Electronic leak detection
Photo courtesy National Research Council Canada
Low- and high-voltage electrical leak detection methods can be used to identify the source of a leak in a roof assembly. Both techniques use electrical conductance to test the integrity of roof membranes. They can detect a breach based on the flow of current to a conductive surface below the membrane.
Electronic leak detection requires three conditions for accurate testing, including:
- grounding medium beneath the membrane to receive the electric current;
- electrically nonconductive membrane; and
- lack of electrically insulated materials between the membrane and the ground.
The low-voltage method uses the presence of water to identify a membrane breach, whereas high-voltage is a ‘dry’ method. Both tests may be faster, safer, and more economical to specify than flood testing of the roofing system.
According to the National Roofing Contractors Association (NRCA), flood testing is a membrane-integrity test conducted by plugging or closing any drains and erecting temporary dams where required to retain water on the surface of a waterproofing membrane. The surface of the roof is then flooded to a maximum depth of 51 mm (2 in.) at its highest point. This water must be retained for a minimum of 24 hours or as long as required by the manufacturer.
NRCA does not recommend flood tests as part of a routine quality control/assurance (QC/A) program for a new roof system. One reason is flood tests are sometimes solely and incorrectly relied on to determine the quality of a roof system. Flood testing alone does not forecast a properly designed or installed roof system. For example, the test will not provide information about service life or evaluate a roof system’s ability to resist wind or impact loads.
Flood testing also is not appropriate for identifying potential leak sources. Roof systems are designed to be weatherproof and not waterproof. While a weatherproof roof resists the passage of water with a minimal amount of hydrostatic pressure (flowing water), waterproofing systems prevent the passage of water under hydrostatic pressure (standing water). For example, water leakage may occur at roof drain flashings with flood testing that exposes drains to hydrostatic pressure. It is important to note roof drains are not designed to be leak-free under such unrealistic imposed conditions.
When using the Low Voltage–Wet Method per ASTM D7877, Standard Guide for Electronic Methods for Detecting and Locating Leaks in Waterproof Membranes, a conductor cable loop is installed around the perimeter of the area to be tested. The cable loop is connected to a low-voltage pulsating generator and the upper electrical plate is formed by dampening the area within the loop. By grounding the conductive deck, it acts as the lower electrical plate, and the roof membrane acts as the insulator. When a breach is present, current will flow through the opening of the membrane to the deck, completing the circuit.
Photo courtesy Sika Sarnafil
The low-voltage method can identify the leak source. It cannot detect moisture accumulation in roof insulation or measure the moisture content in the roof system. Therefore, this technique is not applicable to roof systems containing insulation because it blocks the electrical field. Similarly, low-voltage detection is inappropriate for use in roof systems containing a vapour retarder that will mask the breach by blocking the electrical field.
This technique has historically been of greatest value when investigating protected membrane roof (PMR) assemblies or inverted roof membrane assemblies (IRMAs) or when the membrane is applied directly to highly conductive materials such as steel or structural concrete deck. Low-voltage evaluation with existing black ethylene propylene diene monomer (EPDM) and butyl membranes or assemblies with aluminized protective coatings is ineffective due to the high electrical conductivity of these materials.
At the same time, roof coverboards will block the electrical field unless a conductive material (wire grid or primer) is placed directly under the membrane.
Not all roofing manufacturers have approved the installation of wire grid directly under the membranes. Additionally, when using conductive primers, some manufacturers have not performed compatibility testing with the membranes.
On roof systems where overburden has been installed, only low-voltage evaluations may be specified, as high-voltage testing requires direct contact with the membrane.
Many factors can adversely influence the accuracy of low-voltage testing. For example, vegetative roofs and other assemblies with overburden are typically fitted with on-demand leak detection systems. Part of the process includes the installation of conductive wire loops on the surface of the membrane after the overburden is applied. Connection boxes are used above the overburden to provide access to the wiring at a later date. These existing electronic detection devices can interfere with low-voltage watertightness evaluations. Also, the membrane surface must be wet, which may create serious difficulties with some overburden systems.
Photo courtesy GAF
When used under ideal conditions (i.e. highly conductive materials below the roof membrane), there is still the potential for false positives. The operator’s experience is important for interpreting results accurately. This is particularly true when low-voltage testing through overburden, where the operator is required to interpret the relatively subtle patterns achieved with low levels of voltage.
When using the High Voltage–Dry Method per ASTM D7877, an electrical lead is connected to the roof deck, while another is attached to the device (resembling a push broom with copper bristles). The membrane acts as an insulator. When a breach is present in the membrane, the electricity will flow through the defect and ground to the conductive roof deck.
Drains provide good grounding components, as the drain lines are secured to the structure. However, those with polyvinyl chloride (PVC) piping are ineffective. Metal vent pipes, metal flashings, and exposed rebar secured to the structure are additional grounds.
Similar to the ‘wet’ method, this high-voltage technique can be used to identify the leak source, but cannot detect moisture accumulation in roof insulation or measure the moisture content present in the roof system.
For roof membranes other than black EPDM, the surface must be completely dry and exposed. If water is present behind flashings or under the membrane from adjacent surfaces (e.g. windows, storefronts, porous masonry, or unsealed base flashings), readings cannot be obtained, as breaches will not be detected.
Additionally, more false positive results have been reported using this method compared to low-voltage testing (For more information, read “Everything Leaks: Testing roofs to ensure watertightness at the outset” by Ronald J. Ray, RA, CCS, CCCA, CSI, AIA, in the February 2017 issue of The Construction Specifier. ).
