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The general aim of this thesis is to develop an efficient and accurate model to simulate upward flame spread in unventilated and ventilated cladding systems. The validated model is then used to assess the current Euroclass system of selecting external materials and proposes guidelines on how to improve external material selection while still within the simplistic framework of regulations. The model consists of the following main sub-models: a one-dimensional combined heat and mass transfer model to compute gaseous fuel production due to decomposition, and a simplified model to calculate surface heat flux and transient flame growth. To demonstrate the capability of the new model, vertical burning and upward flame spread on 2.4m high panels of wood and 5.0 m high panels of PMMA are carried out and compared with experimental results and FDS simulations. The new model calculation results, including propagation of the pyrolysis front, total heat flux, and heat release rate are in good agreement with experimental and FDS's results. However, the new model requires a fraction (a few minutes of CPU time) of that for carrying out the corresponding FDS simulation (CPU time of 2 days). The validated fire spread model was used to quantify the realistic ranges of combustible materials represented by the same SBI classification. The material properties were obtained by inverse analysis so that their combinations would exactly achieve the relevant thresholds for Class B and Class C materials according to SBI test. This thesis assessed fire spread behaviour of these materials in Full-scale test (BS 8414) when used as cladding, either on its own, or in combination with another material, both with and without cavity ventilation. The majority, but not all Class B materials would be able to pass the BS 8414 full-scale fire test criterion for flame height. The Class B materials that do not pass the acceptance criterion are combinations of high density (>120kg/m3) and high heat of combustion (>16 MJ/kg). Many Class C materials could still pass the BS 8414 full-scale fire test criterion for flame height. Such materials are characterized by low density (< 60kg/m³) and low heat of combustion (< 16 MJ/kg). The SBI-based material specification for multi-layered façade is severely restrictive if the insulation material is encapsulated by a non-combustible external surface. Instead of achieving Class A2, even Class C insulation materials can be used provided the thickness of the external layer is not lower than a critical thickness. This critical thickness is no more than a few mms and can be easily achieved in practice.