Nowadays, FDS (Fire Dynamics Simulator) is a tool for researchers and engineers to study the behavior of fire, and a help to solving problems. Moreover, it is often used in performance-based design in order to prove safety in innovative design. Although, FDS has solved many problems in engineering approach, some questions about validation have remained without answer. One of the main questions for fire safety engineers and researchers are what details of geometry are essential to model, and how these will influence the results if some of the details are ignored in the modeling.