A structural crack is where the foundation wall feels uneven or the width of the crack is about ¼” or wider. Some structural foundation cracks will require engineer specifications in order to do a basement foundation repair. The obvious thing to look for is major cracks on the foundation wall. A horizontal crack means that there is a problem. Though all cracks on the basement wall are generally bad, but horizontal cracks are serious. Horizontal cracks occur with the soil [mainly clay] constantly expanding and contracting putting the basement wall under a lot of pressure. Some walls especially block foundation walls tend to bow.