Chester-11 arrived o/s reporting heavy fire from front to rear, 1st floor of a 2-1/2 story eor vacant dwelling. Fire extended to the 2nd floor and thru the roof. The 2nd alarm was requested with extension in the "B" exposure, which was also a vacant. Companies went to an exterior attack on the original fire dwelling and the "B-1" exposure. Crews stretched a line into the "B-2" exposure, which was occuppied, and opened up, preventing spread of the fire into that bldg.