Steel gable frames, or pitched roof portal frames, have been widely used for industrial building applications due to their ease of erection and fabrication. The roof slope is one of the most important geometric characteristics of gable frames. The roof slope affects wind loads, snow loads, rainwater drainage, and the optimal weight of the frame. The purpose of this study is to find the optimal roof slope of gable frames with tapered members for different spans. A novel meta-heuristic optimization algorithm called the Enhanced Crystal Structure Algorithm (ECryStAl) is utilized to optimize the steel frames. In this broad study, all practical load types, load combinations, and design constraints are considered for the structural optimization process. The results of this study show that up to 39.50% of the project material cost can be reduced by the proper choice of the roof slope depending on the span length and column height as the optimal roof slope depends mainly on them. In addition, mathematical models, which can be used in practical design situations, are developed to determine the optimal roof slope for different span lengths and column heights.