A Reminder to Cut the Roof

Stuart L. Raney Level III Certified Infrared Thermographer

It is a typical roof inspection using an infrared imager to locate hidden moisture. The roof is in pretty good shape and no exceptions have been located on the first two sections. Walking across the third roof section, the first exception is spotted. It is a small one, roughly 2’ x 2’, and appears to be half of a 2’ x 4’ Perlite board.

Stepping in the middle of the exception reveals the softness created when board type insulation becomes wet. With a small exception like this, it is tempting to mark it and move on to the next, but first let’s check it with our capacitance meter. Sure enough, the meter pegs the needle, but to make sure we whip out the pin-type moisture meter. Inserted into the center of the area, it also pegs the needle. So now we have a footstep and three advanced pieces of technology that all agree the roof is wet, or do they?

The footstep only tells us the roof was slightly softer in that area. The infrared imager only reports that the radiated energy was slightly higher. The capacitance meter only reports that the electrical impedance of the area is different from the area around it. The pin-type meter only reports that it encountered a different electrical resistance.

In order to confirm the presence of moisture we take a core sample of the roof. What we found was a piece of sheet metal laid below the membrane, apparently to cover the opening left by an old vent pipe that had been removed. The metal changed the radiated energy seen by the imager, the impedance seen by the capacitance meter, the resistance seen by the pin-type meter and small hole in the deck changed the firmness felt by the footstep. All these were good reasons to suspect a wet area but none good enough to verify one, even when all four agreed.

This is an old tip, but one worth revisiting. This exception was actually encountered on a recent inspection and could have been misinterpreted had the roof not been cored to confirm or deny the other results.

Perhaps a good way to understand the importance of core cuts is to realize that the visible evidence of a core is the only method of investigation that determines if a roof is wet or dry. Infrared imagers, nuclear gauges, capacitance meters and even pin-type resistance moisture meters can only be used to narrow down areas of the roof and limit the number of cores that must be taken. So if you are in the business of roof moisture surveys, your primary tool is a core cutter. You just use the fancy equipment to tell you where to do the real work.

First published as an Tip of the Week for August 31, 2009