lets say i have a staraptor use roost,
will that make it a almost pure normal type which means it will loose its 50% power boost from stab moves?
Sort of but not really. Roost only takes away flying type for the turn you use it, so while you would technically lose the flying STAB you would not be able to move twice on the same turn anyway.
Example: Turn one Staraptor uses Roost, Staraptor is now a Normal type. Opponent uses move move that is not important. Turn is over Staraptor is now both Flying and Normal type again.
For more info on Roost: http://bulbapedia.bulbagarden.net/wiki/Roost_(move)