Knock down the wrong wall in an old house and the ceiling may sag. That wall was load-bearing. A load-bearing wall supports weight from above — floors, roof, or other walls — in addition to its own weight. Remove it without adding a beam or header, and the structure above loses its support.

Load-bearing walls are usually easy to spot in a basement or attic. They run perpendicular to floor joists, sit directly above a foundation wall, and feel solid when tapped. But appearances can deceive. A wall that looks structural might be a partition. A wall that looks like a partition might carry a beam.

Renovation projects often require an engineer to confirm which walls are load-bearing. If one must come out, the load is transferred to a beam or header sized by the engineer. The beam carries the load to columns or adjacent walls. This is routine work, but it must be done correctly.

In high-rise construction, load-bearing walls are less common. Steel and concrete frames carry the loads instead, and walls are non-structural infill. The principle remains: something must carry the weight.