Here's how freeze works this generation (as per Bulbapedia):
A frozen Pokémon has a 20% chance of being thawed out each time it attempts to move. If a frozen Pokémon is successfully hit by a damaging Fire-type move, in addition to Scald and Steam Eruption, it will be immediately thawed. A frozen Pokémon can still use the moves Flame Wheel, Sacred Fire, Flare Blitz, Fusion Flare, Scald, Steam Eruption, Burn Up, Pyro Ball, and Scorching Sands while frozen; these moves will thaw the user, then execute normally. Heal Bell, Aromatherapy, Natural Cure, Hydration, Shed Skin, and Healer can also cure freeze.
As you can see, Roost doesn't directly do anything to cure freeze, but your Pokemon does naturally have a 20% chance of being thawed out each time it attempts to use it.