Rooftop storm and commanders

[Prplprince]

Example: My {Thraximundar} has just been exiled back to the zone so his tax is 2. I play  {Rooftop Storm} thrax would cost me 7+2 normally is rooftop's alternate cost of zero affected by the tax as well?

[Remillo]

Reductions then additions

7 down to 0
0 up by 2

Final cost = 2 (this time)

The end result is correct, the reasoning is not.  Rooftop Storm creates an alternative cost of Zombie spells you control.  If you choose to pay that alternative cost, that becomes the base cost, similar to if you're casting a spell for Morph, Overload or 'without paying it's mana cost.'

So, when you calculate the cost of your Thraximundar, you take the base of 0, then the plus 2 for commander tax, leaving you with a cost of 2 colorless.

[Prplprince]

Ok thx guys