Appeal 33426

ORSC R302.3, ORSC AK102/103, ADU Code Guide D.2

FIRE- AND SOUND SEPARATION OF MAIN HOUSE AND ATTACHED ADU: R302.3 requires dwelling units in two-family dwellings to be separated by wall and floor-ceiling assemblies listed and tested for minimum 1-hour fire resistance and for 45 STC/IIC sound transmission (AK102/103). EXCEPT, for an attached ADU only, existing floor-ceiling and/or wall assemblies with minimum ½” gypsum board in sound condition may remain without upgrading fire or sound separations (ADU Code Guide D.2).

Allow a ground floor primary dwelling to be 1-hour fire-separated and 45 STC/IIC sound-separated from a second floor ADU above it by enclosing an existing interior stair opening connecting the two floors with alternate, non-listed wall and floor-ceiling assemblies.

The upper floor ADU and the lower floor primary dwelling unit are separated by a single stairway (130” long, 32” wide) that climbs along the north-most wall of the building. It is proposed that the stairway opening be closed with an assembly that extends the upper stairway landing out until it intersects with the lower unit’s ceiling. This small extension would be secured into the north wall and into the upper unit’s east stairway wall (which is located above the stairway opening). The southwest corner of the extension would be supported by a wooden leg standing on a stair step.

The stair opening (30” long, 32” wide) connecting the floors (23% of the floor-ceiling area) requires enclosure with 1-hour/45 STC floor-ceiling and wall assembly. The assembly can be described in 3 parts: A) the floor extension, B) the wall extension, and C) the support.

Part A would consist of a horizontal hollow wooden frame made of 2x6’s, filled with 3” fiberglass insulation, with a 5/8" thick Type X gypsum wallboard secured to the underside of the frame, and then a layer of sound matting (QuietSound: 5mm, 50 IIC rated) on top of the frame, and a plywood sheet secured to the frame on top of the mat. This would become the floor of the ADU.

Part B would consist of a vertical hollow wooden frame made of 2x4’s with 2 sheets of 5/8" thick Type X gypsum wallboard secured to the west-facing side of the frame (facing away from the stairway opening; facing in toward the ADU stairway landing). This would become the wall of the ADU.

Part C would consist of a 2x6 wood leg standing on a stairway step beneath the assembly, screwed into the bottom of the assembly to support the northwest corner of the assembly. This wood leg would be wrapped in (2) layers of 5/8” Type X gypsum board.

It is proposed that a sound rated mat be put on top of the floor assembly in lieu of the resilient channels on the underside of the floor joists. The proposed alternative is more spatially and economically practical than the prescriptive floor-ceiling assembly under the ADU Code Guide.

ORSC R302.3.1, ADU Code Guide D.2

FIRE-RATED SUPPORTING CONSTRUCTION FOR 1-HOUR-RATED FLOOR-CEILING ASSEMBLY SEPARATING MAIN HOUSE FROM ADU: R302.3.1 requires that where a floor-ceiling assembly separating two dwelling units must be 1-hour fire-rated per R302.3, then all Supporting Construction for that floor-ceiling assembly must likewise be minimum 1-hour fire-rated.

Allow the use of alternate, non-listed methods to achieve a 1-hour fire resistance rating for all Supporting Construction for a new 1-hour floor-ceiling assembly separating a primary house from an ADU above it.

The upper floor ADU and the lower floor primary dwelling unit are separated by a single stairway (130” long, 32” wide) that climbs along the north-most wall of the building. It is proposed that the stairway opening be closed with an assembly that extends the upper stairway landing out until it intersects with the lower unit’s ceiling. This small extension would be secured into the north wall and into the upper unit’s east stairway wall (which is located above the stairway opening). The southwest corner of the extension would be supported by a wooden leg standing on a stair step.

The stair opening (30” long, 32” wide) connecting the floors (23% of the floor-ceiling area) requires enclosure with 1-hour/45 STC floor-ceiling and wall assembly. The assembly can be described in 3 parts: A) the floor extension, B) the wall extension, and C) the support.

Part A would consist of a horizontal hollow wooden frame made of 2x6’s, filled with 3” fiberglass insulation, with a 5/8" thick Type X gypsum wallboard secured to the underside of the frame, and then a layer of sound matting (QuietSound: 5mm, 50 IIC rated) on top of the frame, and a plywood sheet secured to the frame on top of the mat. This would become the floor of the ADU.

Part B would consist of a vertical hollow wooden frame made of 2x4’s with 2 sheets of 5/8" thick Type X gypsum wallboard secured to the west-facing side of the frame (facing away from the stairway opening; facing in toward the ADU stairway landing). This would become the wall of the ADU.

Part C would consist of a 2x6 wood leg standing on a stairway step beneath the assembly, screwed into the bottom of the assembly to support the northwest corner of the assembly. This wood leg would be wrapped in (2) layers of 5/8” Type X gypsum board.

The large majority of the existing floor-ceiling assembly and supporting construction will remain as-is: un-rated. Only the supporting construction for the new floor-ceiling assembly needs to be 1-hour rated. Each supporting Part will be wrapped in (2) layers of 5/8” Type X gypsum board, from the perspective of the primary dwelling unit. The stairs would remain in existing condition.