It doesn't. Rooftop Storm only affects Zombie creature spells, and creatures don't get flashback.
Additionally, Rooftop Storm creates an alternate cost. Because flashback is also an alternate cost, and because you can only pay either the mana cost or one alternate cost per spell, you wouldn't be able to flashback for another alternate cost anyway.
November 25, 2015
8:51 a.m.
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Epochalyptik says... Accepted answer #1
It doesn't. Rooftop Storm only affects Zombie creature spells, and creatures don't get flashback.
Additionally, Rooftop Storm creates an alternate cost. Because flashback is also an alternate cost, and because you can only pay either the mana cost or one alternate cost per spell, you wouldn't be able to flashback for another alternate cost anyway.
November 25, 2015 8:51 a.m.