The issue of Coverage in visual sensor networks (VSNs) has attracted considerable attention due to sensors unique directional sensing characteristic. It answers the question that how well the target field is monitored by a network of sensors with video/image capturing capability. In floorplan scenario the network is to monitor a plane parallel to the sensors' deployment plane. Coverage probability estimation based on both the sensors and the network related parameters is a fundamental issue in this field. For a large scale application in which the sensors' deployment is done by dropping sensors from an airplane, random sensors' placement and orientation according to their respective distribution is a practical assumption. Although some studies exist on the coverage problem of floorplan VSNs, none of them has derived a closed form solution for the coverage estimation, which is the main contribution of this paper. The Simulation results validated the proposed mathematical solution.