In principle, your roof shingles should be approximately the same temperature as the outside air. Why? Because your shingles are on the outside of the house, specifically outside of the insulation. In winter, if the shingles are significantly warmer than the outside air, your home is losing heat. So what diagnostic tool can we use to determine if we have heat loss? Do you really have to carry a thermometer around your icy roof in January? There has to be an easier way!
After a snow-fall, most shallow-pitch roofs should be covered with an even layer of snow. In below freezing temperatures there should be very little snow melting. Pockets of thawed snow are generally indicative of heat loss.
What about roofs where snow doesn’t uniformly accumulate such as those with steep pitches, or metal? In this case, you can check your eaves and gutters for large icicles. See wiki’s ice-damming article here.
To complicate matters a bit, keep in mind that roof ventilation is designed to lower roof shingle temperature. So while hot patches and ice-damming almost certainly indicate heat loss, the reverse is not always true. A well ventilated roof may mask heat loss problems.
Earlier this week, with the temperatures hovering around freezing, we took a few photos of local roofs. Keep in mind that all of the photos below were taken in the same hour.