Roof Survey Techniques: Infrared

« Back

Photo of rooftop
Photo of infrared image display

During the day, the entire surface of a typical roof reaches very close to a uniform temperature. A two-man crew (minimum, for safety) will go up on a roof after dark when the roof has started to cool off. Because of the thermal storage characteristic of water, wet areas will hold heat for a period of time while dry areas will cool off relatively rapidly. At some time there is a difference in the surface temperatures of the wet and dry areas. That period of time is described as the window. An infrared camera produces images of these temperature differences.

A good rule of thumb for how long a window will last is 4 hours. Some types of construction when combined with a particular weather pattern can make the window longer or shorter. Structural concrete with a gravel surfaced built up roof on a hot summer day tends to lengthen the window. A white single ply with minimum insulation of a wood deck on a mild day can shorten the window significantly.

Typically, the perimeter of the suspected wet areas are painted on the roof and a roof plan showing the location and outline of suspected wet areas is supplied.

The Infrared technique has limitations. “Hot spots” can also be caused by interior heat sources, excessive gravel or roof top equipment. The camera “sees” apparent surface temperature, so the roof surface should be dry and not covered by leaves or other debris. Also, an infrared camera will not produce usable images if a 10 to 15 mph breeze is blowing across the roof or if the surface has a fresh aluminum coating on it.

On the positive side of the technique, it surveys the entire roof as the camera operator pans the roof surface. This means that even small localized areas of subsurface moisture can be found. Infrared surveys are very effective on large roofs, since the operator is “scanning” the roof and only needs to “look closely at hot (suspected wet) spots.