The reduction of energy demand for space cooling requires adequate solutions at building and built environment scales in order to achieve sustainability goals. Since many existing buildings have inadequate envelopes for reducing heat gains in summer and heat losses in winter, environmentally friendly renovation techniques have to be considered. The roofs of existing buildings were identified as a field of intervention which could contribute to providing both energy savings and environmental benefits. The aim of the paper was to evaluate the contribution of green roofs to energy savings for space cooling depending on the building shape factor. Two groups of building models, with vertical and horizontal expansions, were analysed. The comparative analyses of the building models with conventional and green roofs showed small reduction of energy consumption-less than 1% in case of well-insulated roofs with the addition of the extensive green roof. The comparison of different building models with green roofs of the same volume, which is in this case the same cooling area, indicated a more effective solution for green roofs implementation with the aim to improve energy performances of existing buildings using this passive design technique.