As there wasn’t technological equipment during those days to make the work more labor-saving, Noah built the ark over a century. He may have had the help of other people. His sons began to be born 100 years before the flood, and as they grew up, they were able to help their father in building the ark. Noah’s own grandfather Methuselah was also alive during the construction of the ark (he died in the year of the Flood) so he may have helped his grandson, too. Aside from being a huge boat, it was also a dwelling for Noah and his family as well the remaining animals on Earth.
The ark had a rooftop, three decks as well as rooms within them. The ark was designed and built not for sailing or steering but only for floating. It only drifted under God’s direction.