This report describes methods of predicting heat flux exposure to external walls due to the impingement of flame issuing through a window opening. A heat transfer model was set up for the purpose of predicting the geometry of the emerging flame and the resultant heat flux exposure to the wall surface. An existing flame spread model implemented in the BRANZFIRE model was selected for characterising the flame spread on exterior wall cladding materials, as a function of the heat flux exposure (from the projecting flame to the wall) and the material flammability properties of the wall material. Modifications were made to the flame spread model. The result was a prediction of rate and extent of the upward flame spread as a function of time and the heat release rate of the burning cladding material. It is concluded that the flame spread model has the potential to determine the flame spread characteristics associated with four different cladding materials. The flame spread model gave conservative prediction for three of the tested cladding materials. Overall, the heat transfer model seems to predict the total heat flux density received by the exposing wall with reasonable accuracy. Further validation of the heat transfer model is needed before it can be successfully integrated into the flame spread model to provide a useful tool for characterising flame spread and estimating the heat flux exposure conditions.