The shape of a building can be recorded through various technologies, such as three-dimensional (3D) laser scanners and photogrammetry. The 3D measurement technology facilitates the mensuration of unevenness of a reinforced concrete structure, horizontality of the floor, and drainage gradient. In particular, measuring devices with a tilt sensor (e.g., 3D laser scanners) allow the evaluation of the horizontality of buildings using point cloud data. In this study, a method is introduced to evaluate the drainage gradient using 3D polygon data and a physics engine. The point cloud data were acquired via a 3D laser scanner and converted into polygon data. The polygon data were imported into a virtual space through the physics engine. Subsequently, the drainage capacity was evaluated by placing water particles on the polygon data and observing their movement according to the physics engine. The aim of this procedure is to evaluate the inclination angle of all parts of a building (e.g., rooftop) and develop a method for identifying the locations where puddles can frequently occur. Our simulation software was created using Unity and NVIDIA FleX. In this research, a simulation was conducted in which the polygon data were imported into the developed software, and the drainage situation was predicted.