A method of locating a leak in a roof, where the roof comprises a generally horizontal roof support deck with a water impermeable membrane which is electrically conductive applied onto the upper surface of the support deck; the method comprising: providing a first conductor brush arrangement and a second conductor brush arrangement for engaging the roof above the membrane; providing a return conductor arrangement for electrical connection to the support deck; applying an electrical voltage between the first conductor brush arrangement and the roof support deck and between the second conductor brush arrangement and the roof support deck; wherein the first brush arrangement is an inner brush arrangement and the second brush arrangement is an outer brush arrangement forming an outer shielding member surrounding the first inner brush arrangement with both arrangements (8, 7) engaging the roof; the first inner and second outer conductor brush arrangements (8,7) being mounted on a common carriage which is moved over the roof so as to scan the first and second conductor brush arrangements (8,7) over selected areas of the roof; and independently sensing by first and second sensing systems (6, 5) respectively the current flowing from the roof support deck to the first inner and second outer conductor brush arrangements (8, 7) to detect the changes in current as the first inner and second outer conductor brush arrangements (8, 7) are scanned over the selected areas of the roof to locate the leak site in the membrane; characterized in that the membrane is moderately electrically conductive as defined in ASTM D4496-13 and in that the electric voltage difference between the roof support deck and the first sensing system is less than the electric voltage between the roof support deck and the second sensing system; and in that the electric voltage difference between the roof support deck and the first sensing system is adjusted relative to the electric voltage between the roof support deck and the second sensing system based on the conductivity of the membrane. providing a first conductor brush arrangement and a second conductor brush arrangement for engaging the roof above the membrane; providing a return conductor arrangement for electrical connection to the support deck; applying an electrical voltage between the first conductor brush arrangement and the roof support deck and between the second conductor brush arrangement and the roof support deck; wherein the first brush arrangement is an inner brush arrangement and the second brush arrangement is an outer brush arrangement forming an outer shielding member surrounding the first inner brush arrangement with both arrangements (8, 7) engaging the roof; the first inner and second outer conductor brush arrangements (8,7) being mounted on a common carriage which is moved over the roof so as to scan the first and second conductor brush arrangements (8,7) over selected areas of the roof; and independently sensing by first and second sensing systems (6, 5) respectively the current flowing from the roof support deck to the first inner and second outer conductor brush arrangements (8, 7) to detect the changes in current as the first inner and second outer conductor brush arrangements (8, 7) are scanned over the selected areas of the roof to locate the leak site in the membrane; characterized in that the membrane is moderately electrically conductive as defined in ASTM D4496-13 and in that the electric voltage difference between the roof support deck and the first sensing system is less than the electric voltage between the roof support deck and the second sensing system; and in that the electric voltage difference between the roof support deck and the first sensing system is adjusted relative to the electric voltage between the roof support deck and the second sensing system based on the conductivity of the membrane.