Genesis says that Noah waterproofed the Ark with pitch. Genesis 6:14 instructs him to make the Ark from gopher wood and to coat it with pitch inside and outside.
Pitch is a thick, waterproofing substance traditionally made from bitumen, tar, or resinous materials. In the ancient Near East, similar substances were used to seal boats and protect wooden structures from water.
The Hebrew wording behind “pitch” is related to the idea of covering or atonement, and translations vary slightly in how they render the material. English Bibles commonly use “pitch,” “bitumen,” or a comparable term.
This detail is separate from the Ark’s dimensions and animal cargo. The construction instructions identify both the timber and the coating, giving the vessel a practical feature intended to keep water out during the Flood.