TT #11 · Parashas Noach · First Aliyah · Passuk 6:16
Make a tzohar for the ark, and finish it to a cubit upward; put the entrance of the ark in its side; make it with lower, second, and third decks.
Torah Temimah points to Sanhedrin 108b (reason for the sloping roof to a cubit).
E-Daf image — Sanhedrin 108b (size=3)
Shas / HebrewBooks PDF of the daf
English: “And finish it to a cubit upward” — Why? Because only in that form does it stand (i.e., a sloping roof that narrows to a cubit so that the water runs off).
Custom (practical / structural reason for the architectural detail given in the verse).
The ark’s roof was designed to slope and terminate at a cubit so that water would run off and the structure would remain stable.