7 CLEAR LAYER
7.1 Industrial Buildings excluding High Racked Warehouses
7.1.1 The minimum clear layer should be 3.0m. This is
to permit the escape of personnel and the entry
of fire fighters. Note, however, that the
maximum clear height which car be physically
achieved is 80-90o of the building's height,
due to the presence of the ceiling jet in the
smoke layer (see Appendix 2 Reference 7).
7.1.2 Where the chances of spontaneous ignition and
flashover present a considered risk, and the
maximum clear height available permits it, the
clear layer should extend at least SOOmm above
the stacked goods.
7.1.3 To avoid disruption of the smoke layer in any
single compartment, the base of the smoke layer
should be at least 1-2m above any door/opening
which might either by design or chance be a
source for replacement air, unless the incoming
air velocity is less than lm/s, in which
instance it should be no less than SOOmm above
the opening.
7.1.4 Where the floor level changes within a
compartment, the height of rise should be
measured from the shallowest end for safety
purposes and the overall height from the
deepest end for the entrainment calculation.
7.1.5 Special considerations are necessary when
designing for-mezzanine floors. Note the
comments contained within the Design Parameters
for Mezzanine Floors.
7.2 High Racked Storage Warehouses
7.2.1 In sprinklered warehouses, the system
objectives must be clearly defined for a design
to be produced. These objectives may be to.
(a) Protect personnel within the building, by
ensuring that their escape routes remain
unaffected by smoke;
(b) Protect stock and materials which may be
combustible or salvageable;
(c) Facilitate fire-fighting operations.
7.2.2 For objective (a) to be achieved the design
approach can be either 7.2.2.1 and 7.2.2.3 or
7.2.2.2 and 7.2.2.3.
7.2.2.1 (1) Ensure that the smoke layer is is
established well above head height
ie >lOm above the floor or highest
exposed walkway (other than those used
for infrequent maintenance purposes).
(ii) Provide no reservoir compartment subdivisions but ensure that the additional
heat losses from the smoke layer due to
the extended layer area are accounted
for, using a calculation technique which
takes into account convective and
radiative heat transfer from the layer
to the buildings fabric and atmosphere.
The final smoke layer temperature should
not be less than 20oC above the average
roof level ambient temperature.
The relaxation on compartmentation is
based on the assumption that escaping
personnel will be able to travel hrough,
and out of, the building without being
detrimentally affected (psychologically)
by the presence of the smoke layer above
their heads, by virtue of its height
above the floor and the nature of their
escape routine, (ie regimented and
practiced escape drill).
(iii) Ensure that an adequate fail-safe low
level inlet air supply is available at
all times, except where reservoir subdivision are used for this purpose (see
clause 12). Note. It should be
remembered that low level doors/windows
may be secured and low level inlets may
be partially blocked by stored materials.
7.2.2.2 (i) Where it is desirable for the smoke
layer to descend below lOm above the
floor, then the minimum height of rise
should be 3m, and screen separation of
the roof space into reservoirs not
exceeding 60m in the direction of escape
is required. Note, if the plan area of
the reservoirs is greater than 2000m²,
consideration should be given to the
heat losses sustained by the layer.
(ii) In exceptional circumstances it may be
impractical to install such screens. In
these instances alternative smoke ontrol
solutions may be considered, and etailed
calculations shall be provided in
support of any alternative strategy.

7.2.2.3 All life safety systems should operate on
smoke detection and where possible sprinkler
flow switch.
7.2.3 For objective (b) to be achieved the design
approach can be.
(i) Ensure that the design smoke layer is
established above the salvageable stock
levels (except in the instance where this
will result in a smoke layer depth less
than 1/lOth the building height,
see 7.2.5 below).
(ii) Provide screen separation of the roof
space into reservoirs not exceeding
3000m². Where for practical purposes it
is desirable to exceed this area, the
heat losses from the layer should be
taken into account (see 7.2.2.1 (ii)
above). The final design smoke layer
temperature should not be less than 20C
above the average roof level temperature.
(iii) The inlet air supply may be provided from
adjacent, unused reservoirs, or dedicated
low-level openings.
(iv) The venting system should preferably be
operated by a smoke detection system, but
as a minimum should operate on sprinkler
flow switch for in-rack sprinklers, smoke
detectors for roof mounted sprinklers or
on both.
7.2.4 For objective (c) to be achieved the design
approach can be as per objective (a) (see 7.2.2
above), with the exception that smoke reservoir
screens will not be required except to provide
inlet air, and the system may be operated by
sprinkler flow switch and manually by firefighters.
7.2.5 The minimum layer depth used in a design cannot
be less than l0% of the building (room) height,
due to the presence of the ceiling jet in the
smoke layer (see Appendix 2 Reference 7).

7.2.6 Where the floor level changes within a
compartment, the height of rise should be
measured from the shallowest end for safety
purposes and the overall height from the
deepest end for the calculation.
7.2.7 In unsprinklered warehouses, the current fire
experience has been the total involvement, and
subsequent loss, of the building. The speed of
fire growth in all but the least combustible of
materials (eg. steel) is such that venting is
ineffective for practical considerations of
stock protection and fire-fighting operations.
See 9.2 below.