Why ice forms at the gutter
Ice dams form when heat escaping from the house warms the upper roof, snow there melts, and the meltwater runs down to the cold eaves, where it refreezes. The ice builds up at the roof edge and in the gutter, and the next meltwater backs up behind it, under the shingles and into walls and ceilings. Frozen gutters and downspouts make it worse, because meltwater has nowhere to go. See ice dams and gutters.
University extension services, including the University of Minnesota, are clear that the root cause is heat loss into the attic, and the lasting fix is air sealing, insulation and ventilation. Heat cables treat the symptom.
What heat cables do
Heat cables are electric heating elements designed for outdoor use on roofs and in gutters. They are usually laid in a zigzag pattern along the lower edge of the roof, run along the bottom of the gutter and down inside each downspout. The goal is not to melt all the snow but to keep a channel open so meltwater can drain out through the gutter and downspout instead of refreezing and backing up.
They work best as a targeted fix for problem spots: a north-facing eave that always ices up, a valley over an entry, a low-slope porch roof or a downspout that freezes solid.
Types of cable
- Self-regulating cable: its output changes with temperature, producing more heat when it is cold and less when it is warmer. Manufacturers advertise outputs such as about 5 watts per foot at 50°F and higher in snow and ice. Because it adjusts itself, it is less prone to overheating where cable overlaps, and it can be cut to length.
- Constant-wattage cable: the simple, cheaper kits sold in hardware stores. Output is fixed; they must never overlap or cross, and they tend to use more electricity because they run at full output whenever they are on.
Look for cable listed for roof and gutter de-icing by a recognized testing laboratory, and follow the manufacturer’s instructions exactly.
Running cost
Electricity use depends on cable length, wattage and how long it runs. A rough example: 100 feet of cable at 5 to 8 watts per foot draws 500 to 800 watts, about the same as a small space heater. Left on around the clock for a month, that is roughly 360 to 580 kilowatt-hours. At an electricity rate of about 15 cents per kWh, that works out to roughly $55 to $90 a month. Controls that switch the cable on only when it is cold and wet (temperature and moisture sensors) cut the cost dramatically compared with leaving it on all winter.
Check your own electricity rate and the cable’s rated output to estimate your cost.
Safety
- Plug into a GFCI-protected outdoor outlet, or have an electrician install a dedicated circuit with ground-fault protection.
- Do not use indoor extension cords; use only cords and connections rated for outdoor use.
- Never overlap constant-wattage cable, and keep it away from combustible materials as instructed.
- Use the clips made for the cable; never nail or staple through it.
- Inspect it each fall for cracked insulation, damage from squirrels or snow removal, and loose clips.
- Turn it off when not needed, and follow the manufacturer’s guidance on roof types.
Heat cables and gutter guards
Many gutter guard manufacturers sell or approve heat cables that run under or through their guards. If you have or plan guards, ask whether cable can be installed with them and whether it affects the guard warranty. Some guards trap snow on top, which the cable underneath cannot reach; others let meltwater through more easily. See are gutter guards worth it?
What works better in the long run
- Air sealing the attic floor: sealing gaps around light fixtures, chimneys, plumbing and attic hatches so warm air stays in the house.
- Insulation to recommended levels for your climate.
- Ventilation with soffit and ridge vents so the roof deck stays cold.
- Ice and water shield membrane at the eaves when the roof is replaced, so any backed-up water cannot get in.
- Strong gutter attachment, with hidden hangers about every 2 feet in snow country. See gutter hangers explained.
- Correct slope and downspout size, so water drains quickly before it freezes. See downspout size and placement.
Many utilities and state programs offer energy audits and rebates for air sealing and insulation, which also cut heating bills. That is usually a better investment than running cable every winter.
Heat cables and your roof warranty
Some shingle and metal roof manufacturers have rules about what may be fastened to the roof surface. Clips that attach under shingle tabs or to metal seams may be acceptable; nails or screws through the roofing usually are not. Check your roof warranty and the cable manufacturer’s instructions before installing, and ask the roofer if the roof is new.
When heat cables make sense
- A specific eave, valley or downspout that ices up every winter despite good insulation
- Complex roofs with valleys and dormers where heat loss is hard to fix
- Low-slope porch or addition roofs
- As a stopgap until attic work can be done
- Homes in heavy snow belts such as Northeast Ohio, Milwaukee, Minneapolis and Colorado’s mountains
Signs cable is not working
Icicles and ice forming along the cable run, frozen downspouts despite the cable, or a breaker that trips all point to a problem: a damaged section, a failed connection or a cable too short for the problem area. Test it in the fall before you need it, and replace damaged cable rather than taping it.
Installation
Kits can be installed by handy homeowners on single-story homes, but roofs in winter are dangerous. Installing cable in the fall, before snow, is much safer. Many gutter installers and roofers install cable, and electricians handle dedicated circuits and controls. If you are replacing gutters, it is a good time to add cable to problem areas, because it can be clipped in neatly and routed down downspouts before everything is closed up.