According to Genesis, Noah was told to apply pitch to the Ark inside and out.
Genesis 6:14 describes the Ark as a wooden vessel and instructs Noah to “cover it” with pitch on the inside and outside. Pitch is a waterproofing substance, often associated with bitumen or tar-like materials, that could seal seams and reduce leakage. The underlying Hebrew word is related to covering or coating.
This detail is practical rather than symbolic: a large wooden vessel would need sealed joints to float safely. The passage also specifies compartments, a door in the side, and a roof or opening, although translations differ over some architectural wording.
Readers sometimes assume the biblical text says Noah used modern petroleum tar. It does not identify a precise chemical product. “Pitch” is the conventional English answer, while “bitumen” and “tar” can describe related materials in other contexts. The text’s key point is waterproofing the Ark’s surfaces.