An escape route from a first story office in a commercial building involves traversing a 3.0 metre width deck that is fire rated on the underside, then descending a set of stairs to the ground. Do the underside of the stairs need to be fire rated? A safe path is needed I get that but C/AS2 suggests that if the stairs are more than 2.0 metres away from the external wall (which is fire rated at ground storey below the deck but not first storey above the deck) they do not need to be . Confused as usual. Tim.
Guidance can be taken from C/AS2. The requirements for external stairs are covered in C/AS2 3.11.4 c) ii). These requirements essentially limit the critical radiant flux or the group number on the u/s of the stairs. The only fire rating requirement for stairs comes from C/AS2 4.13.3 which relates to stairs to intermediate floors. This should also be applied to open path stairs within a firecell. These requirements are for stairs inside firecells.
The stairs in question are external stairs and are not within a firecell. I do not believe fire rating to the stair would be required for compliance with C/AS2. You may elect to consider the likelihood on a fire load occurring under the stair and what the consequences of a conflagration would be.