Heat cables are typically considered to be the last line of defense against leakage to due ice dams. They are usually recommended for hard to ventilate areas and architectural details that channel a large volume of water to a small area (the base of valleys, sumps, between dormer walls etc). If more insulation and better ventilation are not feasible the only option left is to keep the water draining from behind the ice dam and heat cables will do this if installed and turned on before the ice forms. Heat cables must be installed to allow continuous drainage off the roof. If you have gutters and downspouts, this means running the heat cables from the roof continuously through the gutters and downspouts all the way to the ground, unless it’s feasible to leave them hanging over the outside edge of the gutters. This is usually not recommended because the original purpose of the gutter was to divert drainage away from this area.
There are two basic types of cables, plug-in cables that are on all the time and self-regulating cables that are hard-wired and thermostatically controlled. They also have optional controls that sense moisture and temperature and are activated automatically. In either case, the cables should be turned on in late fall and turned off when the snow and ice are gone for good in the spring. Plug-in cables are cheaper but are generally more expensive to run because they are on continuously. The self-regulating hard-wired cables are more expensive partly because you will need a licensed electrician to complete the electrical work. But this option will last longer and typically is less costly to run because of the automatic controls. Note: Heat cables are not a substitute for the proper installation of winterizing membrane on the roof deck at the lower edge of the roof, and the best time to install cables is in the warmer months.