The following illustration shows a very typical crack that can happen in both poured concrete and block foundations. Both materials shrink as they dry, and tight 90 degree corners are splendid places for cracks to originate.
The key is to monitor the width of the crack. Make a mark somewhere along the crack and measure it accurately with a ruler that has millimeter markings. If the crack starts to get wider at the top or the bottom, it could be trouble.
Stair step or stepped cracks in concrete block walls or brick walls like this one are trouble. They are telling you that settlement of some type is happening. It could also be caused by possible upheaval of the foundation. Sometimes the settlement or upheaval is minor. Other times, the crack can grow to 1/2 inch or more in width. Monitor the width of these cracks and call in a structural engineer early in the process to get a professional assessment.
Random cracking above interior doors and windows and archways is not always a sign of structural movement or settlement. Sometimes they are simple relief joints within your house. Every house has joints in it just like your knees, elbows and hips. These joints move during the changing of the seasons as indoor and outdoor humidity levels change. Often these cracks get smaller as the outdoor humidity rises.
If you have crown molding in your house on ceilings that are adjacent to the bottom of attic trusses, these cracks are rarely settlement cracks. They are caused by attic trusses that change shape in the winter time and actually raise upwards. This causes a bow in the ceilings closest to the center of the trusses. The cracks close up usually in the middle of summer. The solution is to install special simple clips between interior walls and the bottom of the trusses. Read my past column about this!
Vertical or nearly vertical cracks in poured concrete or concrete block foundations are bad news. They are a sure sign of serious trouble. If the cracks are wide at the top, it usually means that one end or both ends of the foundation are dropping or the middle of the foundation is rising or heaving. If the crack is wider at the bottom then the exact opposite is probably happening. You need a structural engineer as soon as possible to evaluate this type of problem.
Slab foundations are popular in different parts of the nation. They often can crack all the way through. These cracks may be simple shrinkage cracks or they may be actual settlement cracks if one part of the foundation is poured on poor quality soil. If you live in an area of expansive clay soils and have a plumbing leak beneath the slab, I am quite confident you will see cracking like this in many places in a short amount of time.
This is a top view of a basement floor as if you were a bird flying over a house. Many basements have 90 degree offset corners like this. It is common to see a crack in the basement slab floor radiating from such a corner. The cracks often take one year to grow to their full size. At the end of one year you can caulk the cracks with a grey urethane caulk or fill it with an expansive hydraulic cement that swells as it dries.
This crack is one that appears in the horizontal bed joints of either a brick or a concrete block wall. These cracks spell trouble with a capital T. If you see these in your basement block walls, they are a sign that the walls are in the early stages of failure. The pressure of the dirt behind them has bent the wall and the crack is the hinge point. On a brick wall, the cracks can be caused by corroded wall ties or inadequate ties and wind pressure is actually causing the wall to bend.
Brick that is laid directly in contact with concrete foundations can cause angular cracks like this in the top 12 - 16 inches of a foundation. These cracks are caused by the brick expanding and pushing the outer corner of the foundation with it. The cracks are not major structural problems.
It is often pointless to repair them as the next time it gets hot, a crack will likely appear in your repair material.
If you have doors in your house that have wide angular gaps at the top, this is telling you that the original builders were possibly blind, drunk, or horribly inept. It is probably a sign of severe settlement, especially if the crack is 1/2 to 3/4 inch wide in just 3 feet!
If you see cracks or gaps like this consider them to be huge flashing red lights telling you that major problems are happening somewhere else in the house.