Intention Resisting fire spread over external walls

The external envelope of a building should not contribute to undue fire spread from one part of a building to another part. This intention can be met by constructing external walls so that both of the following are satisfied. a. The risk of ignition by an external source to the outside surface of the building and spread of fire over the outside surface is restricted. b. The materials used to construct external walls, and attachments to them, and how they are assembled do not contribute to the rate of fire spread up the outside of the building. The extent to which this is necessary depends on the height and use of the building