Roof Heat Cable Length Calculator

Enter the eave length, overhang and downspouts. You get the cable length, clips, wattage and operating cost.

How the numbers work

Heat cable on a roof edge is laid in triangles that reach up past the exterior wall line so melt channels start over warm roof. The deeper the overhang, the taller each triangle and the more cable per foot of eave. The calculator uses about 2.0 feet of cable per foot of eave for a 12 inch overhang up to 3.6 feet for 36 inches, which matches typical manufacturer layout charts.

It then adds one run in the gutter, one down each downspout plus a foot to exit the bottom, and 10 feet of lead to the outlet. Wattage times length gives the load, and dividing by 120 volts gives amps. A 15 amp circuit should carry no more than about 12 amps continuous, so long runs need more than one circuit.

Install notes

Use self regulating cable listed for roofs and gutters, plug into a GFCI protected outlet, and run it on a thermostat or controller so it only heats when it is near freezing with moisture present. Heat cable treats the symptom. Air sealing and attic insulation fix the cause of ice dams.

Questions

How much heat cable do I need for my roof?

For a zig zag pattern along the eaves, a common rule is about 2.5 to 3 feet of cable per foot of roof edge for a 24 inch overhang, plus cable for each gutter and downspout. The calculator follows the typical layout in manufacturer guides.

How much does roof heat tape cost to run?

Self regulating cable typically draws 5 to 8 watts per foot. 200 feet at 5 watts is 1 kilowatt, so 10 hours a day at $0.12 per kWh is about $1.20 a day.

Should cable go in the gutter and downspout?

Yes. Run one length down the gutter and down each downspout so meltwater does not refreeze and form a new dam.

Do I need a GFCI?

Yes. The National Electrical Code requires ground fault protection for roof and gutter de icing equipment.