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Rashi 6(14) quoting the Medrash and Gemorah Sotah, explains that the Ark was waterproofed both outside and inside because of the force of the waters.

Practically, I do not see what benefit would be gained from applying pitch on the inside.

Unless maybe the idea was that the pitch should penetrate into the wood?

Any sources to answer the question?

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  • What's so strange about two layer of protection?
    – mevaqesh
    Oct 30, 2016 at 18:53
  • Sounds like an engineering question
    – TrustMeI'mARabbi
    Oct 30, 2016 at 18:53
  • This is a question concerning shipbuilding.
    – kouty
    Oct 30, 2016 at 18:56
  • Pitch does penetrate into the wood; it's not just a surface coating. Whether penetration from both sides helps with this I don't know.
    – Monica Cellio
    Oct 30, 2016 at 22:48
  • @Monica Cellio water penetrate from any sides and damages the wood
    – kouty
    Oct 31, 2016 at 4:05

3 Answers 3

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Although this may be better answered by looking into ancient ship building methods, we can also attempt to answer this from chazzal.

See Bava Metzia 40a

באתריה דמר חפו בקירא ולא מייץ טפי באתריה דמר חפו בכופרא ומייץ טפי 

Rashi explains:

חפו בקירא. טחו החביות מבפנים בשעוה כדרך שעושין אנו בזפת: כופרא. זפת:

So we see that zefes does not completely prevent absorption and can actually be penetrated. In contrast to wax which prevents the absorption.

Therefore a second insulating layer was added on the inside to help keep out whatever water made it through into the wooden hull.

Edit Another possibly relevant source about Zephes: In Maseches Keilim chapter 3 mishna 7 we find that Zephes can not be trusted to maintain it's structural integrity in boiling water and can melt. See Bartenura here:

שהזפת ניתך ונמס במים חמין.

So according to the opinion on Sanhedrin 108b that the waters were actually boiling we can suggest why a regular coating of Zephes wouldn't be trusted. Of course we would have to assume the second layer would help. That's why I only say this is possibly relevant.

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  • Also of note is that chazzal pointed out that the lumber used was new, from trees grown specifically for this purpose. See the sugya there amud beis with tosafos bimizufafin that old barrels lined with zefes do not absorb. New wood with zefes will, based on the gemara above.
    – user6591
    Nov 16, 2016 at 19:50
  • And Rashi in Noah 6 14 was kind enough to point out that kopher is zephes in arami, which the gemara calls kuphra.
    – user6591
    Nov 18, 2016 at 1:52
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I'm told that the purpose is to prevent mold/mildew/fungus from getting a hold on the inside surface. Once that happens, the wood can lose integrity and stiffness, and eventually the water will push its way through the spongy wood. In such an environment, particularly where the boat is holding innumerable living animals, and especially where there's not enough ventilation, the likelihood of mold/mildew is very high.

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  • So - not like Rashi. You write, "I'm told"; who told you please?
    – Avrohom Yitzchok
    Nov 1, 2016 at 18:55
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    A friend, Andrew Sadok, with the largest masted vessel on the Great Lakes, The Red Witch. I don't know if he's right or not, but boy, he can make a boat dance.
    – Eliezer Eisenberg
    Dec 4, 2016 at 4:06
  • @EliezerEisenberg Great!
    – Avrohom Yitzchok
    Dec 4, 2016 at 10:13
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Radak (Genesis 6:14) explains that this was intended as two layers of protection; if the outside should crumble, the inside would remain firm. He seems to understand that the point wasn't water proofing, as much as strengthening the ark against the force of the water:

מבית ומחוץ - שאם יתפרקו מה שבחוץ מפני חוזק המים עומד מה שבפנים

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