Final answer:
A crack in the bricks at the top of a doorway or window usually indicates a missing or damaged lintel, which is a structural component supporting the wall above the opening.
Step-by-step explanation:
A crack in the bricks at the top of a doorway or window usually indicates a structural issue. The presence of such a crack is often a sign that there may be a missing or damaged lintel. The lintel is an important structural component that supports the weight of the wall above the opening for a door or window. When it fails or is absent, it can lead to cracking due to the unsupported weight above the opening. This is different from mudcracks which occur in wet sediment that shrinks upon drying, forming desiccation cracks on the surface.
Issues like a water leak, improperly mixed mortar, or a lack of a vapor barrier in the insulation are separate concerns which may cause other types of damage or deterioration, but they are less likely to be directly responsible for cracking at the top of a doorway or window.