"Republicans have a 53-47 majority in the chamber, and can afford up to three defections when the Senate considers whether to call additional witnesses on Friday. In the event of a 50-50 tie, by rule, the vote on witnesses would fail in the Senate. Chief Justice of the United States John Roberts is likely to abstain rather than assert his debatable power to cast a tie-breaking vote."
Justice Roberts’ role is very clearly stated Article 1 of the Constitution - the Senate has the sole power to try the impeachment,...when the President of the United States is tried, the Chief Justice shall preside. Not try, he is from a separate branch of government and does not have voting power.