I think someone has asked this before but not sure on that.
>If a Pokémon that has become a pure Flying-type uses Roost, it will become a ???-type Pokémon until the end of the turn.
>If a pure Flying-type Pokémon uses Roost, it will become Normal-type until the end of the turn.
So no, Ground will not be super-effective. Ground is not super-effective to ??? type or Normal type.
Hope This Helped :D
Depends on what flying type it is, because when you use roost, you loose your flying typing. So that means Earthquake would be super effective on all Flying-Fire, Flying-Electric, Flying-Poison, Flying-Rock, and Flying-Steel, after they have used roost of course.