The placement of the sensors is really at the customers discretion. In a perfect world there would be a sensor everywhere water could be leaking but that may not be practical. The actual sensors should be placed slightly off the floor in most situations so that a damp floor or condensation does not cause a false alarm. The sensors should never be placed on concrete as the minerals in concrete have a conductive nature and will cause false alarms. The installer will have to use his best judgement in most cases to determine the best sensor and transmitter location.