Flames emerging from an opening of a fully developed burning room are the main path of a fire spreading from floor to floor and to an adjacent building. Even though the issue of fire spread to an adjacent building has been incorporated in fire safety engineering design guidelines, performance-based design provides an alternative and accurate method to optimize the distance between the building facade with a burning enclosure and an adjacent building. This situation Was Simulated by performing a series of small scale experiments having a facade wall with flames and an opposite parallel wall representing an adjacent building in order to investigate the physics of the flame behaviour and measure the heat fluxes between two walls. The outcome of this research provides unique information for engineering design for separation distances between adjacent buildings. A new length scales, l(3), representing the length after which the flames turn from horizontal to vertical was developed in this research. The significance of this length scale has been verified by the experimental results for flame heights and heat fluxes and by observations. Similarity correlations for flame heights between the two parallel walls were developed in this research. Similarity correlations for radiative heat fluxes on facade walls were developed by dividing the data into two groups. One is for the distance between two walls being larger than length scale, the other is for the distance being less or equal to length scale l(3). Measurements of the heat fluxes on the opposite wall allow also the determination of the total radiant output. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.