When in play Rooftop Storm allows you to pay zero mana when casting a zombie creature spell. This effect does not apply to spells and abilities that put zombie tokens into the battlefield or otherwise relate to zombies. For example, you still have to pay the mana cost when playing Moan of the Unhallowed from your hand. Similarly, you’d have to pay the full mana cost of Call to the Grave. Neither of these cards have the creature subtype of zombie or are creature spells. They may ‘feel’ like zombie cards and fit right into a zombie deck, but they are not zombie creature spells.

One final note, the effect from Rooftop Storm does not change the cost to use the unearth activated ability either. For example, in order to bring back a Viscera Dragger from the graveyard you’d have to pay 1B.