Such commissioners in each proceeding shall also make or cause to be made an assessment roll in which shall appear the names of the owners, so far as known, the description of each lot, block, tract or parcel of land or other property and the amounts assessed as special benefits thereto, and in which they shall set down as against the city the amount they shall have found as public benefit, if any, and certify such assessment roll to the court before which said proceeding is pending, within sixty days after their appointment or after the date of the order referring said proceeding to them, or within such extension of said period as shall be allowed by the court.