How Long did it Take Noah to Build the Ark?
How long was the Ark in the building? The answer, which is arrived at inductively, is around 98 years; possibly a few years less. In verse 32 of chapter 5 Noah was said to be five hundred years old. It also says that Shem was already born. We have pointed out that this statement is not to be taken as a part of the iteration of the genealogies, because of the literary change in the narrative. The end of the precise defining of the genealogies was reached with Lamech, Noah's father. Lamech lived 777 years and died 4 years before the Flood. With Lamech, the genealogy ceases for a while, as we give attention to Noah, the Ark, and the Flood. The scene in verse 32 of chapter 5 was not exactly in Noah's 500th year. The literary style is that of generalization, so it is neither critical nor inconsistent.
It is not clear from the record whether or not God spoke to Noah at that time. Verses 1 through 13 of chapter 6 intervene before we hear God saying anything to Noah. He may have talked to Noah before this, but there is no way of knowing for certain. But there can be no speculation as to how old Noah was when the Flood came. This is spelled out in verse 11 of chapter 7: “In the 600th year of Noah's life, in the 2nd month, and the 17th day.” If verse 32 of chapter 5 does not intend to be specific, this verse does.
In verse 10 of chapter 11 we read that Shem was a hundred years old, and begat Arphaxad two years after the Flood. This age is to be taken as exact also, because it is a part of a genealogy, and it is telling us, along with verse 11, how long Shem lived. He lived to be exactly 600 years. So we know that there could have been no more than 98 years between the 32nd verse of the 5th chapter and the Flood. 98 years would assume that Shem was born that year, which is reasonable, since he was the youngest of the three, according to chapter 10. If he was born earlier, less time would have intervened. Because of the approximate five hundred year age of Noah, less time is not argued for, though it would not have mattered insofar as whether or not there was enough time to build the Ark.
Arguments have been made that verse 3 of chapter 6 means the antediluvian race would only continue for another 120 years. The argument goes that from the commandment to build the Ark until the flood came was 120 years. They say that it could not have been talking about men after the Flood, because Noah and his sons lived longer than that.
There are a number of problems with that argument. First of all, the normal sense of the genealogies of Seth and Cain would tell us that both Japheth and Ham were already older than 120 when God spoke to Noah. And of course Noah himself was around 500. The limitation of years was not on these men or on those immediately after the Flood, but on the human race in general. It took a few generations for the lives of men to shorten to 120 years, but we see steady decline. Arphaxad lived to be 438. Salah lived to be 433, Eber lived to be 464, Peleg lived to be 239, as did Reu, Serug lived to be 230, and Nahor lived to be 148. Abraham lived to be 175, but it is indicated that he lived an exceptionally long life. By the time of Moses, the age of man was down to 120. If this is not the meaning of God's words in verse 6, it is strange that this is just how it worked out with the race by the end of Genesis. And if this is not what God meant to say, then why did Lamech prophesy that God was going to bless the race by greatly shortening their years? It is always a little frightening to be at odds with imposing people. Even so, it is my duty to point out that it could not have taken 120 years to build the Ark, since the age of Shem, when his son was born two years after the Flood, simply will not allow that exegesis.
Could Noah and his family have built the Ark in 98 years? Let us think about that for a moment. These were gigantic people, skilled in building houses and cities, living in a world where weapons and tools were made of brass and iron. They had lived and developed experience for five long centuries. There were with huge elephants, oxen, horses and other animals, apparently much gentler than today, to use as beasts of burden and to pull windlasses for hoisting. There were multitudes of people around to hire for helpers if needed. If the purpose of asking is to raise doubts, then the question seems ludicrous on its face. One must be ignorant of the matter altogether to suppose that building the Ark in that much time presented any kind of problem.
 Those who preceded Noah began having children around ages 130. If Japeth, the oldest, was born when Noah was 130 and Ham was born when Noah was 260, they would both have been more than 240 years old when God spoke to Noah.