If Deemed-to-Satisfy (DTS) Provision D1.4 can be satisfied by providing one exit, is that enough?
Although in many circumstances it is, DTS Provision D1.2 may require an additional exit and DTS Provision D1.5 will specify further requirements for that exit.
What does the NCC require?
Performance Requirement DP4 in NCC Volume One requires the provision of exits to facilitate safe evacuation. D1.4 specifies the maximum travel distance to an exit and, in doing so, indirectly specifies a minimum number of exits. For instance, a storey with a large floor space will require multiple exits to satisfy D1.4. Whereas a storey with a small floor space could potentially satisfy D1.4 with only one exit.
D1.2 contains specific requirements for the number of exits provided to a building or storey. In particular instances, where there are difficult evacuation conditions, not less than two exits are required, even if D1.4 is satisfied by only one exit.
Further, DTS Provision D1.5 Distance between alternative exits, concurrently applies. D1.5 requires multiple exits to be distributed as uniformly as practicable and not less than 9 m apart, where multiple exits are required, such as by D1.2.
The interaction of these provisions is evident in the following scenario.
How many exits are required in this building?
A single storey Class 9b school building with two classrooms and a total of 52 occupants.
In Configuration A below, D1.4 is satisfied by the provision of one exit on account of all parts of the storey being within 20 m of that exit (D1.4(c)(i)). However, D1.2(d)(vi) requires any storey of a Class 9 building that accommodates more than 50 people to be provided with at least two exits. Therefore, Configuration A does not comply for this scenario.
In Configuration B a second exit is provided to satisfy D1.2(d)(vi). A door is provided between the two classrooms because D1.2(g)(ii) requires all occupants of a storey to have access to at least two exits if two or more exits are required. Further, on account of D1.5, the two required exits are at least 9 m apart and distributed evenly around the storey. Configuration B does comply for this scenario.
Want to know more?
Further information can be found in the Guide to NCC Volume One, available via download in the NCC suite.