A Global Strategy for Evaluating Building Roof Performance and Diagnosing Moisture Intrusion into Roofing Systems Using Infrared Thermography and Other Diagnostic Tools

David Khudaverdian, Eng. Patenaude-Trempe Inc.

ABSTRACT

For the owners and managers of large institutional, commercial and industrial buildings, the potential cost of roof repairs and eventual roof replacement is a major capital expense. To allow for the effective management and budgeting of these expenditures, it is imperative that a detailed and accurate maintenance plan for all the roofing systems be developed. The maintenance program should establish a detailed plan of action for the corrective work required to extend the useful life of the existing roofing systems, forecast the eventual replacement of the roofing system, and establish projected budget costs associated with the repairs and replacement of the roofing system.

In conjunction with other diagnostic tools and an understanding of roofing construction and roofing principles, infrared thermography can play a key role in assisting the experienced building professional to effectively establish the existing condition of most conventional roofing systems and to rapidly diagnose problems of moisture intrusion within the roofing system. With this information, a roof maintenance plan and strategy can be developed to forecast all corrective intervention and budget costs related to maintaining multiple roofing systems in large buildings.

The primary objective of this paper is to provide increased knowledge to the building thermography community about the benefits of using infrared thermography as part of a global strategy for developing an effective roof maintenance plan for building owners and managers.

INTRODUCTION

In Canada, the useful life of a multi-ply conventional roofing system is considered to be approximately 20 years. In a perfect world, the building managers or owners of a large portfolio of buildings would simply require little more than to budget replacement of their roofing systems every 20 years in accordance with this recognized rule of thumb. Unfortunately for those who must foresee and project the capital expenses for their properties, there are numerous factors that may contribute to the premature failure of roofing systems well before the expected 20 year mark. With replacement costs of conventional roofing systems nearly tripling in the last 15 years in Québec, unexpected capital costs in the millions of dollars that are required 10 or even 5 years earlier than budgeted could create substantial financial tremors for many building owners.

The factors that contribute to premature failure may include such latent items as poor original design, deficient workmanship, deficient or inappropriate construction materials and questionable methods of construction. Even in those rare instances when the perfect roof design meets superior workmanship, premature failure of may still occur due to misuse or abuse of the roofing system in the form of accidental damage caused by building users or building maintenance personnel.

If the building owner is fortunate, the defect or compromise of the roofing system results in an immediate localized leak into the premises below. In this circumstance, if the informed building owner acts quickly enough, addressing the problem of water ingress immediately may only cost a few thousand or even a few hundred dollars depending on the scope of the problem, and more importantly, helps to preserve the overall integrity and useful life of the roofing system.

However, today’s modern roofs, especially those in northern climates, are often designed and constructed with moisture-tight vapor barriers to prevent condensation from forming within the insulated roofing systems during the heating season. Water absorptive materials such as fiberboard or perlite panels used as support panels for the membrane above the insulation, and plywood sheets surrounding roof parapets and control joints are also common in the composition and assembly of new roofing systems. The combination of the watertight vapor barrier along the roof deck and the absorptive characteristics of the roofing materials installed beneath the roofing membrane combine to create conditions whereby the water ingress through the roofing membrane does NOT result in an immediate leak into the premises below. Rather, the moisture which has penetrated through the roofing membrane is gradually absorbed by the roofing material and retained or imprisoned by the vapor barrier within the roofing system itself. Eventually the imprisoned water spreads through the roofing system by capillary action and absorption by the roofing insulation and sponge-like support panels beneath the roof membrane. The imprisoned moisture causes the roof elements to degrade and also dramatically reduces the thermal properties of the roof insulation, creating conditions favorable to the formation of condensation inside the building. The end result of all these combined conditions is that a significant portion of the roofing system may become irreparably damaged by moisture well before the visible symptoms of any problems or leaks become apparent and well before the expected expiration of the useful life of the roofing system. Consequently, the building owner is forced to face the unexpected and sudden high cost of corrective intervention for the premature replacement of the roofing system.

To help building owners accurately project and budget the replacement of their roofing systems and avoid having to face the unwanted prospect of investing significant funds towards premature replacement, it is essential that a detailed Preventive Maintenance Program for the roofs be developed and implemented.

The preventive maintenance program for the roofing systems would typically summarize for the building owner the plan of action or scope of roof repairs and associated costs that required in the immediate term, short term, and long term as a means to help preserve or prolong the remaining useful life of the roofing systems. It would also project the year and cost when roof replacement will eventually be required. Essentially, the preventive maintenance program provides a roadmap for budgeting repairs for each and every one of the owner’s roofing systems, greatly reducing the risk of having to deal with the inconvenience and costs of unexpected roofing issues.

Typically the best roof preventive maintenance programs are developed by experienced professional firms and qualified thermographers with a detailed knowledge of roofing construction and building sciences and the ability to carry out the necessary diagnostic analysis and interpretation of data.

Infrared thermography, along with the use of other diagnostic equipment and an understanding of building construction concepts, can play a key role in a global strategy for roof maintenance. It can help practitioners detect non-visible deficiencies in the roofing systems, accurately assess the actual condition of roofing, and achieving the desired objectives of the preventive maintenance program for roofing.

PREVENTIVE MAINTENANCE PROGRAM FOR ROOFING – A GLOBAL STRATEGY

The development of an accurate preventive maintenance program for roofing requires a systematic approach involving a combination of historical information on the roofing system, results obtained from a detailed physical assessment of the roofing system, and knowledge and experience of roof construction and building sciences. Therefore, the best approach toward developing an accurate preventive maintenance program for roofs would involve the following steps:

Review of Architectural Plans of Existing Roofing

Whenever available, the existing roof plans and architectural details of the building should be reviewed prior to the on-site assessment. The information should include such general items as the age, geometry, and location of the roof basins, layout of all drains and mechanical rooftop units, and most importantly, the composition of each of the roof basins to be reviewed. The copies of the roof plans can also be used during the survey on site to transfer information or record deficiencies directly onto the drawings for reporting purposes.

Special attention to the roof composition should also be considered well before proceeding with the on-site assessment. Some roofing compositions and systems readily allow for standard review practices and diagnosis through non-destructive methods such as infrared thermography. However, most inverted roofing systems and even some conventional roofing systems do not allow for an accurate review using infrared thermography. It is good practice to be aware of which roof basins can be analyzed using this technique, and which cannot be prior to initiating the roof assessment on site.

In addition, the original architectural drawings, including details of the air and vapor barrier of the building envelope, should also be reviewed by an experienced professional in order to confirm the suitability of the envelope design in terms of heat transfer and moisture vapor travel within the building given its usage. These first steps are necessary not only to help define which strategies (destructive and non-destructive measures) and diagnostic equipment are required to accurately assess the physical condition of the roofing system, but also to foresee some of the potential causes of water ingress prior to assessment on site. For example, in certain instances, this step could lead to the discovery that reports of water ingress or roofing failure may in fact be due to problems of condensation and not necessarily to the failure of the roofing membrane.

Figure 1. Thermogram showing moisture within the roof caused by condensation due to high humidity levels and absence of suitable vapor barrier in the building below.

Preliminary Visit and Interview with Building Personnel

A preliminary visit to the building roofs and interviews with building maintenance personnel often provide invaluable information such as the historical background of the original roof construction, corrective maintenance previously executed on the roofs, identification and the actual composition of roofs basins which had previously been replaced or repaired, and reported locations of both previous and active areas of water ingress below the roofs. This information simplifies calibration of non-destructive moisture detection equipment during the roof assessment and helps to alleviate possible confusion during the interpretation of results when using infrared thermography. For example, a single roof basin that has been replaced with added insulation or non-absorptive insulation will transmit different readings and images when compared to the adjacent roof basins that have not been modified. Not knowing the maintenance history and the actual composition of all the roof basins to be assessed ahead of time may lead to error in the interpretation of infrared scans. A preliminary visit to the site also gives the infrared practitioner the opportunity to walk the roof during daylight hours to view the roof layouts and help spot membrane and roofing deficiencies, as well as safety issues or tripping hazards that may not be readily visible during the evening. This also helps in determining whether the roof itself is suitable for infrared thermography as a means to assess the condition of the roofing system (see below regarding limitation of infrared thermography for the condition assessment of roofing systems).

Figures 2 and 3. Photos showing signs of water ingress below roof deck (at left) and signs of roof membrane deterioration above

Assessment of Roofing Systems Using Infrared Thermography

Infrared thermography is likely the most cost-effective and efficient tool for non-destructive assessment of conventional roofing systems. In the hands of an experienced professional, the use of infrared thermography can play a key role in helping to quickly determine the location and general scope of moisture imprisonment hidden within the roofing system. The introduction of moisture, whether in the free state or absorbed by the insulation, reduces the insulation's effectiveness and increases its conductivity. During the summer months, under clear sky conditions, the moisture within the roof acts as a large thermal collector and heat sink. At sunset, the roof surface starts to cool rapidly, while the moisture-laden insulation continues to emit heat to the membrane from below as it too slowly starts to cool. For many hours following sunset (this duration depends on several variables) the membrane sections above wet sections of insulation of a conventional roofing system will continue to emit heat and remain warmer than the surrounding roof membrane above dry insulation. Through the use of an infrared camera to detect minute variations in the emission of heat of exposed surfaces and to convert this information into an electrical signal and color image, infrared thermography makes it possible to detect/predict the presence of moisture within the insulation of most conventional roofing systems.

Figures 4 and 5. Schematic representation of heat absorption of wet insulation in roof during the day (at left) and heat emitted by roof membrane above wet insulation detected by infrared camera in the evening.

During the infrared scan of the roofing system, the suspect areas can be delineated with spray paint applied directly on the surface of the roof membrane. These areas are measured and then transferred onto plan drawings of the roofing system as part of the recording process. Often depending on the geometry of the building, up to several hundred thousand square feet of roof area can be scanned during one evening, until the thermal signature from the roof membrane above the wet locations stabilizes with that of the surrounding roof membrane and the signal is no longer detectable.

Figures 6 and 7. Typical infrared scan of wet area within roofing system at left. At right, paint typically used on roof surface to delineate suspect area of roofing system.

Figure 8. Results of infrared scan transferred onto typical plan drawings of the roofing system with suspect wet areas within roofing system indicated in the drawing.

In this way, infrared thermography of roofing systems provides the information necessary to help determine the expected scope of immediate repairs and to predict the expected remaining useful life of the roofing system, which is the core objective of developing an accurate preventive maintenance program for roofing.

However, despite its obvious advantages, infrared thermography alone cannot provide all the information required for the development of a complete preventive maintenance program for roofing. It is critical to understand that several limitations do exist regarding when and how infrared surveys of roofing systems can be used successfully. Conditions that restrict or prevent the use of infrared thermography as a means to effectively assess roofing systems include, but are not limited to, the following:

  1. Inability to review most inverted roofing systems (where ballast and insulation is installed over the roofing membrane);
  2. Inability to effectively review conventional roofing systems when non-absorptive support panels (ex. cement board) and/or insulation form part of the roof composition;
  3. Inability to review systems where ponding water or moisture on the roof surface is present;
  4. Limited window of opportunity--Infrared thermography of roofs is limited to mostly nighttime review during the cooling seasons with light winds no greater than 10mph;
  5. Inability to achieve reliable results when roof areas are surrounded with clusters of installed rooftop mechanical equipment due to reflective surface of units;
  6. Inability to effectively review most upturn detail (membrane flashing).

Given these limitations, it is clear that other diagnostic approaches and tools may be required to provide the global assessment that is needed for an accurate preventive maintenance program for a given roof system under certain conditions.

Non-Destructive Assessment of Roofing Systems to Assist Infrared Thermography

In localized areas or instances where infrared thermography cannot be utilized, other non-destructive diagnostic tools and equipment may be used to address these areas, and to validate the results of the infrared survey. The diagnoses of such locations generally require the use of capacitance meters and nuclear meters in order to detect moisture imprisoned within the roofing system.

The nuclear meter used for roofing assessments is specifically designed for performing roof moisture surveys. Its calibration differs from the nuclear density meters used to measure in situ soil moisture. A roof moisture gauge sends a cone of fast neutrons into the roof. The neutrons bounce off materials in the roof and some return to the sensor located inside the nuclear device (see figure below), which detects and displays the results. The speed of the neutrons is slowed if they come into contact with hydrogen atoms. Typically the presence of hydrogen atoms in roofing systems is due to the presence of water (H2O) within the roof composition. Thus the sensor within the nuclear moisture meter is able to confirm water presence within the roofing system. Other roofing materials may also contain hydrogen atoms and therefore must be differentiated from the hydrogen atoms contributed by the presence of water. Usually readings are taken on a 5’ by 5‘ grid marked directly on the roof surface.

Figures 9 and 10. Use of nuclear meter used to detect moisture in the roofing system.

A capacitance meter creates an alternating current electrical field in the materials below it (see figure below). The ability of those materials to store and dissipate electrical energy is related to their dielectric properties. The dielectric properties of water are significantly different from those of roofing materials. The dielectric constants of most materials used in roofing range from 1-4, whereas the dielectric constant of water is 80.

Numerical readings are obtained at grid points about 5 feet apart. Capacitance meters are quite sensitive to moisture just below their base. It is essential that the surface of the membrane and any gravel on it be dry when readings are taken. The presence of a small amount of water within the plies of a bituminous built-up membrane will be noted more easily with a capacitance meter than with a nuclear meter or infrared camera.

With the exception of inverted roof systems, both the nuclear meters and the capacitance meters offer an alternative to assess roofing systems and detect imprisoned moisture in roof areas or in conditions or locations where infrared thermography cannot be used effectively. They can also provide a second validation to infrared results wherever moisture is detected within localized areas of the roofing system. The only drawbacks to these two systems are that they require multiple readings to be taken and significantly greater time to assess the condition of an entire roof basin than compared with infrared thermography.

Because of the different limitations and varying advantages of these approaches, a thorough roof assessment and preventive maintenance plan for conventional roofing systems should use a combination of nondestructive equipment including an infrared camera, a capacitance meter, and a nuclear meter.

Exploratory Openings - Validating Results of Non-Destructive Assessment of the Roofing System

Despite the advancements and benefits provided in moisture detection equipment such as infrared thermography, nuclear meters, and capacitance meters, over large areas, exploratory openings in select locations in the existing roofing system are nonetheless required to provide final confirmation and validate the results obtained from the non-destructive assessment of the roofs. In addition, the exploratory openings provide confirmation of the actual roof composition. The exploratory openings, which vary in size, are executed in a few select areas where moisture detection equipment has registered possible moisture within the roofing system, as well as in areas where moisture was not detected. These openings also help to confirm the depth at which moisture is imprisoned in the roofing system or within the plies of the roofing membrane. An experienced roofer should be mandated to execute the required openings and subsequent repairs under the supervision of the roofing professional. In cases where the roofing system is still within the warranty period, the exploratory openings and repairs should be executed by the same roofer who is providing the warranty in order not to risk nullifying the roof warranty.

Figures 13, 14, and 15. Exploratory openings executed in roofing system to confirm roof composition, to validate results of infrared thermography study, and to determine actual location of moisture within the roof components.

PRESENTATION OF PREVENTIVE MAINTENANCE PROGRAM FOR ROOFING

Once the roof assessments have been completed, an experienced professional should develop a preventive maintenance program for the roofing. This is often presented in the form of two tables: one table provides the synopsis of the results of the roofing assessment and severity of the deficiencies detected or observed; and a second table provides a summary of the recommendations for corrective work over the next 5 or 10 years, with associated budget cost estimates for all the corrective work recommended.

The information provided in the tables for each of the roof basins reviewed generally consists of the following:

This information is also presented with recommendations for typical maintenance items involving roofing including: maintenance to roof drains; repairs to all areas of visible deterioration of the roofing membrane such as membrane blisters, ridging, fishmouths, tears or splits; replacement of damaged metal flashing and stack jack sleeves; and the removal of existing debris or vegetation present on the roof surface, as a means to preserve or prolong the remaining useful life of the roofing system.

DOSSIER: T-0202A

The tables and recommendations of the preventive maintenance program are often presented along with a schematic view drawing of the roofing system whereby all the roof basins and all the deficiencies recorded during the roof assessment and areas of recommended roof repairs are also identified.

Level of Severity (medium or high):REDLevel of Severity (low): GREEN

InfraMation 2007 ITC 121A 2007-05-24

DOSSIER: T-0202A

Level of Severity (medium or high):RED actions : Replacement (REF) Garanty (GAR)Expertise (EXPERT)Level of Severity (low): GREEN Repairs (CORR) Visual (VIS)

Table 2 – Typical Sample of Summary of Recommendations

SUMMARY

The combined expertise of an experienced building or roofing professional and a qualified thermographer is key to providing building owners with an accurate and efficient preventive maintenance program for multiple buildings and roof basins. A qualified professional should review the architectural plans and historical information of the roofing system to obtaining the necessary information on what strategy and diagnostic tools will be required for the roof assessment. The roof assessment should make use of infrared thermography as a means to most effectively and efficiently assess large numbers and areas of roof basins. Other nondestructive diagnostic tools such as nuclear and capacitance meters should also be used in order to validate the results of the infrared scans, and to review specific roof areas that are unable to be scanned by infrared thermography. Exploratory openings at localized areas in the roofing system are also critical as part of any roof assessments in order to validate the interpreted results of the non-destructive surveys. The results and recommendations which form the basis of the preventive maintenance program are best presented in the form of tables and schematic drawings that illustrate the areas of roof repairs or replacement, and the associated budget cost estimates.

ABOUT THE AUTHOR

David Khudaverdian is a senior engineer and associate of Patenaude-Trempe Inc., a privately owned engineering consulting firm in Québec, Canada, with over 15 years experience specializing in the field of building science and building envelope diagnosis and restoration.