The inspection of many—but not all—low-slope roofs is straightforward. If you are dealing with a built-up roof (BUR) or a “bald” single-ply membrane that has aged several years, you should have good success using one of two basic strategies in which the wet insulation reveals itself thermally due to its higher heat capacitance or its greater thermal conductivity. Ballasted single-ply roofs and newer ones that utilize low-absorbency insulation are both quite challenging to inspect. Additionally, many bald single-ply membranes are quite reflective in the long-wave band.
In all cases, you’ll be working up on the roof—so safety is a primary concern. Plan on a preliminary daylight walk around to help you identify potential hazards and to visually inspect the roof. Day or night, it is critical to always be with a qualified escort on a roof! Especially when working near the edge of the roof, take precautions to prevent a fall. When working at night, go slowly, as your eyes will adjust for the imager’s view screen leaving you “night blind!”
While ideally the work is done on a calm, clear evening after a sunny day, it may be possible to do it on any evening, or on a heavily overcast day when the outdoor air temperature is 50°F or less. The roof surface, including any gravel, must be dry. It is also important to know what kind of insulation is used, how many layers there are and to have an idea of where the leaks are occurring.
Walk the roof slowly and systematically until the patterns begin to differentiate themselves. As viewed from the roof, the thermal pattern of the wet insulation will be warmer than the dry insulation. Absorbent insulations will have a characteristic right-angled, “board-stock” pattern (see below), while low-absorbency insulations will show a “picture-frame” pattern.
There may also be, of course, many other thermal patterns not associated with wet insulation, including flashings, drain pans, gravel piles, exhaust from HVAC units and structural elements in the roof system, among many. Once you determine a certain pattern indicating wet insulation—and testing with a moisture meter is recommended—you can keep looking for similar patterns until you’ve covered the entire roof.
Don’t worry about finding the actual leak during the inspection, though. It will typically be somewhere within the boundaries of the wet insulation and is often easier to find during a daytime follow-up visit. Ideally, you will find a thermal indication for every known leak. Mark the thermal patterns as accurately as possible using a high-quality aerosol marking paint, and number each section for easy reference. When conditions are right and the roof is not too complicated, you can typically survey 100-200 squares in an evening! At some point the signatures may dissipate indicating it is time to pack up and head home.
The investment in an inspection can pay huge dividends. We’ll talk more about the value of roof moisture inspections next week.
John Snell—The Snell Group, a Fluke Thermal Imaging Blog content partner