Australia's national energy system is heavily reliant on coal-fired power plants and requires quick action to decrease CO2 emissions in order to meet the Paris climate change agreement by 2030. As a good alternative to coal power, renewable resources can produce the required energy for Australia while keeping the environment clean. The annual amount of falling solar irradiation in Australia is a few thousand times higher than its energy consumption, but the amount of the collected solar energy is unsatisfactory. In Perth, the sunniest city in Australia, one-third of the residential dwellings have roof-mounted solar collector systems. However, there have been limited studies into the integration of these systems in building design and it is still unknown which roof design can deliver maximum efficiency based on the local environmental conditions. Therefore, the aims of this study are to determine the optimum residential roof design in Perth, Western Australia and to maximise extracted solar power throughout a year. To reach this aim, a combination of analytical modelling, numerical simulation, and evolutionary algorithm methods have been selected. At first, a series of detailed mathematical calculations were performed in Excel to find the optimum solar collector mounting settings. In the numerical simulation, the examination was repeated in the Rhino-Grasshopper interface to verify the earlier findings and calibrate the chosen simulation package with the help of parametric architecture. In the last step, a wide range of housing roof shapes has been tested by evolutionary algorithm plugin-Galapagos, to find the optimum roof shape design in the given context. Our results show that, unlike the traditional approach and common belief, a roof shape with a 25-degree tilt angle, 170-175 azimuth angle, and aspect ratio of 1:1.2 is optimum to gain maximum solar gaining annually. Moreover, this study advances the knowledge of solar capturing by suggesting a new parameter -oblique angle- in finding the optimum roof shape design. After evaluating the importance of the mentioned factors, it is found that, after tilt angle, the oblique angle is the most effective parameter in rooftop solar gaining, higher than roof aspect ratio and azimuth angle. In conclusion, our results demonstrate that an ‘oblique shed roof design’ can generate 15.7% higher solar radiation compared to the base case. The findings of this study can be applied by architects, construction companies, and householders to optimize the solar gaining in the future of the residential building in Perth.