Identifying watertightness of low-slope roof membranes

by nithya_caleb | January 23, 2019 12:00 am

by Dominique Lefebvre and Bas A. Baskaran, PhD, P.Eng.

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An accurate assessment of the watertightness of new and existing roofs can potentially save building owners hundreds of millions of dollars annually. The challenge for roofing specifiers is choosing the most effective exterior-to-interior watertightness evaluation techniques.

The fact is there is no single, straightforward method to accurately evaluate water ingress from the exterior roof membrane to the interior of the building. This has led to confusion among specifiers as to which method is most appropriate for a particular type of roof, and what information each test provides.

This dilemma has led the Roofing Industry Committee on Weather Issues (RICOWI) to review existing water-detection techniques to identify:

• methods of operation;
• capabilities of the tests;
• test compatibility with various components; and
• benefits and the shortcomings of each test procedure.

RICOWI’s Moisture Control and Green Committee led this investigation, as the group focuses on moisture-control issues and identifies specific roof performance metrics. Committee membership consists of manufacturers, property owners, and academics to ensure a diverse roofing community representation.

Addressing watertightness of roofs

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The main function of any roof is to prevent water entry into a building. This leads to the fundamental question of how to assess the watertightness of a roof assembly.

Watertightness detection is needed for both new and existing roof systems. For new construction, this watertightness evaluation provides confirmation to the property owner the roof was properly installed, and as such, can be used as a field commissioning tool.

For existing construction, a watertightness evaluation can be employed for the following two purposes:

• to assess the current roof condition and determine if replacement is required after confirmation from test cuts and professional insight; and
• to evaluate if the watertightness capacity of a roof was compromised after a major weather event.

From the perspective of the roofing specifier and property owner, effective nondestructive roofing and waterproofing testing can provide various practical benefits. For example:

• pinpointing, documenting, and repairing wet roof areas to limit further damage to commercial low-slope roofs;
• reducing the risk of structural damage and potential failure of new or existing roof systems;
• identifying sound roof sections and replacing wet insulation to conserve time, materials, and energy use; and
• targeting problem areas to accurately budget roof maintenance and improve specifications for competitive bidding procedures.

Electronic leak detection

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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.

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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.

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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[6]” by Ronald J. Ray, RA, CCS, CCCA, CSI, AIA, in the February 2017 issue of The Construction Specifier. ).

Infrared thermography

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The infrared thermography method uses infrared imaging to identify temperature differentials between dry and wet locations to indicate the presence of water in the system. This method operates on the principle wet insulation has a higher thermal mass and, therefore, retains heat longer than dry insulation. Although this method is not time consuming and allows the operator to sample the entire roof, it can provide misleading information. Since the testing relies on differences in temperature, mechanical equipment, heating/cooling systems, and shaded areas can adversely influence the results.

Infrared imaging per ASTM C1153, Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging, and Testing Application Standard (TAS) 126-95, Standard Procedures for Roof Moisture Surveys, is used to determine the location of wet insulation in contact with the membrane in the roofing system. This test method is often used at night when the roof begins to cool because the wet insulation (higher mass) retains heat longer than the dry area (lower mass). The infrared camera is able to capture the temperature differential between the dry and wet roof areas.

[8]Unlike electronic leak detection, infrared thermography is unable to identify the source of the leak. However, moisture content can be measured using core samples.

Infrared thermography can be specified to analyze the presence of moisture in all types of roof membranes. However, it is incompatible with ballasted roof assemblies or PMR/IRMA varities. During testing, the membrane must be dry and devoid of condensation. This test method is not suitable on insulations that do not absorb water, such as expanded polystyrene (EPS) and closed-cell sprayed polyurethane foam (SPF). Additionally, infrared thermography should not be specified when the existing roof deck is capable of retaining significant amounts of water. These wet-applied decks include lightweight concrete and poured gypsum.

Similar to other methods of moisture detection, infrared thermography can generate false positives, and the operator’s experience is important for interpreting results accurately.

INCREASING RESILIENCY OF ROOFING SYSTEMS

The federal government has initiated a mandate to increase the resiliency of the built environment through the Climate Resilient Buildings and Core Public Infrastructure project at the National Research Council Canada (NRC). The initiative includes two major projects relevant to the roofing industry. The Guidelines for Commissioning and Certifying the Resiliency of Roofs Subjected to Extreme Weather Events project involves developing field protocols to perform in-situ assessments of wind uplift resistance, watertightness, and thermal performance. These tools will allow the industry to assess the capacity of a new roof, ensuring it was installed to meet the design requirements and withstand climatic events. The guidelines will also provide the industry with a method of assessing the remaining roof capacity after either an extreme event or field aging. The Codification of Material Properties for Building Adaptation to Climate Change project includes evaluation of the properties of more than 20 common building materials for various climatic zones to develop a database of climate-dependent material properties. One of the major outcomes of this initiative is the development of an online database tool to improve access and ease of use for the building envelope community. Both NRC projects are aimed at increasing the resilience of the building envelope and roofing. These initiatives were identified through an industry consultation on building resiliency held in 2016. The methodologies derived from the climate adaptation projects are scheduled for inclusion in the National Building Code of Canada (NBC).

Nuclear detection techniques

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The nuclear water detection method employs radioisotopic thermalization to emit high-velocity neutrons and measure backscatter. Although this method is directly detecting the presence of hydrogen, and therefore water, in the roof assembly, it requires operation by licensed personnel due to its complexity (e.g. unit angle sensitivity and baseline reading requirements).

Radioisotopic thermalization (TAS 126-95 and American National Standards Institute/Single Ply Roofing Industry [ANSI/SPRI]/RCI NT-1-2017, Detection And Location Of Latent Moisture in Building Roofing Systems by Nuclear Radioisotopic Thermalization) involves a process where a nuclear moisture meter emits high-velocity neutrons and measures backscattered ‘slow’ neutrons that have lost much of their energy in collisions with hydrogen atoms. Thus, higher levels of slowed neutrons are recorded at wet areas, as water contains a significant amount of hydrogen atoms.

Nuclear leak detection is also able to identify the accumulation of moisture in insulation, but cannot pinpoint the source of the leak(s). Core samples may be taken of dry and wet locations to determine moisture content.

This technique is applicable to all conventional roof assemblies except metal roofs. Nonconventional PMR (upside down) roofs are incompatible with nuclear watertightness evaluations. Roofs with overburden may be tested only if the ballast or pavers are removed from the test area. In most cases, the practicality of using this time-consuming method of watertightness detection beneath vegetative roof assemblies would be of questionable value to specifiers.

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As with other methods of watertightness detection outlined here, there is the potential for false positives.

Typically, a baseline reading to calibrate the nuclear meter must be taken in a known dry area of the roof. The nuclear meter samples about 0.6 m² (6.5 sf) at each grid point in a 0.9 x 0.9-m (3 x 3-ft), 1.8 x 1.8-m (6 x 6-ft), or 3 x 3-m (10 x 10-ft) pattern. The equipment has a depth limitation of 1.8 to 2.4 m (6 to 8 ft). However, unlike several other methods of watertightness evaluation that are employed, nuclear scanning is not dependent on climatic conditions.

It is important to note areas of ponding water on the roof surface will result in increased readings from the nuclear meter. Readings can also be affected by inconsistencies seen among roofing components, such as joints where the different elements meet one another.

Further, technicians may need to comply with Canadian Nuclear Safety Commission (CNSC) regulations.

Electrical impedance evaluations

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The electrical impedance method uses a device to create an alternating electrical field for penetrating the roofing material. Since wet insulation provides less resistance to electrical current than dry, the current can be correlated to the presence of water. The alternating current flowing through the field is inversely proportional to the impedance of the moisture-absorbing materials. On the downside, the electrical impedance unit contains a scanner sensitive to interply and surface moisture and inconsistencies in the roof system.

Like the nuclear method, this technique is applicable to most conventional roof assemblies except for black EPDM membranes or assemblies treated with aluminized protective coatings. PMR roofs are incompatible with electrical impedance watertightness evaluations, and any roofs with overburden may be tested only if the ballast or pavers are removed from the test area. It is critical for the membrane to be free of surface moisture to obtain accurate readings.

Similar to the nuclear and infrared methods, the impedance method is also able to identify moisture accumulation in insulation, but cannot pinpoint the source of the leak(s). Again, core samples may be taken of dry and wet locations to determine the moisture content.

As always, the operator’s experience is essential for interpreting results accurately. Other potential issues reducing instrument sensitivity include aggregate-ballasted and/or aggregate-surfaced membranes with variable size and weight. As mentioned, the scanners are also more sensitive to interply moisture and water closer to the scanner electrodes, which can make readings further below the membrane difficult. Roof patches dissimilar to the system under testing may also result in erroneous readings.

While the electrical impedance watertightness method is easy to use and less complex than other evaluation techniques, the presence of dew, rain, snow, and ice significantly affects the readings.

Conclusion

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None of the moisture-detection methods described in this article are able to quantify the moisture content without performing core samples, which is undesirable due to the destructive nature of this sampling.

For all water-detection methods, there is the potential for false positive readings. The experience and/or licensing of the operator is essential for performing the test and interpreting the results accurately. All the methods investigated are also sensitive to the type of roof assembly under evaluation.

Based on the review conducted by RICOWI’s Moisture Control and Green Committee, it is evident a generic method does not exist to detect exterior water entry into all types of roof assemblies.

A common industry practice is to employ a combination of applicable methods to both detect the presence of water in a roof assembly and identify the source of the leak. Since infrared thermography is the most simple and convenient method, it is often used in conjunction with another, more technical method to detect the presence of water depending on the roof type.

It is important watertightness evaluations are not only specified in the aftermath of a major weather event or roof leak, but also included as part of a regular maintenance program.

Without performing regular watertightness evaluations, property owners will either be left guessing whether there is a problem with their roofs or will only be made aware of an issue once it has progressed to visible interior damage.

However, by combining water-detection methods suitable for a particular roof, one can obtain reliable information on the watertightness resistance and condition of an existing roof system (Special thanks to David Hawn of Dedicated Roof and Hydro-Solutions for his comments on this article. The authors acknowledge the members of the Moisture Control and Green Committee of RICOWI for their input, especially David Balistreri of Building Envelope Consultants, Greg Keeler of Owens Corning, Peter Brooks of IR Analyzers Vector Mapping, Shaun Katz of Detec, Tom Kelly of 2001 Company, and William Tipton of Roof Maintenance Systems.).

REFERENCES

1. ASTM D7877, Standard Guide for Electronic Methods for Detecting and Locating Leaks in Waterproof Membranes. 2. ASTM D7954, Standard Practice for Moisture Surveying of Roofing and Waterproofing Systems Using Non-Destructive Electrical Impedance Scanners. 3. The paper “Climate Change Adaptation Technologies for Roofing” by B. Baskaran, S. Molleti, D. Lefebvre, and N. Holcroft for the 33rd RCI International Convention and Trade Show. 4. “Electronic Leak Detection: Sound Science, Not a Magic Wand” by P. Brooks in the July 2017 issue of RCI Interface. 5. Testing Application Standard (TAS) 126-95, Standard Procedures for Roof Moisture Surveys. 6. “Electronic Leak Detection: A Quality Assurance Tool” by D. Honza for RCI Interface. 7. Infrared Roof Moisture Surveys Accurate Assessment of Roof Condition. Visit www.iranalyzers.com/infraredroof.htm for more information. 8. Nuclear Roof Moisture Surveys. Retrieved from IR Analyzers Vector Mapping at www.iranalyzers.com/nuclearroof.htm. 9. “A Comparison of Three Different Technologies for Performing Nondestructive Roof Moisture Survey” by J. Robinson, D. Bradford, J. Mitchell, and P. Majkowski, published in the Proceedings of the North American Conference on Roofing Technology. 10. The American National Standards Institute/Single Ply Roofing Industry (ANSI/SPRI)/RCI NT-1, Detection and Location of Latent Moisture in Building Roofing Systems by Nuclear Radioisotopic Thermalization.

 

[13]Dominique Lefebvre is a research officer with the National Research Council Canada (NRC). Her research area focuses on the evaluation of the interface of various roofing materials, as well as the development of tools and techniques for climate adaptation of commercial roofs. Currently, she is working on developing the performance requirements of coverboards in low-slope membrane roofing for the creation of a harmonized standard. Lefebvre can be reached via e-mail by contacting at dominique.lefebvre@nrc-cnrc.gc.ca[14].

[15]Bas A. Baskaran, PhD, P. Eng., is a group leader at NRC, where he researches the performance of roofing systems and insulation. He is an adjunct professor at the University of Ottawa, and a member of Roofing Committee on Weather Issues (RICOWI), RCI Inc., Single Ply Roofing Industry (SPRI), and several other technical committees. Baskaran is a research advisor to various task groups of the National Building Code of Canada (NBC). He was recognized by Her Majesty Queen Elizabeth II with a Diamond Jubilee medal for his contribution to fellow Canadians. Baskaran can be reached at bas.baskaran@nrc-cnrc.gc.ca[16].

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