Computer-Aided Design models have generated new possibilities in the sustainable design of buildings. Computer models assisting different aspects of architectural design have been developed and used for several decades. A review of contributions of computing to architectural design is given by Gero. Most of the conventional simulation computer programs do not actively support design development and optimization, specially at the formative design stages. It is well established that most decisions that affect comfort and building energy use occur during the formative design stages of the project. Furthermore, the efforts required to implement those decisions at the beginning of the design process are small compared to the effort that would be necessary later on in the design process. Therefore, if sustainable design issues are going to receive an appropriate level of consideration at the beginning of the design process, they must be presented in a way which is useful to the architect and fits with other things the architect is considering at that time. Design is seen as a problem-solving process of searching through a space of design solutions. The process of finding a solution to a design problem involves, identifying one or more objectives, making design decisions based on the objectives, predicting and evaluating the performance to find the acceptable decisions. Each of these activities can be performed inside or outside the formal model. In designing a roof, an architect or building designer has to make many decisions on the materials. The arrangement of these materials determines the aesthetic appearance of the roof and the house. Other considerations that affect the choice of roofing materials are thermal performance, rain, fire protection, cost, availability and maintenance. Recyclability of materials, hazardous materials, life-cycle expectancy, solutions, and design options as they relate to the environment also need to be considered. Consequently, the design of roof has become quite a complex and multifaceted problem. The principal need is for a direct design aid which can generate feasible solutions and tradeoff performance in conflicting requirements and prescribe the optimum solution. This paper presents a conceptual Computer Aided Design model for dwelling roof. It is based on generation and optimization paradigms of Computer Aided Design, which is diametrically opposite to conventional simulation. The design of roof (design goal) can be defined in terms of design objective as "control radiant and conduction heat." This objective must be satisfied to achieve the design goal. The performance variables, such as roof ceiling surface temperature or new thermal performance index (TPI*) must acquire values within certain ranges which will satisfy the objective. Given the required inputs, this computer model automatically generates prescriptive quantitative information to design roof to achieve optimum thermal comfort in warm humid tropics. The model first generates feasible solutions based on the decision rules, next it evaluates the thermal performance of the roof taking into account design variables related to the building's roof and finally it applies numerical optimization techniques to automatically determine the optimum design variables, which achieve the best thermal performance. The rational and methodology used to develop the proposed model is outlined and the implementation of model is described with examples for climatic and technological contexts of India and Australia.