In the Genesis account, Noah was told to coat the Ark with pitch to make it watertight.
Genesis 6:14 instructs Noah to cover the ark inside and outside with pitch. Pitch is a thick, waterproofing substance traditionally made from tar or bitumen, although the exact ancient material intended by the biblical term is debated. The coating would seal the wooden structure and help keep water from entering through seams and gaps.
This detail fits the practical construction language of the passage, which also specifies gopher wood, internal rooms, a roof or opening, a door, and three levels. Genesis does not describe modern sealants, metal fasteners, pumps, or a steering system. Later reconstructions may choose different materials based on engineering assumptions.
“Pitch” is sometimes confused with wood or with a type of paint. In the biblical instruction, it is the waterproofing coating, and it is applied both inside and outside the vessel. The answer is therefore a material rather than a dimension or architectural feature.