ABSTRACT A similitude approach was used to develop predictive graphs for the ventilation rate due to the stack or chimney effect. Using a half scale model of an open side wall structure with a continuous and unrestricted open ridge, it was found that (a) ventilation rate was approx-imately proportional to ridge outlet width; (b) outlet Reynolds number response, i.e., ventilation rate, to changes in Grashof number was a function of the ratio between building height and ridge width.