Efficient use of attic area or mansard is determined by proper usage of the slope angle of roof planes. The paper deals with the determination of values of geometric parameters for optimal design of mansard in the buildings with gable roof during both building renovation and planning a new design. The optimization analysis regarding the useful floor area or available mansard volume is performed with respect to the angle of the slope of roof planes. Obtained nomograms will allow architects and customers make the final decision on building’s roof concept at the early design stage based on both economic considerations and architectonic impressions.