Final answer:
Galvanized steel is the sheet metal used to protect buildings from water seepage, offering prophylactic and cathodic protection due to its zinc coating.
Step-by-step explanation:
The name of the sheet metal used to protect a building from water seepage is galvanized steel. Galvanization is a process where iron or steel is coated with a layer of zinc to prevent corrosion. This protective zinc layer acts as a barrier, providing both prophylactic protection, by hindering oxygen and water from making contact with the iron, and cathodic protection, as the zinc will oxidize before the underlying steel if exposed.
Galvanized steel is an excellent material for preventing water damage and is commonly used in various applications, including construction.