The traditional method of leak testing flat roofs is to flood test them. This is costly and time-consuming (it can take several days for the leaks to appear) and relies on establishing substantial leakage through the roof that can cause damage to internal fixings and finishes.
There are a number of non-destructive electrical leak detection techniques, which can be used, that cause minimal disruption or damage to the building. 

If a roofs waterproof membrane is continuous, it may act as an electrical insulator (some membranes such as EPDM are electrical conductors and this technique cannot be used).  However, if any leak positions exist, water that has seeped into it will act as a partial conductor for any electrical current to flow to the deck and from there to earth.  The test equipment introduces an electrical pulse over the wet roof membrane;  by measuring the potential difference of the roof surface, an experienced technician can pinpoint the leaks by tracking any current flow.

If a roofs waterproof membrane is continuous, it may act as an electrical insulator of an electrically conductive substrate below (some membranes such as EPDM are electrical conductors and this technique cannot be used).  However, if any defect in the membrane (leak position) exists this will allow a pathway for an electrical current to be discharged to earth.  The test equipment introduces a stabilised direct current (d.c) over the dry roof membrane; by moving over the dry waterproof membrane with the portable test instrument an experienced technician can pinpoint defects which will allow water to enter into the roof construction. 

A wet insulation, screed or deck has different electrical properties from when it is dry.  This can be determined non-destructively from above the membrane by using an electrical capacitance meter.
The test instrument transmits electrical pulses into the roof construction and measures the ability of the substrate to hold an electric charge.  Relative damp areas hold less charge and both an audible and visible signal indicates that water may be present.  The area of entrapped water can be identified by moving the instrument across the roof surface.  The affected areas can be marked on the roof surface using a wax crayon and on a roof plan in the accompanying report.

Non-destructive leak detection can quickly and accurately identify water entry pathways and areas of entrapped water within a roof construction.  Used individually or in combination these techniques can be used as quality control on new projects or investigations prior to remedial works where they can offer significant cost savings over complete roof refurbishment or remove any liability for damage from following trades.