This article presents the characterization of dielectric properties of common building materials such as window glass, ceiling board, floorboard, wood, plywood, plasterboard, brick, marble, concrete, and chipboard between 100 and 400 GHz. The characterization of these materials is accomplished with a frequency domain spectroscopy system using a custom quasi-optical beam-guide measurement path, and a signal processing algorithm for the accurate dielectric characterization. Deviations from the ITU standard are noted for several materials, and new building materials such as vinyl-based and HDF-based floorboards present entirely new categories of materials to characterize.