Although you may have had your home's foundation inspected when you built or purchased it, problems can develop that were not there during the initial inspection. Soil conditions may change with the weather, and external factors such as tree roots can affect an otherwise stable foundation. The key to knowing when your foundation is settling as it should or when it is faulty is in knowing the signs of trouble.
When a foundation settles, it is because of the movements of the building or its components to a lower point. This is usually caused by shifting or compacting of the soil beneath the foundation. Soil often shifts naturally because of moisture content and changes in the environmental conditions around it. Most homes will settle to a certain degree within the first few years after construction. This is normal and does not affect the foundation's stability because builders take this into account when designing the home. While settling, the foundation may develop cracks which are small and do not represent a cause for concern.
When foundations move significantly more than is normal, problems in the house itself occur, such as stuck windows and doors that won't fully close. Your plumbing and wiring can also be affected. If a crack is more than 1/4 inch wide, it is typically a sign of foundation problems. Small cracks may also indicate problems, but there are typically more than just one. New cracks or those that were once very small but got bigger over time could indicate a serious structural problem with your foundation.
Foundation movement may be caused by environmental factors or building errors or a combination of the two. Building a foundation on soil that contain gaps or filling with soil that hasn't been compacted enough causes some movement. Often this is minor and doesn't indicate a faulty foundation. Over-excavating and then inadequately back-filling can also result in some settling. Settlement that is a result of normal soil compaction or expansion is minimal and will reach a stable point. If the soil consists of materials that deteriorate continuously, then the settlement will continue as well. Foundation problems also occur if the soil suddenly becomes wet and swells beyond normal levels, causing the foundation to heave. This moisture is typically caused by plumbing problems, over watering plants along the foundation or extreme weather events such as flooding.
Most concrete and block foundations will have some cracking because concrete shrinks as it cures. Hairline cracks located in an L-shaped section of your foundation, such as where the foundation must be built down a slope, are often caused by shrinkage and aren't indicators of a faulty foundation. Hairline cracks formed in the mortar between blocks are also minor and not a concern. Cracks that move in a stair step pattern in masonry joints can indicate structural problems, particularly if the wall bulges or the cracks exceed 1/4 inch in width. Horizontal cracks are more serious, indicating moist soil has frozen and pushed into the foundation, breaking it. Also look for cracks in your interior walls around doorways, windows, or where the walls meet the ceilings. In areas where flooring has been laid over a concrete floor, buckles and cracks may develop. While these signs may result from normal settling, more often they are signs of a more serious foundation issue.