A copper down conductor may never be ...

A copper down conductor must never be attached to aluminum flashing due to the potential for galvanic corrosion that can occur when dissimilar metals are in contact. This corrosion can significantly reduce the lifespan and effectiveness of the lightning protection system. Since copper and aluminum have different electrochemical properties, the moisture present can lead to accelerated deterioration of the aluminum, compromising the integrity of the flashing and potentially leading to failures in the protection system.

In contrast, while there are considerations for attaching down conductors to wooden structures or employing burying techniques when dealing with underground installations, those are not outright prohibitions and can be managed with proper techniques and insulation if necessary. Similarly, connecting to steel reinforcements can be done under controlled circumstances, typically employing protective methods to mitigate corrosion risks. The primary concern with aluminum flashing stands due to the direct and immediate risk associated with the combination of these two metals.