According to Genesis, Noah waterproofed the Ark with pitch.
Genesis 6:14 tells Noah to cover the Ark inside and outside with pitch. Pitch is a thick, sticky waterproofing material historically made from natural bitumen, tar, or resinous substances. Coating wooden vessels with such materials helped reduce water seepage through joints and gaps.
The verse’s wording is notable because it specifies both sides of the Ark’s wooden structure. Genesis does not provide a recipe, identify the pitch’s exact source, or describe how the coating was applied. The instruction is practical and brief, appearing alongside directions for the Ark’s compartments, door, and opening.
Pitch is sometimes confused with “gopher wood,” but they are different parts of the construction instructions. Gopher wood is the stated building material; pitch is the waterproofing coating. The biblical account also does not mention modern caulking, steel fasteners, fiberglass, or paint.