The roof angle of a building can influence its energy consumption. To evaluate this influence, we have used a computer program, BRE-ADMIT, based on the admittance method. An additional program was written in order to analyse sloping structures. The new software transforms the data on buildings with one or two-pitched roofs to that of a building of cubical shape such that it can be analyzed by BRE-ADMIT. From this cubical-shaped building, the thermal behavior of the our pitched-roof building could be obtained by running BRE-ADMIT. To verify the validity of our mathematical model, experiments were performed on one-pitch roof buildings. For a roof sloped north with an optimal slope angle, we have found that one can save up to 20% of the cooling energy. For a one-slope building facing south, when the roof has an optimal slope angle, 11% of the heating energy is saved but for a building with a two-pitch roof, energy consumption during winter does not depend on the angle of the slope. This new program can give useful information to building designers on roof slope angle in order to obtain the lowest cooling and heating energy consumption.