Also known as an interlocking seam.  Adjacent steel deck panels interlock with one another during installation.

How Interlocking Side-Lap Joints Work:

Steel deck panels have a male and female side edge. The side-laps or interlocking seams are normally raised above the supports and are fastened together using button punches, screws, or welds.

Why Interlocking Side-Lap Joints Matter:

Steel deck panels that are fastened together add to the strength of the overall structure and prevent concrete or water from leaking through the metal deck.