Noah used pitch to waterproof the Ark inside and out, according to Genesis.
Genesis 6:14 instructs Noah to make the Ark from gopher wood and to cover it with pitch inside and outside. Pitch is a sticky, waterproofing substance traditionally associated with bitumen, tar, or resinous material, although the exact ancient substance is not identified with certainty.
This instruction concerns sealing the vessel, not the material from which its frame was built. The same verse separately names gopher wood, a term whose precise meaning has been debated for centuries.
The waterproofing detail reflects practical ancient boatbuilding knowledge. Bitumen and related compounds were used in the ancient Near East to seal boats and containers. The Genesis text does not provide a recipe or explain how much pitch Noah used, so modern reconstructions must make assumptions about the material and construction.