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The use of a #8 self-tapping screw for attaching steel sheathing to a metal structure is based on the specific balance of strength, thread engagement, and application compatibility involved in securing metal materials. The #8 size provides adequate holding power while also being suitable for the thickness of both the sheathing and the metal structure in typical applications.

Self-tapping screws are designed to create their own mating threads in the material being fastened, which is crucial when dealing with metal structures. The threading on a #8 screw is optimized for penetrating metal without excessive force or pre-drilling, while also ensuring a tight fit that can withstand environmental stresses such as wind or thermal expansion.

Selecting a screw that is too small, like a #6, may not provide enough strength, leading to potential failure under load. Conversely, using a screw that is too large or of a heavier gauge, such as a #10 or #12, may lead to threading issues or increased installation difficulty as well as causing damage to the material being fastened. Therefore, the #8 self-tapping screw represents a balanced choice, aligning well with both the requirements of structural integrity and ease of installation.