The weight of a Zero
Jelly, 8 bytes

Z¬Þ€UZṚḌ


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How it works

Z¬Þ€UZṚḌ  Main link. Argument: M (2D array / matrix)

Z         Zip; transpose M by reading columns as rows.
 ¬Þ€      Sort each row of the transpose by logical NOT, pushing 0's to the end.
    U     Upend; reverse all sorted rows of the transpose.
     Z    Zip again, restoring rows from columns. Overhangs rise to the top.
      Ṛ   Reverse the order of the rows.
       Ḍ  Decimal; convert the rows from base 10 to integer.