There are cracks in stucco, drywall, concrete, and cracks in masonry and mortar. Most buildings have them; many of them are too small to be noticed. These are some of the factors leading to cracks and why some are of concern and others are not.
Rigid Building Materials
The fundamental reason cracks occur is the lack of flexibility in relatively rigid building materials. That flexibility would be unnecessary if there were no movement in buildings. The problem is that there is always movement in buildings:
- All structures settle and conform to their loads when constructed;
- Most building materials shrink as they cure or dry; and,
- Thermal expansion and contraction can cause structures to swell and shrink between day and night, summer and winter.
Other Impacts on Buildings
Long-term settlement of soils in response to building lands and seasonal movement of expansive soils can cause flexing that exceeds the capacity of rigid building materials to adapt. Environmental stresses on buildings due to minor earthquakes, wind, or snow impact structures. Even changes in loads, such as furniture, large numbers of people, remodeling, adding/removing walls, counters, etc., can cause building elements to shift or warp over long periods.
While the above events are mostly considered “normal,” there are also more severe events:
- Building movement due to excessive differential settlement;
- Significant lateral or vertical earth movement;
- Significant earthquakes;
- Failure of structural elements or connections; and,
- Impacts or forces from moving objects, such as falling trees, floods, or vehicles.
Those unusual building movements can be minor, resulting in only cosmetic damage, or it can result in structural damage that requires significant repair to restore the building’s previous integrity.
Outdoor Cracks
Most outdoor cracks are due to a combination of shrinkage/curing of materials and thermal expansion/contraction. For instance, stucco walls frequently crack in the area where large panels join narrow panels. That’s because the tension that develops from the shrinkage in the large panel makes it pull away from the small one.
This cracking frequently occurs between the small panels above doors and windows and the large panels of adjacent walls. This same crack pattern appears in concrete slabs if large panels are cast contiguously with small or narrow ones.
Some of these cracks are virtually invisible at first but widen as repeated thermal changes cause movement. Stucco on a house may have the same initial shrinkage cracking all around the house, but the extra thermal activity on the sunny side might cause the cracks there to widen and become more obtrusive.
Newer vs. Older Construction
New houses today seem to suffer more cracks than those of the past. There are several reasons for this. One is the faster construction speed for new homes. When houses took longer to build, the structure had time to adjust to its load, and the initial settlement of the foundation had time before rigid finishes were applied. In addition, older construction used cured lumber. Typical materials experiencing curing/shrinkage, such as masonry, concrete, and wood, had time to complete their initial shrinkage before applying the final finishes.
Concrete
Concrete slabs often have cracks. Most of these are due to the differential shrinkage of the concrete. Concrete shrinks as it cures and dries. If the shrinkage occurs before the concrete has developed enough tensile strength to resist the internal tension caused by the shrinkage, the concrete cracks.
A concrete panel that is significantly longer than it is wide has more shrinkage in the long direction than the shorter width. Cracks are more likely to cross the shorter dimension (more tension, less sectional area to provide tensile resistance). Shrinkage cracks are often crested; that is, there is a high point in the slab at the location of the crack. Crested cracks come from the differential curing and shrinkage between the top and the bottom of the slab. The top dries, shrinks, and develops tension before the bottom does, and the pieces of the slab tend to curl, causing lift at the crack. For example, this outcome is similar to the process that causes curled pieces of soil in a dried mud puddle.
Proper curing involves maintaining moisture and temperature in concrete for several days after casting the slab. The wetter the concrete, the more it shrinks. The concrete with higher proportions of fine-grained material shrinks more than that with larger aggregate. In addition, the temperature is a factor in overly rapid drying, but the wind is just as important, and humidity is a crucial factor, too.
Differential Movement
Cracks appearing from differential movement in buildings and slabs are frequently harder to interpret. For example, a 45-degree crack in a wall could be due to:
- Downward movement of the foundation below the wall;
- Settlement of one end of the wall; or,
- Movement of the rest of the building relative to the wall.
Displacement along the crack is a significant clue. If a jagged crack is wider in sections going in one direction, then movement in the direction aligned with the narrow segments can be inferred. If the crack is of relatively even width, then tension perpendicular to the crack is likely.
Cracks often appear in slabs with relatively minor differential movement. A floor-level survey is frequently necessary to determine what movement occurred. Slabs with a hump in the middle may be due to the heaving of expansive soils below. Or, it could be from the settlement of the footings around the perimeter.
If the slab is flat but tilted, there could be settlement of one side. Or the slab is out of level. Truly random and closely spaced cracking can be an indication of a cement-aggregate reaction. The swelling of aggregate or formation of crystals causes stresses in the slab resulting in cracking and/or flaking of the concrete.
A Problem or Not
Some cracks may only indicate a problem; some may be a problem. Most cracks are cosmetic. However, cracks can also be detrimental to the system’s functionality. One of the purposes of stucco is to keep water from the interior of a wall. Therefore, a crack in the stucco can allow water access. Properly installed flashing and papering under the stucco shouldn’t cause an immediate problem. But if the papering is faulty or if the water is allowed access for an extended period, problems can result from this crack.
The life of a pavement depends, to a degree, on protecting the soil subgrade from water. Soil saturation can result in a subgrade with little resistance to flexing and eventual deterioration of the pavement. Cracks become potholes. Cracks in floor slabs can allow access to bugs and moisture; two items houses are built to keep out. Cracks in structural members can weaken a structure, making it less resistant to earthquakes or other forces.
What To Do
Dont’s take cracks lightly but don’t panic either. They are most often the result of a complicated interaction of building systems, and their cause is not always obvious or easy to determine.
Knowledgeable professionals should evaluate cracks, especially if there are other signs of soil movement. However, to fully understand a cracking building, it may be necessary to get structural engineers, specialty contractors, building material manufacturers, or others involved.
Note: Earth Systems originally published this article in a newsletter in 2004 and re-edited it for the web.