Instead of welding the outside edges of the overlap (which can look messy and trap moisture), you drill or punch a hole in the top sheet. You then direct your welding arc into that hole, creating a "puddle" of molten metal that bonds the top sheet to the underlying parent metal. When you finish, the hole is filled flush with weld metal.

When most people picture a weld, they imagine a long, glowing bead tracing a seam between two plates of metal. But in the worlds of auto manufacturing, shipbuilding, and heavy fabrication, there is another, less glamorous but equally critical player: the puddle weld (also known as a plug weld or a rosette weld).

If you’ve ever wondered how the floor pans are attached to the frame of your car, or how a sheet metal bracket is secured to a hollow tube, you’ve likely seen a puddle weld. Here is your complete guide to what it is, why it’s used, and how to do it right. A puddle weld is a method of joining two pieces of metal—usually where one piece overlaps the other—by melting through a hole in the top layer to fuse it to the bottom layer.