A method of detecting a leak in a roof where the roof comprises a generally horizontal roof support deck with a water impermeable membrane applied onto the upper surface of the support deck so as to define an upper surface of the membrane and a lower surface of the membrane, the method comprising: applying electrically conductive shielding elements on the upper surface of the roof membrane in an X and Y grid pattern so as to be in electrical communication with any moisture on the upper surface; the X and Y grid pattern of the electrically conductive shielding elements providing separation of the membrane into a plurality of zones each being bounded on two first sides by two of the electrically conductive shielding elements extending in the X direction and being bounded on two second sides by two of the electrically conductive shielding elements extending in the Y direction; generating an electrical potential difference between the electrically conductive shielding elements on the upper surface of the membrane and a conductive component at or adjacent the roof deck on the underside of the membrane; characterised by in each of the zones locating a respective one of a plurality of sensor conductors on the upper surface of the membrane; each of the sensor conductors being separate from the electrically conductive shielding elements; while the electrical potential difference is applied between the electrically conductive shielding elements and the conductive component, generating an electrical potential difference between the sensor conductor on the upper surface of the membrane and the conductive component on the underside of the membrane such that, in the presence of a leak located within the zone, current flows between the sensor conductor and the conductive component through moisture at the leak; and detecting said current flowing between the sensor conductor and the conductive component through moisture at the leak to determine the presence of the leak. applying electrically conductive shielding elements on the upper surface of the roof membrane in an X and Y grid pattern so as to be in electrical communication with any moisture on the upper surface; the X and Y grid pattern of the electrically conductive shielding elements providing separation of the membrane into a plurality of zones each being bounded on two first sides by two of the electrically conductive shielding elements extending in the X direction and being bounded on two second sides by two of the electrically conductive shielding elements extending in the Y direction; generating an electrical potential difference between the electrically conductive shielding elements on the upper surface of the membrane and a conductive component at or adjacent the roof deck on the underside of the membrane; characterised by in each of the zones locating a respective one of a plurality of sensor conductors on the upper surface of the membrane; each of the sensor conductors being separate from the electrically conductive shielding elements; while the electrical potential difference is applied between the electrically conductive shielding elements and the conductive component, generating an electrical potential difference between the sensor conductor on the upper surface of the membrane and the conductive component on the underside of the membrane such that, in the presence of a leak located within the zone, current flows between the sensor conductor and the conductive component through moisture at the leak; and detecting said current flowing between the sensor conductor and the conductive component through moisture at the leak to determine the presence of the leak.