Appeal 34816

1206.2

Interior walls, partitions, and floor-ceiling assemblies separating dwelling units must provide an airborne sound insulation rating of not less than STC 50 (or STC 45 if field-tested).

Approval to maintain and restore existing demising wall construction between units without increasing sound transmission class (STC) ratings to meet current OSSC 1206.2 levels, based on original construction and feasibility constraints.

The existing demising walls between dwelling units are composed of wood stud framing, likely 2” x 4” or 2x4, at 16" o.c., with single layers of lath and plaster or gypsum wallboard on both sides. There is no known existing batt insulation in the wall cavities. Under the plumbing rehabilitation project, selective removal of interior finishes is necessary to access domestic water piping, including limited removal of finishes on certain demising walls. All affected finishes will be restored to a uniform 1-hour fire-resistance-rated assembly (UL U305) using fiberglass batt insulation, Type X gypsum wallboard, mud, tape, and matching texture. No modification is being made to the structural, spatial, or occupancy of the building.

This Chapter 13 building was constructed in 1910, prior to the adoption of STC requirements. Achieving STC 50 would require full demolition and reconstruction of existing demising walls. Given the dimensional limitations and existing stud configurations, compliance is infeasible without major disruption to occupied units. The current scope involves limited finish removal for plumbing access. Restoring affected areas with Type X gypsum board to a UL U305-rated assembly enhances acoustic separation. This targeted restoration provides functional equivalency within the scope and practical constraints.