NCC 2022 Volume One - Building Code of Australia Class 2 to 9 buildings
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9

Specification 9 Cavity barriers for fire-protected timber

Specification 9 Cavity barriers for fire-protected timber

This Specification sets out requirements for cavity barriers in fire-protected timber construction.


Specification 9 sets out requirements for cavity barriers used for fire-protected timber. A cavity barrier is a barrier/enclosure provided to cavities within, around or adjacent to fire-protected timber. The purpose of a cavity barrier is to limit the spread of fire, smoke and hot gases to other parts of a building in the event of a fire. A cavity barrier is used under C2D13 to permit fire-protected timber to be used in lieu of non-combustible material.

The requirements for cavity barriers in Specification 9 include but are not limited to:

  • the location of cavity barriers, i.e. junctions between fire-resisting floor/ceiling assemblies and fire-resisting walls;
  • minimum thickness of timber used as a barrier;
  • minimum thickness of insulation; and
  • the method to determine the FRL of the cavity barrier. 

(1) Cavity barriers must be provided in the following locations where fire-protected timber is used in any of the listed elements:

  1. At concealed cavities adjacent to junctions between fire-resisting floor/ceiling assemblies and fire-resisting walls.
  2. At concealed cavities adjacent to junctions between fire-resisting floor/ceiling assemblies and fire-resisting or non-combustible external walls.
  3. At concealed cavities adjacent to junctions between fire-resisting walls and fire-resisting or non-combustible external walls.
  4. Around the perimeter of door and window openings in fire-resisting construction.

(2) Cavity barriers must be installed so they are tight fitting and are able to withstand thermal expansion and structural movement without the loss of seal against fire and smoke.

(3) In addition to cavity barriers required by (1), horizontal and vertical cavity barriers are to be provided to wall cavities within, around or adjacent to fire-protected timber elements as follows:

  1. Horizontal cavity barriers — at not more than 5 m centres.
  2. Vertical cavity barriers — at not more than 10 m centres.

(4) Cavity barriers must—

  1. achieve the performance specified in Table S9C2 based on the highest FRL of the elements they are mounted within or seal against; or
  2. consist of—
    1. timber with the minimum thickness specified in Table S9C2; or
    2. polythene-sleeved mineral wool or mineral wool slabs or strips placed under compression to achieve the minimum thickness specified in Table S9C2.

(5) Cavity barriers provided around openings may be formed by the window or door frame if—

  1. the frame is constructed of steel or timber with the minimum thickness specified in Table S9C2 for timber; and
  2. the frame is tightly fitted to rigid construction and mechanically fixed in position.

(6) The FRL of cavity barriers in fire-protected timber construction must be determined in accordance with Specifications 1 and 2 applying the criteria for control joint systems specified in Section 10 of AS 1530.4 with the cavity barrier system fitted within an opening between timber members exposed directly to the furnace heating conditions.

(7) Notwithstanding anything to the contrary in Specifications 1 and 2 or AS 1530.4, the test results from (6) may be used when the fire-protected timber is constructed from timber having a nominal density at least equal to the tested timber.

Table S9C2 Cavity barrier requirements
System required FRL –/60/60 or –/90/90 –/120/120, –/180/180 or –/240/240
Cavity barrier required FRL –/45/45 –/60/60
Timber, required minimum thickness 45 mm 60 mm
Mineral wool, required minimum thickness 45 mm 60 mm
Table Notes

Minimum thicknesses are to be measured in the direction of heat flow.