This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
247. A party wall is the wall separating two adjoining buildings, and carrying the floor and roof beams of both of them. A party wall is sometimes owned jointly, when two persons own adjacent property - in this case the center line of the party wall marks one of the boundary lines of the lot - or the right to use the wall for floor and roof beams may be purchased at the time of the erection of an adjoining building.
The floor loads on party walls are obviously twice as great as the load on any outside wall; besides this, the necessity for thorough and complete fire protection must be greater in party walls than in outside walls, because the outside ones can easily be reached in case of fire, while party walls, being enclosed by other walls, are more difficult of access.
Building regulations differ materially in regard to the thickness of party walls. The general agreement is that no such walls should be less than 12 inches thick, and the best guide for determining the thickness of party walls is to make them 4 inches thicker in each story than the outside wall.