A vote is taken on a motion and after it is counted it is discovered that the vote is 16 for and 16 against. Now what?
When a tie vote occurs on a motion that requires a majority vote for adoption, the motion is defeated.
If the presiding officer is a member of the assembly, and has not voted, he or she may — but is not obligated to — vote in favor of the motion to break a tie (it makes no sense for them to vote against the motion, because it is already defeated with the tie vote). The chair has the right, as a member of the assembly, to vote whenever his vote will effect the outcome of the vote, either to make or break a tie vote. However, to maintain the appearance of impartiality, the chair should exercise this right judiciously — when it is absolutely necessary in the chair’s mind that the tie vote needs to created or broken.
If the vote was by ballot, The chair should vote at the same time as all of the other members. The chair can never vote twice: once as a member and then subsequently in his role as presiding officer.
For more information, see RONR (11th Ed.) pp. 53-54;258, 405-406; 421.