I've been told by a friend that Rooftop Storm allows you to ignore the "commander tax" if your commander is a zombie, say Grimgrin, Corpse-born, allowing you to go arbitrarily large with a sacrifice outlet. He's convinced this works because he says he saw it happen on Magic Online. I don't see how this is possible, the Storm still has you cast the zombies as spells, and its effect only replaces the mana cost, not any additional costs. Is there something I'm missing?
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
The Commander tax is an additional cost to casting a commander from the command zone. While you would be able to cast the commander without paying its mana cost, that "free ride" does not include any additional costs that are required to cast the spell. They would still be required to pay the commander tax, assuming they are casting the card from the command zone.
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