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The Difference Between On-Roof and Aerial Infrared Thermography for Roof Moisture Surveying

Everything wears out. Your roof is no exception. The ravages of sun, wind, rain, snow, chemicals, leakage and time will eventually cause your roof to fail. You might as well face the fact that you will have to maintain it. Do not think your roof warranty is going to protect you. Vaguely written roof warranties, that do not define words like "regular", or "routine" maintenance will come back to haunt you. You should have some kind of agreement with your roofer or roofing consultant to inspect the roof at least once a year. Waterproofing problems manifest themselves two ways: Leakage and entrained water contamination. Leakage is pretty simple, although the leak inside the building rarely directly relates to the exact spot on the roof, which you already know if you ever tried to track one down. Since most types of insulation absorb a certain amount of water, its harder to find the water contamination in the insulation because it may not leak until it has absorbed all the water it can hold. OK, what tools can we use to find water in the roof. There are three types at your disposal. Nuclear gauges-which count neutrons, capacitance meters-which measure resistance, and infrared-which measure heat. Both nuclear gauges and capacitance meters take readings on a 10' X 10' or 20' X 20' grid on the roof. These measurements are used to extrapolate where the water is. They are good for types of roofs that do not gain or lose much solar energy and therefore, do not lend themselves to infrared. Some roofs and insulation types or combinations do not even absorb water.

During the day, the sun radiates energy onto the roof of your building and into the roof substrate, then at night, the roof radiates the heat back into outer space (See figure 1-1). This is called radiational cooling.

Areas of the roof that are of higher mass (wet) retain this heat longer than the lower mass (dry) areas and therefore radiate this heat for a longer period of time, because it takes longer to cool.

Infrared cameras can detect the heat and "see" the higher mass, during this "window" of uneven heat dissipation.

To perform an on-roof survey, you need a crew of three people: one experienced thermographer, one thermographer's helper and one building owners' representative for access and security. You need access to the roof and plenty of time to collect data. Depending on how many problems are found (very dependent), a crew can survey a 200,000 square foot building roof in one night. Areas that contain subsurface moisture are outlined with marking paint directly on the roof. The infrared images are stored on videotape or flashcards and printed so that the photos can be lined-up in the report. The next day, the crew goes back on the roof to take visual photographs of the areas that are considered to contain subsurface moisture.

The cameras that we use for on-roof surveys are not of sufficient thermal or spatial resolution to obtain good imagery from our minimum flight altitudes of 1000 feet AGL (above ground level). Most modern infrared cameras have 256 X 256 pixels, a total of 65,536 pixels. The infrared cameras that we use for Aerial IR have 512 X 512 pixels, which total 262,144 pixels. This is needed because like the human eye, the farther away something is, the harder it is to discern differences. We use a specially modified Cessna 182 to perform the surveys. A helicopter will work, however high operating costs, vibrations, and slow ferrying speeds make a fixed-wing the best option for us. Very little time is required to obtain the infrared data, once the aircraft is over the building. Usually, a 200,000 square foot building is imaged in less than 10 minutes. This will include multiple passes over the building from varying altitudes, attitudes, speeds and angles. The imagery is recorded on videotape and/or captured directly onto a computer. The visual photographs are taken earlier in the day or the next day. Once the thermographer returns to the office, the photos are developed and the thermographs (infrared images) are saved on the computer. The infrared images and "scanned-in" photographs are used to make an edited videotape of the passes over the building. Both visual and infrared images are used to do the analysis by overlaying the infrared images "over" the AutoCAD, to draw in the areas of suspected moisture contamination. The result is a report where visual, infrared and AutoCAD components (printed and video) are well matched and lined-up.

The same laws of physics apply to both aerial IR and on-roof IR. We need a dry roof, low winds, no rain and few clouds on the night of the survey. Of course the "window" when the roof is radiating heat differently from wet and dry areas, has to be longer in order to do an on-roof survey. The biggest advantage to aerial is angle of view. We used to climb up ladders with $80K cameras to get as high above and as close as possible, to a 90 degree angle over the particular area, to avoid reflections. Aerial has the advantage of looking straight down over the target, and access to multiple levels is never a problem. The drawing dimensions are close to perfect, which was a big problem with on-roof. There are three distinct advantages that on-roof surveys have over aerial. If you are on the roof, you can look underneath rooftop equipment. Also, if you take a roof consultant, he/she can perform destructive verification right then & there. Finally, no one has devised a method yet, to paint the roof while flying by @ 80 MPH. I had one client who wanted the roof painted. I simply took the scaled AutoCAD drawings (see example drawings figure 2) up on the roof and marked it for him. Aerial infrared works best on large buildings or multiple buildings, because it is an expensive operation and the costs have to be spread out among all the targets that night.

The cost of either on-roof or aerial survey is usually the same, between 1 and 2 cents per square foot, depending mostly on what level of report is needed. Since it cost in the dollars per square foot to repair roofs, knowing where the water is (it got there though a hole, tear, or leak around a penetration), is a very cost effective means of planning the extent of repairs. You have a tremendous investment over your head. Extending its life will save you a lot of money. Also, your motivation may be, for example, to use this information to plan budgets, look at a building before you buy it, or to put some teeth in that new roof warranty. That's right, a lot of our work is on new roofs. You get baseline information to compare at any time in the future, and catch any installation problems. This is real roof asset management

Gregory R. Stockton

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