In a method of detecting a leak in a water impermeable membrane applied on a roof substrate, an electrical potential is created between a conductive detector on top of the membrane and the roof substrate causing current to flow between the roof substrate and the conductive detector through moisture in any leak in the membrane. The membrane is attached to the roof support substrate by an intervening layer therebetween which can be a primer or an adhesive layer which is caused to be electrically conductive by the addition an electrically conductive material. This allows the conductor on the roof substrate to which the potential difference is applied to be attached onto the roof substrate and covered by the layer underneath the membrane.