrinkage crack - something that occurred at or close to the time of construction, not a crack that occurred as a result of stresses, loads, or building movement.

2. Assess any impact of the diagonal foundation crack on the structure or its stability. For the diagonal shrinkage crack above it is unlikely that there has been any measurable impact on the rest of the building structure.

But because multiple forces or stresses can be at work at a building at the same time, a shrinkage cracked foundation might also show signs of settlement or actual movement. If this crack also showed signs of ongoing or cyclic building movement, such as due to frost pressure, thus converting it into a structural crack, we'd expect to see breakage across that discontinuous point in the crack shown in our photo, and we might also see lateral dislocation - that is, the foundation wall on the two sides of the crack would no longer be flush. And if there is ongoing settlement we'd expect the crack to be wider at its top than at its bottom (in most cases).

3. Seal the diagonal shrinkage crack against water leakage. If the crack is confirmed to be only due to concrete shrinkage, and to stop water leaks through the foundation, an expert might recommend sealing using epoxy injection. The appeal of that approach is that the cost is much less than a foundation waterproofing effort involving exterior excavation.

Watch out: 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. See WATER BARRIERS, EXTERIOR BUILDING and WATER ENTRY in BUILDINGS.

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

Photograph of diagonal cracking in concrete block (C) Daniel Friedman

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.

In this photograph, 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 probably have to be rebuilt.

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 shrinkage 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.

Watch out: even though this wall will probably be rebuilt, as for the concrete foundation discussed earlier, 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.

See WATER BARRIERS, EXTERIOR BUILDING and WATER ENTRY in BUILDINGS.

Reader Question: How can we distinguish a diagonal structural crack from a cold pour joint in a poured concrete foundation wall?

I am hoping you can help me out here, the home inspector was not very helpful to me for this one. I have attached an image of the foundation cracks in the basement, I am wondering if you can help me identify if there is any structural problems. The outside of the wall is backfill. Thank you so much for your help! - C.C. 9/5/12

Photograph of contraditory diagonal "cracks" in a poured concrete foundation wall (C) InspectAPedia & C.C.

Reply: Unfortunately your initial photos were quite blurry and small, making any detailed examination impossible. I cannot see if you are actually showing photos of actual foundation cracking (such as due to settlement or movement) or if in fact we are looking at foundation wall leak stains along cold pour joints. Certainly the large dark diagonals in your photographs are COLD POUR JOINTS, CONCRETE. We will continue the discussion in added detail in that article.

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.

 

Continue reading at EARTHQUAKE DAMAGED FOUNDATIONS or select a topic from the More Reading links shown below.

Or see BULGED vs. LEANING FOUNDATIONS

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DIAGONAL FOUNDATION CRACKS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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