It has been suggested that the Model Byelaws for the fire requirements for the external walls of multistorey flats are too restrictive. Model Byelaw 40 specifies that the fire resistance of these walls be one hour and Model Byelaw 47 specifies that either the vertical separation between windows be three feet or the horizontal projection between storeys be two feet. It would be an economy to use types of external walling which do not conform to these conditions in the construction of multistorey dwellings, and the Joint Fire Research Organization undertook to investigate the effect on the fire hazards of relaxing these standards for external non-load bearing walls. The two types of hazard considered were, the spread of fire from floor to floor and the exposure of one building to radiation from an adjacent burning building. Full-scale experiments were not possible and so work with models was carried out instead. The scaling laws which these obey as yet have only been partly investigated, but the experiments were useful in revealing the factors likely to affect the fire hazard in a full-scale room, though not necessarily their relative importance.