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Roof heat cable doesn't stop ice dams; it keeps a drainage path open so meltwater can get off the roof. It goes on in a zig-zag above the eave, out over the overhang and back, then along the gutter and down every downspout to the ground. Measure the eave run and multiply by the factor for your overhang: EasyHeat's ADKS method uses 4 for a 12 in overhang with a gutter, so a 40 ft eave with two 10 ft downspouts needs 200 ft of cable. At 5 W per foot that is 1,000 W, about 8.3 A on a 120 V circuit. Install it on a dry day before the snow, plug it into a GFCI-protected outdoor outlet, and switch it on only when ice is forming.
Measure the length of every eave that ices (usually the ones over heated rooms and below valleys), the overhang depth from the wall to the drip edge, and the height of each downspout. The cable goes up the roof in triangles to a point just inside the warm wall, so the overhang sets how much cable each foot of eave takes.
| Overhang | With gutter | No gutter |
|---|---|---|
| 12 in | 4 | 3 |
| 24 in | 5.3 | 4.3 |
| 36 in | 6.8 | 5.8 |
| 48 in | 8.1 | 7.1 |
| 60 in | 9.6 | 8.6 |
| 72 in | 11.2 | 10.2 |
Worked example: 40 ft of eave with a 12 in overhang and a gutter is 40 × 4 = 160 ft; two 10 ft downspouts add 40 ft, so 200 ft in all and a 200 ft kit. The roof heat tape calculator does this for dormers and valleys too.
Constant-wattage cable such as EasyHeat ADKS must not cross, touch or overlap itself, or it overheats; self-regulating cable is more forgiving, but follow the sheet that comes with the cable you buy.
Plug it in before the first freeze and feel the cable warm along the run. Turn it on when snow is on the roof and temperatures swing around freezing, and off in long dry cold spells. The example's 1,000 W running 12 hours a day for 90 days uses 1,080 kWh, about $187 at EIA's 17.3¢ per kWh winter average.
HomeAdvisor puts cable at $1–$6 per foot and professional installation at $12–$20 per linear foot, so the example is $200–$1,200 in cable or $2,400–$4,000 installed. Our best roof heat cable guide compares self-regulating and constant-wattage cable.
Heat cable treats the symptom. The cure is a cold roof deck: air-seal and insulate the attic floor and balance the vents; see the ice dam prevention guide and the attic ventilation calculator.
Eave length × the overhang multiplier (EasyHeat ADKS: 4 for a 12 in overhang with a gutter, 5.3 for 24 in), plus twice each downspout's height and about 6 ft per valley, rounded up to a kit length. The heat tape calculator does the math.
Yes. After the zig-zag on the roof, the cable runs along the bottom of the gutter and down each downspout, so meltwater has a thawed path all the way to the ground.
No. Use shingle clips that slide under the tabs and gutter clips in the trough. A nail through the cable is a fire and shock risk, and nails through shingles leak.
No. Use a thermostat or controller, or switch it on when snow is on the roof and temperatures cross freezing, and off in long dry cold spells.
Yes. Plug-in cable goes into a GFCI-protected outdoor receptacle with a weatherproof cover, and fixed outdoor de-icing equipment needs ground-fault protection under NEC Article 426.
Reviewed . General guidance, not engineering or electrical advice: a sagging roof needs a structural engineer, and hard-wired de-icing circuits need a licensed electrician.