|   at least equivalent to a door
    Within 20 feet of: any timbered areas   |   constructed of no less than
     or combustible rock, provided that the |   one-quarter inch of plate
     timber and combustible rock within the |   steel with channel or
     20 foot distance are coated with one   |   angle-iron reinforcement to
     inch of shotcrete, one-half inch of    |   minimize warpage. The
     gunite, or other material with         |   framework assembly of the door
     equivalent fire protection             |   and the surrounding bulkhead,
     characteristics and no other           |   if any, shall be at least
     combustible material1                  |   equivalent to the door in fire
     within that distance                   |   and air-leakage resistance,
                                            |   and in physical strength.
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    Within 20 feet of: timbered areas,      |  Control door that serves as a
    exposed combustible rock, or other      |   barrier to fire, the effects
    combustible material1                   |   of fire, and air-leakage. The
                                            |   door shall provide protection
                                            |   at least equivalent to a door
                                            |   constructed of two layers of
                                            |   wood, each a minimum of
                                            |   three-quarters of an inch in
                                            |   thickness. The wood grain of
                                            |   one layer shall be
                                            |   perpendicular to the wood
                                            |   grain of the other layer.
                                            |   The wood construction shall
                                            |   be covered on all sides and
                                            |   edges with no less than
                                            |   twenty-four gauge sheet steel.
                                            |   The framework assembly of the
                                            |   door and the surrounding
                                            |   bulkhead, if any, shall be at
                                            |   least equivalent to the door
                                            |   in fire and air-leakage
                                            |   resistance, and in physical
                                            |   strength. Roll-down steel
                                            |   doors with a fire-resistance
                                            |   rating of 1 1/2 hours or
                                            |   greater, but without an
                                            |   insulation core, are
                                            |   acceptable if an automatic
                                            |   sprinkler or deluge system is
                                            |   installed that provides even
                                            |   coverage of the door on both
                                            |   sides.
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1 In this table, "combustible material" does not refer to installed