10 DEPTH OF SMOKE LAYER

10.1 Industrial Buildings Excluding High Racked Storage
Warehouses
In order to avoid the risk of "flashover", sufficient
ventilation should be provided to ensure the smoke
layer temperature does not exceed 600C for the design
conditions (see Appendix 2 Reference 2). However, the
smoke layer depth cannot be less than 10% of the
building (room) height. Where this occurs
consideration should be given to other (additional)
forms of protection, eg sprinklers.
10.2 High Racked Storage Warehouses
10.2.1 Since it is extremely unlikely for an
unsprinklered high racked storage area to
survive a fire, the following applies to areas
which are sprinklered, where flashover is not
likely to occur. However, flashover should
alvaays be considered, as this guide
recommends.
As the smoke layer base will be in many
instances, by design, at a high level within
the building, it is likely that cool smoke
temperatures will result, especially if
extended reservoirs are employed.
Smoke temperatures less than approximately 20
degrees centigrade above the average roof level
ambient temperature have resulted in the smoke
layer de-stratifying and mixing downward with
the ambient air beneath. This is mainly due to
weak cross-draughts and convection currents,
and will occur despite the provision of a smoke
extraction system. This scenario must be
considered by the designer.
10.2.3 The cooling effect of the operation of the
sprinklers must be considered, refer to
9.2 above.
The designer must state the degree'of cooling caused
by the sprinklers and extended layer travel which has
been allowed for in the calculations.