A leak in an electrically conductive membrane attached on top of a horizontal roof deck is located by a manually operable carriage which is swept over the upper surface of the membrane on the roof deck. A measuring and switching circuit generates a voltage having a positive attached to the roof deck and a negative attached to conductive brushes on the carriage. The brushes are arranged to form an outer peripheral contact with an inner contact inside the outer contact. There is provided a diode switched into the sensing circuit from the inner conductive brushes so as to reduce the voltage across the deck and the inner brushes relative to the outer brushes. Thus the presence of a leak is detected by the outer set of conductive brushes at distance from the leak and by the inner conductive brush when directly over the leakage site.