Foundation cracks can be a sign of a serious structural problem. Even a hairline crack can mean trouble. At the very least a foundation crack is a path of least resistance for water and dangerous soil gases to enter the basement. Water in the basement will create a place for mold and mildew to grow, which can cause a meridian of health problems (see mold/mildew). Soil gases (such as radon: a colorless, odorless, cancer-causing gas) can enter your home through concrete cracks and pollute you indoor air.
To determine if a crack is a structural or nonstructural crack, it is best to have a professional foundation repair contractor inspect your basement. Generally, horizontal cracks are structural cracks and vertical cracks are nonstructural.
Concrete naturally cracks as it expands and contracts with the freeze/thaw cycles, which is why most concrete walls and floors are built with control joints (a straight line for the concrete to crack). The problems start when the crack is letting water in to the basement. Even control joints can leak if they are not properly filled with a flexible urethane.
Structural cracks are usually caused by settlement or hydrostatic pressure that is pushing and moving the foundation. Structural cracks should be injected with a rigid epoxy to keep the crack from moving. Drainage and exterior waterproofing systems can help relieve the hydrostatic pressure build-up. Underpinning systems may be needed if settlement is the cause.
Another sign of a structural problem is if the cracks extend to the upper levels of the house. Cracks in the walls and ceilings upstairs can be a sign of structural problems. Slanted door frames and floors, sticking doors and windows, and sunken porches and sidewalks may also be signs of settlement issues.
All foundation cracks should be fixed. Leaving these problems can be dangerous and impact the structural integrity of your house.