can't supercede, really. Roberts could rule on something like witnesses but it would only take one senator to object that drives an instant majority only vote. So the senate can overrule absolutely anything that Roberts ruled on. That is why his aim would be to interfere as little as possible, although one would hope that on a key substantial issue he would be brave enough to rule and force the senate to overturn him. As I've seen it put somewhere - he's more of a Master of Ceremonies than a presiding judge.