How much movement is too much for you? If you are in the process of buying a structure, your tolerance of any settling will typically be lower than those who currently own the structure. Ultimately, the market value, site preparation, design, and construction & maintenance practices will govern any current or future foundation movement. In other words, what is the structure’s history and what has caused the foundation movement.
As a guideline, the foundation should be designed to be flexible, however, it should not bend or break resulting in unacceptable damage. The ratio of 1/240 (one inch in a twenty foot span) is a good starting point when evaluating differential deflections. After the structure is built, any measured differentials approaching or exceeding this ratio are commonly accompanied by interior and/or exterior stress indicators. The amount and size of the indicators in a structure are generally proportionate to the amount of non-uniform differential displacement of the foundation or super structure.
Simply put, if you have cracks in your flooring, slab, interior or exterior walls, the structure has more than likely experienced movement. Simply because you are seeing cracks does not necessarily mean you need foundation repair. The burning question is…how much is too much for you?