Roof Heat Cable Cost to Run (and What Actually Stops Ice Dams)

Every winter in snowy towns you can see two kinds of houses: the ones with a thick lip of ice and a row of icicles along the gutters, and the ones with a thin black wire zigzagging along the eaves. Roof heat cable is cheap to buy and easy to plug in, so it gets sold as the answer to ice dams. It can help at a trouble spot, but it is also an electric heater that sits outside in the snow, and left on all winter it can quietly add a few hundred dollars to your bill.

Here is what roof and gutter cable actually costs to run, how to use it so it only runs when it is doing something, and the attic work that keeps ice dams from forming in the first place.

The short version

• Most plug-in roof cable is about 5 watts per foot. A 100-foot run costs about $2 a day at $0.17/kWh.
• How long you leave it on matters more than anything else. Storm-only use can cost a sixth of what an all-winter cable does.
• You need roughly 4 to 7 feet of cable for every foot of roof edge you protect, so long eaves get expensive fast.
• Cable melts drainage channels. It does not fix the warm attic that causes ice dams. Air sealing and insulation do, and they lower your heating bill too.
• Use a GFCI outlet, keep the cable out of the bottom of the gutter, and never overlap constant-wattage cable.

What roof heat cable costs to run

Find your real electric rate by dividing your bill’s total charge by the kWh used. We use $0.17/kWh as an illustrative US average, with $0.12 and $0.25 for cheaper and pricier areas.

Cost for one full day (24 hours) with the cable on:

Constant-wattage cable draws its full rating whenever it has power. Self-regulating cable is rated at a reference temperature, and it puts out more heat as it gets colder and wetter, so treat those rows as a floor rather than a ceiling.

There is one more number worth knowing. Because the cable runs in triangles up onto the roof and back down to the gutter, Frost King’s sizing table calls for 4 to 6.8 feet of cable for every foot of roof edge, depending on how deep the overhang is, plus the downspouts. At 5 watts per foot, every foot of eave you protect costs roughly 20 to 34 watts. A 40-foot eave with a 2-foot overhang needs about 212 feet of cable, which is over 1,000 watts.

How you run it matters more than the cable

The same 100-foot, 500-watt cable over a 120-day winter (December through March):

These are worked examples with round numbers. A thermostat that only senses air temperature can’t tell a dry 20°F week from a snowy one, so in a cold place it ends up running most of the winter. Frost King’s own instructions recommend a control that senses both temperature and moisture, so the cable only runs when there is snow or ice to melt, and they say not to run the cable above 50°F. Sensing controls cost more and are usually wired in by an electrician. For a plug-in kit, the cheap and effective control is you: switch it on when snow is coming and off when the edge of the roof and the gutters are clear.

Why ice dams happen

University of Minnesota Extension describes the recipe plainly. You need snow on the roof, a roof surface that is above freezing higher up and below freezing at the edge, and cold weather outside. Snow melts on the warm part of the roof, the water runs down to the cold overhang, and it freezes there. The ridge of ice grows, water pools behind it, and eventually it finds its way under the shingles and into the attic, walls or ceiling.

The warm part of the roof is almost always warmed by the house. Extension names air leaking into the attic as the biggest cause in many homes, through gaps around ceiling lights, bath fans, plumbing stacks, chimneys and the attic hatch. Thin insulation where the roof meets the walls, ductwork in the attic and exhaust fans that end just above the roof add to it. Recessed lights, skylights and complicated rooflines make ice dams more likely.

That’s why heat cable is a stopgap. It keeps a path open so meltwater can drain, but the snow above it keeps melting for the same reason it did before, and you are now paying to heat both the attic and the gutter.

When heat cable makes sense

Cable is worth it in a few situations:

• A known trouble spot, like a valley, a low-slope porch roof under a steeper one, or a north-facing eave that dams every year.
• Gutters and downspouts that freeze solid and overflow, even when the roof itself is fine.
• Cathedral ceilings and other roof shapes where air sealing and insulating from the inside is hard or expensive.
• A rental, or a house you are fixing up, where you need to stop leaks this winter and do the attic work later.

If you are thinking about covering every eave on the house, that is usually a sign the attic needs attention instead.

What to do first

1. Look at the roof the morning after a light snow. Bare patches and melted stripes between the rafters show where heat is escaping. An even blanket of snow means the attic is doing its job.
2. Check the attic floor for the big leaks: the hatch, recessed lights, the bath fan housing, gaps around the chimney and plumbing vents, and open chases. Our guides to air sealing with caulk and foam, recessed light covers and attic hatch covers cover the common ones.
3. Then look at insulation depth, especially out at the eaves where it is often thinnest. Our attic insulation guide explains when it is a DIY job and when to call a pro.
4. Make sure bath and kitchen fans vent outdoors, not into the attic.
5. If you still have a trouble spot, add cable to just that spot, and put it on a switch you will actually use.

What not to do

• Don’t chip at ice dams with a hammer, ice pick or shovel. You will damage the shingles and gutters faster than the ice will.
• Don’t throw rock salt or other chemicals on the roof. Minnesota’s Commerce Department warns that chippers, chemicals and heat can damage shingles, gutters and other parts of the house.
• Don’t climb onto a snowy or icy roof. Work from the ground, or hire someone with the right gear.
• Don’t add powered attic vents to fix ice dams. Both Extension and the Commerce Department say they don’t solve the problem and can make it worse.
• Don’t let constant-wattage cable touch, cross or overlap itself, and don’t lay it in the bottom of the gutter. Frost King requires at least 2 inches between the drip loop and the gutter bottom.
• Don’t use pipe heat tape on a roof or roof cable on water pipes. They are different products with different listings. For pipes, see our pipe heat tape guide.
• Don’t leave the cable plugged in all spring and summer.

Which one should you buy?

If you have an asphalt shingle roof and a straightforward eave and gutter, a constant-wattage kit like Frost King’s RC series is the simple, inexpensive choice. Buy enough length for the zigzag, not just the length of the gutter.

If you have a metal roof, a valley, or a run where the cable might touch itself, a self-regulating cable is more forgiving. Check that the listing names your roof type.

If you need a long run along a big eave, a 200-foot constant kit costs less per foot, but it draws 1,000 watts and should be on a switch.

If you know you will forget to turn it on and off, add a plug-in thermostat. If you’d rather only run it for snow, an outdoor smart plug is the cheaper way to run it.

And if you mostly have snow piling up above the gutters, a roof rake may do more good than any cable.

Our picks

These are good starting points. Read the instructions for clip spacing, layout and approved roof types before you buy, and plug every roof cable into a GFCI-protected outdoor outlet.

Four example homes

These are worked examples with round numbers, not measurements. Your climate, roof and habits will change the results.

The ranch with one bad valley. A single valley over the front porch dams every winter. A 36-foot self-regulating cable, drawing about 350 watts in cold, wet weather, goes on a smart plug and runs for about 15 days over the season. That’s around 126 kWh, roughly $21 at $0.17/kWh.

The north-side gutter. The roof is fine, but one shaded gutter and downspout freeze solid and overflow. A 60-foot, 300-watt Frost King kit runs about 25 days a winter, switched on for storms and off once the gutter flows. That’s 180 kWh, about $31 at $0.17/kWh and $22 at $0.12.

The cold-climate story-and-a-half. Knee walls and a half-insulated slope keep the upper roof warm, and ice dams form along a long eave. 200 feet of 5-watt cable, 1,000 watts in all, sits on a 35°F plug-in thermostat from December through March and runs about 2,200 hours. That’s 2,200 kWh, roughly $374 at $0.17/kWh. This is the house where attic work pays.

The cable nobody unplugged. A 100-foot, 500-watt cable gets plugged in at Thanksgiving and unplugged in mid-April, about 140 days. That’s 1,680 kWh, about $286 at $0.17/kWh and $420 at $0.25. An outdoor smart plug and a few taps around each storm would have cut that to well under $60.

Installing it so it works

• Measure the eave, overhang depth, downspouts and valleys first, and buy for the zigzag, not the straight-line length. Roof cable can’t be cut to fit.
• Test the cable before it goes on the roof. HEATIT, for example, says to uncoil it fully so it doesn’t touch itself, plug it in, and check that it feels slightly warm after about 5 minutes, then unplug it.
• Follow the layout in the instructions. Frost King’s kits use triangles with a 15-inch base that run from the gutter up past the overhang onto the part of the roof above the heated house, held with the supplied clips.
• Run a loop down the inside of the downspout so meltwater has a path all the way to the ground. A cable that stops at the gutter just moves the ice dam to the downspout.
• Keep the drip loop at least 2 inches above the gutter bottom on Frost King kits, and never let constant-wattage cable touch or cross.
• Plug into a GFCI-protected outdoor outlet. Frost King calls for a circuit no larger than 20 amps and recommends against extension cords, so plan the starting point near an outlet. Keep the plug connection out of standing water.
• Check the cable each fall for cracks, crushed spots or clips that have pulled loose, and replace damaged cable rather than taping it.

Troubleshooting

Is fixing the attic cheaper?

Heat cable never pays for itself in energy terms. It adds to your bill every winter you use it. The attic fix works the other way: air sealing and insulation cut the heat that melts the snow and also cut your heating bill, every day of the winter, whether it snows or not.

Take the story-and-a-half above. If an air sealing and insulation job costs $3,000 and lets that household stop running $374 a year of cable, it pays back in about eight years on the cable savings alone. The lower heating bill shortens that. Quotes vary a lot by house and region, so get a home energy assessment with a blower door test first. Many utilities offer them at a discount, and they show exactly where the attic leaks.

Bottom line

Roof heat cable is a reasonable tool for one stubborn valley, a shaded gutter or a downspout that freezes shut. Run it only when snow is on the roof, put it on a switch you will remember to use, and plug it into a GFCI outlet. If you find yourself wanting cable along every eave, the money is better spent in the attic. Seal the attic floor first, then add insulation, or work through the priority checklist for the rest of the house.

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