The interior walls and ceilings of the original house might look, at first glance, to be made of drywall, such as sheetrock. They are not, and you can see evidence of this in the hallway, for example, where the walls are somewhat wavy from hand troweling. This material is rock lath, a sort of hybrid between drywall and conventional plaster over lath that was used briefly in the 1950s. It is vastly stronger than drywall and has other good properties that make it far superior to drywall (eg, and to name only one, it uses no joint compound, which did not exist back then). Do not make the mistake of tearing out the rock lath to install plasterboard.
