The present work shows that weak blast waves that are considered as being harmless can turn to become fatal upon their reflections from walls and corners inside a penetrated building. In the experimental part, weak blast waves were generated by using an open-end shock tube. A three level building model was placed in vicinity to the open-end of the used shock tube. The evolved wave pattern inside the building rooms was recorded by a sequence of schlieren photographs; also pressure histories were recorded on the rooms' walls. In addition, numerical simulations of the evolved flow field inside the building was conducted. The good agreement obtained between numerical and experimental results allows running the used code for identifying safety and dangerous places inside the building rooms penetrated by the weak blast wave.