This paper describes the effect of outward opening the sash of a window on local and overall windpressures. Wind tunnel experiments were used for the purpose of evaluation. A centre-pivot roofwindow on a pitched roof in a modelled scaled building was used in the analysis of wind pressures.The wind pressures were defined in terms of wind pressure coefficients. Traditionally, wind pressurecoefficients are extracted from the analysis of a sealed plane surface. These wind pressurecoefficients are used to estimate the natural ventilation rate through windows/openings due to windeffect. Surface averaged wind pressure coefficients do not accurately estimate the airflow rates. Therefore, local wind pressure coefficients are needed, especially for dynamic calculation of airflow rates. From the wind tunnel experiments, it is concluded that outward opening the sash can significantly affect the wind pressure distribution near to the window. The use of wind pressurecoefficients from the analysis of a sealed plane surface may lead to erroneous estimation of airflow rate.