When self-tapping screws are manufactured, the thread rolling process produces an outside thread diameter greater than the shank diameter, which in turn is greater than the minor (or “root”) diameter.

Since the outside thread diameter is larger than the shank diameter, holes in steel plates need to be sized to accommodate the outside thread diameter (while for full-body fasteners like lag screws, the hole is sized to the shank diameter). Kombi screws have a tapered shoulder underneath the head which is equal to the outside thread diameter to provide sufficient bearing against steel plates, which makes them especially suitable for steel to wood connections.

With regards to which diameter size to use in the Yield Limit Equations as specified in NDS 2018, (Cl. 12.3.7.1), more information can be found in the blog post, ”Which Diameter Do I Enter in the Yield Model Equations?”