You could explicitly state that this was to keep out water. AT: "spread tar on it to keep water from getting into it" (See: [[rc://en/ta/man/translate/figs-explicit]])
This is a sticky brown or black paste that can be made from tree sap or from petroleum. Therefore, "pitch" would include not only "bitumen" but also plant-based resins. It too can be used to keep out water. AT: "tar" or "resin" (See: [[rc://en/ta/man/translate/translate-unknown]])