If Tornadus used Roost, what type would he be?

Question

I know it can't learn it, but it just makes me curious.

Answer 1

The reason he can't learn it.

Roost taking away your flying type side, would make you Nothing

You wouldn't have any weaknesses or Resistances, nothing would touch you, do to you not having any type, and with the current style of battling, it would probably glitch up your game like no other, being pretty much the main reason they didn't give this move as an option to him.

Comments

Josh, look at this:
http://bulbapedia.bulbagarden.net/wiki/Roost_(move)

Generation V
If a pure Flying-type Pokémon uses Roost, it will become Normal-type until the end of the turn.

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in generation IV, the pokemon turned into ??? type

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No Pokemon in Gen IV is pure flying type!

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Also a typeless Pokemon takes neutral damage from every type of attack. It doesn't glitch up the game. https://bulbapedia.bulbagarden.net/wiki/Type

Answer 2

Tornadus becomes the Normal type for the turn.
This means that it receives neutral damage from all types, except Fighting which it is weak to, and Ghost which does not have any effect.

Comments

Only true in Gen IV (when the move was introduced). In Gen V and later, pure-flying types become Normal for the turn when they use Roost.

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Did you not read the comments on the other answer?

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Was Arceus not able to become a Flying type in Gen 4?
Sorry I remembered a BH battle a long time ago, where I used a Tornadus. Seems like my memory is wrong. Corrected.