What should the head of a fastener not be tightly driven against?

The head of a fastener should not be tightly driven against the vinyl siding nail hem because doing so can compromise the integrity and functionality of the siding system. The nail hem is designed to allow for the expansion and contraction of the vinyl material with temperature changes. If the fastener is driven too tightly against the nail hem, it can prevent the siding from moving freely, potentially leading to warping, buckling, or other forms of damage over time.

In the case of wood siding, metal flashing, or brick siding, while it's important to install fasteners correctly, the specific concern about tightly driving against the nail hem primarily applies to vinyl siding due to its flexible nature. Each of these other materials has different properties and installation guidelines, but the interaction of vinyl siding with fasteners is particularly sensitive due to its design and material characteristics.