This is called a scarf (or scarph) joint.  It's intended to allow for movement of the beams.  Among other uses, it was employed in wooden Japanese buildings to keep them standing during earthquakes.

The zig-zaggy joint at the top of the photo is called a scarf (or scarph) joint. It's intended to allow for movement of the beams. Among other uses, it was employed in wooden Japanese buildings to keep them standing during earthquakes. By the way, the floor joist on the right side of the photo is really that shape. It's not a camera distortion.

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