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In this study the data for the ignition frequency of structural fires was derived from statistics for different building categories in Finland. Following a model obtained from literature, the power law function of the total floor area was observed to be at variance with the data, but working heuristically, it was found that a sum of two power functions led to a fairly good fit with the available statistical data. Since theoretically power law dependence is a special case, a generalized theoretical model was proposed, starting from the initial distributions of the buildings involved in fires and the total stock at risk. Good agreement with the observations was obtained using this ansatz. The general method shed new light on ignition frequency, and good theoretical explanation was achieved for the observed phenomena. It showed that ignition frequency varied with the floor area, and has maxima and minima depending on the form of these size distributions. Estimation of systematic errors yielded values of partial safety constants needed for the calculation of the ignition frequency for design purposes. The model is useful for determining the ignition frequency of buildings with a floor area of between 100 and 20 000 m2.