When Salem finished, he boarded up the front entrance with a large, metal desk. The rear entrance was fine, since it was a fire escape that had a large metal door with a metal bar to keep it in place.
Once he also sanitized, he went up onto the roof.
Salem: Well, that siren just saved us. The 1st floor is secure. Only way to get out is using the fire escapes. I'm sure that someone remembered to pull up the ladder?