Fire spread in high rise buildings from floor to floor occurs if flames emerge and extend on the façade of the building to cause ignition in the floor above the floor where flashover has developed. Even though considerable effort has been exerted to address this issue, proposed relations for heat fluxes on façade are incomplete and contradictory because the relevant physics have been poorly clarified. By systematically performing a series small scale experiments having various enclosure geometries, door-like openings and fire locations, the physics and new relations are underpinned for the emerging flames on inert facades in ventilation controlled (under-ventilated) fires at the floor of fire origin. To limit the variables and uncertainties, propane and methane gas burners create a controlled (theoretical) heat release rate at the source. Gas temperatures inside the enclosure and at the opening, heat fluxes on the façade wall, flame contours (by a Charge Coupled Device camera-CCD camera) and heat release rates (by oxygen calorimetry) inside and outside the enclosure have been measured. The gas temperatures inside the enclosure were uniform for aspect ratio (length to width) of the enclosure from one to one to three to one. Previous relations for the air inflow and heat release rate inside the enclosure were verified. The flames are highly radiative because soot can be formed at high temperatures inside the enclosure before the combustion gases and the unburned fuel exit the enclosure. The heat fluxes on an inert façade, both at the centreline and off-center above the opening, have been well correlated by identifying two length scales. One related to the effective area of the outflow ( 1 ), and the other represented the length after which the flames turn from horizontal to vertical ( 3 ). Finally, Correlation of the maximum flame width which is valid for the case having aspect ratio of the opening (width to height) between 0.375 and 2 was proposed in this paper. The results can be used for engineering calculations for real fires and for validation of new large eddy scale simulation models. KEYWORDS