Abstract: An underlayment for supplementing one or more mechanical properties of conventional wall, ceiling, or floor board is used to in a wall, ceiling, or floor assembly that may be fabricated on site. The assembly includes a first and second wall, ceiling, or floor boards which each have mechanical properties, and the underlayment which has at least one supplemental mechanical property for supplementing the physical properties of the first and second boards. The second board is secured to the first board with the underlayment sandwiched between the second and first boards to form a wall, ceiling, or floor assembly with the supplemental physical property or properties of the underlayment. The supplemental physical property or properties of the underlayment may include, but are not limited to, sound transmission reduction.