A wireless sensor network system, GPS on Every Roof is presented in this paper. This system is used for post-seismic building-wise damage detection. Sensor node equipped with GPS is installed on the top of the roof of each and every building. The position of this sensor node is measured before and after earthquake. This information is collected through wireless communication. Superposing this information on GIS, building-wise damage distribution due to earthquake can be obtained and possible road closure due to the debris from the collapsed buildings can be estimated. Since this system is used under emergency situation, toughness and robustness of the wireless communication should be given the highest priority. Also, enhancement of GPS positioning accuracy is essential. GPS relative positioning algorithm to achieve the accuracy in the order of several centimeters is presented in this paper. The system overview, hardware and some of the key components of the system are described in detail. Also, the results from a field experiment using a wireless sensor network with 39 sensor nodes are presented.