A Fine Mess
The soil that sits under a building's foundation is technically part of the foundation. For this reason, foundations are only as good as the soil they're built on. Some soils are worthy elements in a foundation system. Others can cause big problems.
Cracks in foundations are amazingly common. In fact, you can pretty much count on them happening. For the most part, they're nonstructural and cosmetic in nature. As such, foundation cracks are rarely disasters. However, they can indicate factors such as construction defects, shifting ground, or moisture problems in the ground under and around the foundation, all of which can cause structural problems.
Foundation cracks can also lead to other household disasters, including wet basements, basement floods,and pest infestations. For more on dealing with these problems, turn to Bailing Things Out after a Flood, Animals in the Home, Spiders in the House, Ant Infestations in the Home, and Cockroaches in the Home.
Small Crack, Big Problem?
As mentioned, discovering a foundation crack or two is rarely cause for panic. There's really not much you need to do about them on an immediate basis as long as water or pests aren't streaming in. However, if you suddenly see some strange condition changes in your house, there's every good reason to be concerned about foundation cracks.
Here are some of the "strange" things that are cause for alarm:
Door sills and frames that have suddenly come apart
Doors and windows that suddenly don't open or close properly
Big cracks in interior walls near door or window corners (or both)
Diagonal cracks that suddenly appear in brick fireplace walls or on exterior brick or stone veneers
Nails that have suddenly popped out of the wall
Cracking or waving in floor finishes, or feeling like you're walking uphill or downhill when you walk across rooms
Caulking pulling away from exterior surfaces, or caulked joints on exterior surfaces separating
These are all indications of possible structural damage due to differential foundation settlement, meaning that one part of the foundation has moved more than the rest, and they all warrant calling out a structural expert for further diagnosis.
Why Cracks Happen
Lots of things can cause foundation cracks. However, there are two main culprits: soil problems and construction defects. While both can and do happen, soil tends to be a problem more often than not, especially in newer construction.