External cladding systems offer a potential route for fire spread through multi-storey buildings. This third edition of BR 135 sets out the principles and design methodologies related to the fire spread performance characteristics of non-loadbearing cladding systems, and provides revised guidance in the light of the growing market for cladding systems and increasing thermal performance requirements. It also updates the guidance on fire performance of currently used materials and technologies. The illustrations and scenarios are based on typical examples of current practice, but, in view of the rapid changes in designs and materials, the guidance aims to enable designers to understand the parameters relating to fire safe design and construction of external cladding. The guide also consolidates the fire performance classification systems for full-scale fire tests in BS 8414.