Roof Heat Cable Calculator
Work out how much de-icing cable a roof needs, how high the zigzag has to reach to actually work, and what it costs to run for a winter.
How to use this calculator
- 1Measure the overhang horizontally from the outside face of the wall to the fascia - that, with the pitch, is what sets how high the cable must go.
- 2Leave the zigzag height at 0 and let the calculator work it out. Entering a height taken from a product instruction sheet is how installations end up too short.
- 3Include gutter and downspout runs. Cable on the roof alone makes water with nowhere to drain to.
- 4Put it on a thermostat, ideally one with a moisture sensor. The running cost is dominated by hours, not by watts.
How the calculation works
Required zigzag height = overhang x slope factor + 6 in
Cable per foot of eave = 2 x sqrt(height^2 + (spacing / 2)^2) / spacing
Total = eave zigzag + gutter run + downspouts x (height + 1) + lead-in
Watts = total feet x watts per foot- Slope factor
- sqrt(rise^2 + 144)/12 - converts a horizontal overhang into the distance measured up the roof surface
- Height
- How far the zigzag apex reaches up the roof from the eave edge, measured along the surface
- Spacing
- Horizontal distance between successive peaks, typically 15 to 24 inches
- + 1 ft
- Extra cable past each downspout outlet, so the elbow at the bottom cannot freeze shut
The zigzag multiplier is pure geometry: each V spans one spacing of eave using two hypotenuses. At 24 in spacing and 24 in height it is 2.24 ft of cable per foot of eave; taller triangles cost proportionally more.
The height requirement uses the slope factor because the interior wall line sits further up the roof surface than the overhang projects horizontally. On a steep roof that difference is substantial.
Self-regulating cable is quoted at its nominal output at 50 F. Real draw rises in cold weather and falls as the cable warms, so the wattage figure is a design maximum rather than a constant.
Worked example
A 40 ft eave with a 16 in overhang on a 6/12 roof
- 1.A 6/12 pitch has a slope factor of 1.118, so a 16 in horizontal overhang is 17.9 in measured up the roof surface. Add 6 in of margin and the zigzag needs to reach about 24 in.
- 2.At 24 in high and 24 in spacing, each V uses 2 x sqrt(24^2 + 12^2) = 53.7 in of cable to span 24 in of eave - a multiplier of 2.24.
- 3.40 ft of eave is therefore 89 ft of zigzag, plus 40 ft of gutter, plus 2 x 21 ft of downspout, plus 10 ft of lead-in: 181 ft.
- 4.At 5 W/ft that is 906 W, 7.6 A on a 120 V circuit, and 815 kWh over a 900 hour winter - about $150 at 18.44 cents.
Result: About 181 ft, 906 W, and roughly $150 a winter
What a deep overhang costs
- 1.A 30 in overhang on an 8/12 roof: the slope factor is 1.202, so the wall line is 36.1 in up the surface, and the apex needs about 42 in.
- 2.At 42 in high the multiplier rises to 2 x sqrt(42^2 + 12^2) / 24 = 3.64 - well over half again the previous case.
- 3.The same 40 ft of eave now takes 146 ft of zigzag rather than 89.
- 4.Deep overhangs are excellent for keeping rain off walls and expensive to heat, which is one of the arguments for fixing the attic instead.
Result: About 238 ft - the overhang nearly doubles the roof run
The installation that does not work
- 1.The same roof, but the zigzag is run only 12 in up from the eave edge - which is what a 12 in triangle template out of the box produces.
- 2.The wall line is at 17.9 in up the slope. The cable stops well short of it.
- 3.Everything the cable heats is overhang, which was already cold and was never where the melting happened.
- 4.The dam forms just above the cable instead, out of sight from the ground, and the homeowner concludes that heat cable does not work.
Result: 12 in is short of the 24 in needed - the dam just moves up
What heat cable is actually doing
It is not melting the ice. A few watts per foot cannot melt a dam of any size, and anyone who has watched a cable running against a serious ice build-up has seen it fail to. What it does is keep a continuous open channel through the ice, from up the roof, down through the dam, along the gutter, down the downspout and out at the bottom.
That channel is the whole point. Ice dams cause damage because meltwater ponds behind them and finds its way under the shingles. Water that has somewhere to go does not pond, and a dam with a drain through it is largely harmless.
This reframes the design question. The cable does not need to be everywhere the ice is; it needs to trace one unbroken drainage path from above the warm part of the roof to the ground. A gap anywhere along that path - a cold downspout, an unheated gutter section, a zigzag that stops too low - breaks the whole thing.
Why the zigzag has to go so high
Melting happens over the heated part of the house, where escaping heat warms the roof deck. Freezing happens over the overhang, where there is no heated space beneath. The boundary between the two is the interior face of the exterior wall, and it is further up the roof than most people assume.
A 16 inch overhang sounds like a 16 inch problem. But the overhang is measured horizontally, and the roof surface climbs as it goes - on a 6/12 pitch, 16 horizontal inches is nearly 18 inches of shingle. On a 10/12 it is over 20. The cable has to reach past that line, not to it, or every foot of heating is applied to the part of the roof that was never melting anything.
The failure mode when it stops short is quiet and complete. The lower ice clears, the homeowner sees bare shingles at the eave and assumes it is working, and a new dam forms a foot higher up where the cable ends. That dam still backs water under the shingles, and it is harder to see from the ground.
- Roof zigzag — From the eave edge up past the interior wall line, with peaks 15 to 24 inches apart.
- Gutter run — The full length. A frozen gutter under a heated roof is a dam with a heater above it.
- Downspouts — Full height plus a foot past the outlet. The bottom elbow freezes first and blocks everything above it.
- Valleys — Cable up the valley as well - they concentrate meltwater and ice heavily.
Running cost, control, and the case for fixing it properly
A typical residential installation draws somewhere between 500 and 1,500 watts. Left switched on from November to March that is two to five thousand hours, and a bill in the low hundreds every year, permanently. On a thermostat that only closes below about 38 degrees, and better still with a moisture sensor so it runs when there is actually water to move, the hours drop by two thirds or more.
Set against that, air sealing an attic is a one-off cost that runs for nothing afterwards and also lowers the heating bill. Where an attic exists and can be reached, it is nearly always the better investment, and heat cable put in ahead of it tends to remove the pressure to do the real work.
Where heat cable genuinely earns its place is where there is no attic to fix: cathedral ceilings, finished attic spaces, complex valleys between wings, north-facing sections that never see sun. In those cases it is the practical answer, and it is worth doing properly - correct height, continuous drainage path, thermostat control, its own GFCI circuit, and an inspection of the whole run each autumn before it is switched on.
What this assumes, and where it stops
Assumptions
- The zigzag is laid at a uniform height and spacing along the eave, which is how these systems are installed.
- Cable is run in the gutter for its full length and down each downspout with a foot spare at the outlet.
- Self-regulating cable is rated at its nominal output at 50 F; real draw varies with temperature.
- Running hours are the dominant cost variable and are entirely dependent on how the system is controlled.
Limitations
- It sizes cable. It does not tell you whether cable is the right answer, and on a house with an accessible vented attic it usually is not the first one.
- Valleys, dormers, skylights and roof-to-wall junctions all need extra cable that a straight eave calculation does not include.
- Metal roofs, tile and slate all have their own attachment requirements and some are incompatible with clip-mounted cable.
- Electrical work needs to comply with local code and de-icing circuits require ground-fault protection. This is not a wiring guide.
- Constant wattage cable must not be cut, shortened or allowed to overlap. If in doubt about which type is in hand, treat it as constant wattage.
Common questions
How high should the heat cable go up the roof?
Past the interior wall line, by six inches or so. That line is further up the roof than the overhang projects horizontally, by the slope factor - a 16 inch overhang on a 6/12 roof is nearly 18 inches up the shingles, so the apex wants to be about 24 inches from the eave edge. Cable that stops lower heats the overhang, which was already cold, and the dam forms above it instead.
Does heat cable actually stop ice dams?
It does not prevent them; it keeps a drainage channel open through them. That is enough to stop the damage, because the damage comes from meltwater ponding behind the dam rather than from the ice itself. It is a management tool, and on a house with a reachable attic, air sealing is a better use of the money.
What does it cost to run?
Roughly a thousand watts on a typical house, so the bill is set almost entirely by how many hours it runs. On a thermostat with a moisture sensor, perhaps 900 hours and something in the tens of dollars a winter. Switched on in November and forgotten until March, three times that.
Can I cut heat cable to length?
Self-regulating cable, yes - it is sold by the foot and terminated in the field. Constant wattage cable, no: its resistance is set by its length, and shortening it changes the power it draws. Constant wattage cable also must never touch or cross itself, because it cannot throttle back at an overlap and the hot spot is a real fire risk.
Why did my heat cable stop working?
Most often physical damage - cable pulled by sliding snow or ice, or nicked during roof work - followed by a tripped GFCI, which de-icing circuits are required to have and which trips readily on a damaged cable. Self-regulating cable also degrades over years of thermal cycling and gradually puts out less. Test it every autumn before the first storm rather than discovering it in January.
Sources
- Roof and gutter de-icing cable installation guidance — King Electric
- Ice dams - prevention and management — University of Minnesota Extension
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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