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A leak in a membrane on top of a horizontal roof deck is located by applying conductive wires on the membrane underneath the aggregate in a grid pattern. A measuring and switching circuit generates voltage having a positive attached to the roof deck and a negative attached to the wires. The circuit

A leak in a membrane on top of a horizontal roof deck is located by applying conductive wires on the membrane underneath the aggregate in a grid pattern. A measuring and switching circuit generates voltage having a positive attached to the roof deck and a negative attached to the wires. The circuit has a relay for each wire which can be switched between a current sensor system and the negative potential. The sensor system is arranged to sense at each of the wires in turn the current flowing from the roof deck through any leak in the membrane to the wire. A micro-processor operates the relays in turn to connect all the other wires to the negative as a shield while each wire is sensed. From the output of the grid the changes in current in the x and y directions are analyzed to locate the leak in the membrane.

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1. A method of locating a defect in a water impermeable membrane of a roof, where the roof comprises a generally horizontal roof support deck with the water impermeable membrane applied onto an upper surface of the support deck, the method comprising: locating a conductive detector on an upper surfa

1. A method of locating a defect in a water impermeable membrane of a roof, where the roof comprises a generally horizontal roof support deck with the water impermeable membrane applied onto an upper surface of the support deck, the method comprising: locating a conductive detector on an upper surface of the roof membrane;defining a shielding zone on the upper surface of the membrane by providing at least one shielding conductor on the upper surface and by generating an electrical potential difference between said at least one shielding conductor and the roof deck such that currents generated outside the zone are drawn to said at least one shielding conductor;generating an electrical potential difference between the conductive detector and the roof deck such that, in the presence of a defect located within the zone, current flows between the roof deck and the conductive detector through moisture at the defect;sensing the current between the roof deck and the conductive detector so as to detect any defect located within the zone;and arranging the electrical potential difference between said at least one shielding conductor and the roof deck so as to be greater than the electrical potential difference between the conductive detector and the roof deck. 2. The method according to claim 1 including applying bare wires to the upper surface of the membrane to operate as said at least one shielding conductor and said conductive detector. 3. The method according to claim 1 including covering said membrane with said at least one shielding conductor and said conductive detector thereon by an aggregate. 4. The method according to claim 1 including covering said at least one shielding conductor and said conductive detector with insulating sleeves to allow another one of the conductive detectors to pass over the sleeve at right angles thereto. 5. The method according to claim 1 including arranging a plurality of conductive detectors on the upper surface of the roof membrane in a grid pattern providing indications of distance in x and y direction across the membrane with each conductive detector being electrically insulated from each other; selecting each one of the conductive elements in turn and analyzing the sensed currents in the x and y directions to locate the defect in the membrane. 6. The method according to claim 5 including, when each one of the conductive elements is selected in turn, connecting at least some of the other conductive elements so as to form shielding conductors. 7. The method according to claim 1 including causing said at least one shielding conductor to receive current generated by un-insulated roof fixtures. 8. The method according to claim 1 including providing as said at least one shielding conductor a plurality of shielding conductors arranged at spaced positions relative to the conductive detector. 9. The method according to claim 1 wherein the electrical potential difference between said at least one shielding conductor and the roof deck and the electrical potential difference between the conductive detector and the roof deck are generated by a common voltage source and the difference therebetween is generated by an additional voltage source connected to said at least one shielding conductor. 10. The method according to claim 1 wherein the electrical potential difference between said at least one shielding conductor and the roof deck and the electrical potential difference between the conductive detector and the roof deck are generated by a common voltage source and the difference therebetween is generated by providing an electronic component which generates a voltage drop connected to said at least one shielding conductor. 11. The method according to claim 1 wherein the electrical potential difference between said at least one shielding conductor and the roof deck and the electrical potential difference between the conductive detector and the roof deck are generated by a common voltage source and the difference therebetween is generated by providing a zener diode which generates a voltage drop connected to said at least one shielding conductor.