I don't expect to get kills inside a building until the roof is visibly gone. A fire mission of short duration usually isn't long enough to collapse the roof, even with 155s. If I want to make sure no one survives I flatten the building. But sometimes that isn't even enough. I recently played a game where a Tornado dropped its heaviest bomb on a two story building a few seconds before the mission ended (it probably caused the enemy to surrender). The building was completely destroyed, but there were still a few greens and yellows left alive.