A leak in the membrane of a generally horizontal roof support deck includes dividing the membrane into separate zones and locating a sensor to detect moisture underneath the membrane. A movable scanning device carried by a worker has a power supply antenna for supplying power by an electromagnetic wave to the sensor and the worker moves the scanning device to selected ones of the zones. At each selected zone the power supply antenna activates the sensor in the zone and the sensor to emits signals related to the moisture underneath the membrane as detected by the sensor device which are detected and recorded at the mobile scanning device.