(a)   Purpose.  The purpose of a twin-single dwelling is to combine two one-family dwellings in a single structure, separated by a party wall, which is constructed across a property line.
   (b)   Definitions.
      (1)   FIRE WALL:  A fire resistance rated wall, which restricts the spread of fire.  The wall shall have sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall.  The wall shall be continuous from foundation to two feet eight inches above the roof surface, except as provided in A. or B. below.  Firewalls shall be made smoke-tight at their junction with exterior walls.  In exterior wall construction employing studs, the firewall shall extend through the stud space to the exterior sheathing.
         A.   NONCOMBUSTIBLE ROOFS:  The wall is permitted to terminate at the underside of the roof deck where the roof is of noncombustible construction and is properly firestopped at the wall.
         B.   COMBUSTIBLE ROOFS:  The wall is permitted to terminate at the underside of the roof deck where all of the following conditions are met.
            1.   The wall is properly firestopped at the deck.
            2.   The roof sheathing or deck is constructed of approved noncombustible materials, or fire-retardant treated wood, for a distance of four feet on either side of the wall.
            3.   Combustible material does not extend through the wall.
            4.   The roof covering has a minimum of Class C. rating.
      (2)   PARTY WALL:  A masonry or concrete two-hour rated firewall on an interior lot line used or adapted for joint service between two buildings.  Each wall built as a part of a twin-single and placed on the dividing line between lots, and any wall replacing the same, shall be built as a party wall.  There shall be no windows, doors, openings or other penetrations in the party wall.
      (3)   TWIN-SINGLE DWELLING:  A structure with two single-family dwellings.  The one-family dwellings are separated by a party wall.  The structure is located so that each single-family dwelling is entirely on its own lot.  Only a single one-family dwelling may be erected or maintained on any lot.  Each one- family dwelling shall be attached to another one-family dwelling.  The one- family dwellings may have common roofs, siding or both.
   (c)   Detached Accessory Structures.  Detached accessory structures shall comply with Chapter 1274.
(Ord. 08-33.  Passed 5-4-09.)