The objective of this investigation is to analyze the effects of point density on roof shaping. This research focuses on the relationship among point density of lidar, number of noise, complexity of roofs, and accuracy of generated roofs. We simulate the point clouds of roof in different densities and different shapes. Variant noise levels inside the roof are also considered in the simulation. We include the common roof shapes like flat, slat, gable, cylinder and sphere roof. The generated points are used to define a new shape using the least squares fitting and data snooping techniques. Then, the accuracy of generated roof can be determined by comparing the original and the rebuilt ones. Experimental results indicate that the accuracy is improving when the point density is increasing. In addition, the flat and non-flat roof may reach the accuracy of simulated point cloud when percentages of noise are 35 % and 15 %, respectively.