Provides a test method for assessing the behaviour of non-loadbearing external cladding systems, such as curtain walling, glazed elements, infill panels and insulated composite panels, fixed to and supported by a structural steel frame when exposed to a fully-developed (post-flashover) fire in a room, venting through an opening such as a window aperture that exposes the cladding to the effects of external flames, or from an external fire source.