Another type of crack is a horizontal foundation crack. These cracks are almost always structural in nature. These cracks are usually located in the upper third of a concrete or block wall. Often this crack creates a “kinked” wall where it looks like the wall is bulging in at this point. Different types of lateral pressure cause these cracks. Can be cause by driveways and vehicle loading. In freezing climates, they can also be caused by a combination of surface or subsurface water and the frost line. In northern climates, if there are cracked mortar joints in the top third of a block wall or a horizontal crack at about the same depth as the frost line in the area, frost has most likely caused the damage.

Horizontal foundation cracks that are located at mid-wall height on a concrete foundation, block, or stone wall that is bulging inwards at middle height on the wall have probably been caused by vehicle traffic or earth loading. If there’s a driveway near the wall or if the site’s history includes the movement of heavy equipment near the wall, vehicle loading is the likely cause. Backfill damage, such as excessive height or premature backfill before the first-floor framing was in place, is another potential cause of this type of crack. If the home is located on a hillside, earth loading or earth loading exacerbated by water or frost could have caused the cracks. If your home is in an area with wet soil, earth loading or earth loading exacerbated by water or frost could still be the cause.

Next, there are horizontal foundation cracks that are located low on a foundation wall. These are caused because the forces exerted by soils against a foundation wall increase exponentially as you go from the surface level of the soil against the wall to the areas near the bottom of the wall. Essentially, earth pressure is greatest at the bottom of the wall. A wall that has been laterally dislocated at or near its bottom has probably been damaged by earth loading. Horizontal dislocation of a masonry block or brick wall may appear first as a crack and then later as a lateral movement.

There are many clues that help diagnose the probable cause of diagonal foundation cracks in buildings. Here are some of the most common.

Some cracks extend from the corner of a wall towards an adjacent opening and are wider at the top than the bottom. Usually, these are caused by foundation settlement, expansive clay soil, frost damage, or damage from a shrub or tree close to the wall.

Some cracks originate under a ground floor window and extend from the sill to ground with the sill bowed up. These cracks are often due to foundation heave, clay soil, frost, shallow or absent footings.

If you live in a colder climate, you may have a crack in a corner of your home. Frost heave, frost lensing, shallow footings, water problems, or insufficient backfill could have caused these.

Vertical or diagonal cracks that open suddenly after rain are serious problems. They can be caused by the settlement of the structure over sinkholes.

Sometimes straight or diagonal cracks appear over windows or doors. They could appear as horizontal along the top or bottom of the header and vertical at the ends of the header. This may be due to differences in thermal expansion between the header materials and the wall materials. These cracks could also be either vertical or diagonal at the center of the header or the corners, which is a loading failure. If the cracks are vertical or diagonal at the corners, this could be a point-load failure.

While it is curing, concrete shrinks. With poured concrete, this shrinkage causes cracks. It’s common for there to be minor shrinkage cracks that are hairline, random, intermittent, and meandering in the concrete. If the concrete was not properly mixed, or if control joints were omitted, the shrinkage cracks will be larger and appear more frequently. Sometimes cracks form because of the omission of or the placement of steel reinforcements (tie-rods).

Most of the time shrinkage cracking is a result of a poor concrete mix, rapid curing process, or other factors with the original construction. It’s less likely that shrinkage cracks will require structural maintenance or repairs because these cracks aren’t expected to continue to change after the initial curing.

We say these cracks are “least” concerning because they are generally NOT structural in nature. They can be, but usually are not. When compared to horizontal and diagonal cracks, which generally ARE structural in nature. Most simply require waterproofing by doing an interior crack injection.