This is the most structurally significant cause. When the soil beneath a foundation settles unevenly, the structure drops in specific areas.
In rare cases, foundation movement can cause a crack that starts at a corner of the window and runs horizontally. However, true structural cracks are usually wider at one end, appear with other foundation symptoms (sticking doors, floor sloping), and often run diagonally, not perfectly horizontal. horizontal crack under window sill
If a window wasn't properly supported or anchored during installation, everyday movements like doors slamming can eventually result in horizontal gaps. When to Worry: Serious Structural Signs This is the most structurally significant cause