What causes cracks in the walls and floors of a house?
Sunday, October 21, 2018
There are two unavoidable consequences of the passage of time for any type of construction: 1) buildings move, and 2) materials deteriorate. It’s the architectural version of death and taxes, and the process begins even while a home is under construction.
The ground under a new home compacts slowly as the massive weight on it piles up, the concrete floor slab shrinks slightly as it hardens, and the lumber inside the walls moves and shrinks a little as it dries. All of this happens over the first two years, while the ground and the interconnected components of the structure adjust to the new loads and stresses imposed on them.
So you get minor hairline cracks in a concrete floor slab that are normal and unavoidable. A contractor typically cuts score lines about 1/4-inch deep in both directions across large concrete slabs, which are typically visible in the garage (shown below) and driveway, and encourages the inevitable cracks to run in the notches, making them less noticeable.
Also, the slight bowing or twisting of a few rogue pieces of lumber inside the wall causes fasteners to raise out from the wall slightly and short cracks to appear in the interior walls, usually along the seams between drywall boards near inside corners or doors and windows. The problem is easily repaired and is a one-time event if no other factors are involved.
Minor hairline cracks are considered “cosmetic;” as opposed to larger cracks, which are termed “structural.” A structural crack is defined as one that is 1/8-inch or more wide and/or has “differential,” meaning that one side of the crack is raised above the other side. A simple way to determine if a crack is structural: run your hand across it to feel for any differential, and hold two quarters side-by-side and try to stick them into the crack to determine if it exceeds the 1/8-inch width criterion.
After the initial “settling in” of a home’s structure, other forces can cause cracks over time. Here’s our “Top 10” list of possible causes, and it’s important to note what you see might be caused by a combination of two or more of these factors:
1) SOIL EROSION - Rain and underground water movement through the soil causes particles of soil to move from higher levels to lower levels over time. This movement, called soil subsidence, happens in slow motion over many years, and can gradually undercut the foundation of the home (like in the photo below) and cause settlement, typically at the corners first. An early sign of soil erosion is when an unpainted area at the bottom of a wall becomes visible because the ground level has dropped since the house was painted. Homes that are built on a sloping site are especially prone to erosion over time. See our blog post What is the difference between soil subsidence, heave, creep, and settlement?
2) TREE ROOTS - Roots can cause a lifting action under a home. But, more often, tree root intrusion results in settlement, because the roots absorb so much of the water from the ground in their immediate vicinity. To learn more, go to our blog How can trees damage a house?
3) CLAY SOIL - The sponge-like quality of clay deposits in the soil under a home make the ground swell in seasons of heavy rains and subside in droughts. This up-and-down motion wrecks havoc with the footings, floor slab, and walls of a home. Clay also can run in veins under upper layers of other soils. Go to our blog post How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil? for more info.
4) SINKHOLES - Definitely the most dramatic cause of cracks in a home, sinkholes are created when underground water erodes pockets in the karst underlayment of much of the ground in this area of Florida. At some point in time the pockets collapse, often slowly, but sometimes in a quick, dramatic action. You can visit our blog What is my chance of buying a Florida home over a sinkhole? to find out more about both clay soil and sinkholes.
5) CONCRETE SPALLING - When moisture is able to work its way below the surface of concrete and reach the steel reinforcing bars inside, the ensuing expansion of the steel caused by corrosion causes long cracks a the surface. This is called “spalling,” and once the cracks open and let in more moisture, the process accelerates. The photo below is a typical example of concrete spalling. To read more about this problem, go to our blog There's cracks running along the home's concrete tie beam. What's wrong?
6) THERMAL EXPANSION AND CONTRACTION - Everything in a house expands and contracts with seasonal and day-to-night temperature changes outside and the adjustment of the thermostat inside. Components of a house can handle these changes when their movement is not impeded by other materials around them that have a different coefficient of thermal expansion, are at a different temperature, or the geometry/shape of the adjacent component blocks movement. So, basically, materials that are trapped and don’t have room for expansion are vulnerable to cracking themselves, or can crack the adjacent material.
If not installed correctly, stucco on a wood frame wall can have extensive cracking due to the difference in expansion and contraction of the stucco versus the underlying structure. Visit our blog post Why is my stucco cracking? to learn more.
7) CONCENTRATED LOADS - An example of where a concentrated load occurs would be at either side of two-car garage door. All of weight of the roof structure that is bearing on the beam over the door is transferred to about 8-inches of wall at the side of the door. This load is, in turn, concentrated on a small areas of the thickened edge of the concrete floor slab underneath it. Sometimes this causes diagonal cracks in the floor at either side of the garage door between adjacent walls. See our blog How can I tell if cracks in the garage floor are a problem or not? for more on this situation.
8) UNDERSIZE STRUCTURAL MEMBERS - Two examples of where a structural member becomes overloaded and the sagging creates cracks around it would be a beam over a garage door that is undersize for the span, and when a homeowner stores heavy items in an attic that does not have trusses or ceiling joists rated for the extra load. In both cases, the cracks usually do not appear immediately, but develop with the gradual sagging over time.
9) MOISTURE CHANGES - Building materials absorb and release moisture in relation to the change in humidity of the air surrounding them and, of course, any liquid moisture (water) that they get immersed in. The consequent expansion and shrinkage of the materials are another cause of cracks, although usually minor ones. A home that is air conditioned (which causes dehumidification), then left with the a/c off for a period of time (causing the humidity to rise), then air conditioned again (yes, lowering the humidity back again) can cause cracks to open, especially on interior surfaces.
10) IMPACT DAMAGE - Sometimes we reach for a complex explanation for cracks when the answer is simple: it got whacked by something or a heavy object fell on it.
Once a long crack opens up, it is difficult for a reversal of the forces that opened the crack to close it up again. Both sides of the crack can still move to push against the other side but, because of the disconnection caused by the opening of the crack, neither side can pull the other one back towards it when contracting. Also, cracks in floors and exterior walls tend to get clogged with dust and debris over time.
A knowledgeable, experienced inspector can interpret what’s happening to cause mysterious cracks to appear in a home—to some extent. The location of the cracks, direction, relation to each other, taper shape, and debris accumulation, along with change of planes of the surfaces, tell a story about the forces causing them. It’s a kind of “puzzle” that we enjoy solving.
Usually the cracks that we study are understandable, manageable, and perhaps just need minor repair and to be watched for any further movement over time. But when we are stumped, or the cracking indicates a serious problem that warrants a repair plan, we refer our customers to a licensed structural engineer for further evaluation.
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To learn more about exterior walls and structures, see these other blog posts:
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