Roof Heat Cable for Ice Dams

There are two kinds of heat cable sold for roofs and the difference is not marketing. Constant wattage cable draws the same power per foot everywhere along its length, forever. Self-regulating cable contains a conductive polymer whose resistance rises as it warms, so each individual foot throttles its own output based on how cold that foot actually is.

Two consequences follow, and the second is the one that should decide your purchase. The first is energy: a self-regulating cable backs off on a mild night instead of running flat out, so consumption tracks the weather. The second is safety. If constant-wattage cable overlaps or crosses itself, heat concentrates at the crossing with nothing to limit it. WarmlyYours states explicitly that the self-regulating design helps prevent overlapped or criss-crossed cable from overheating.

On a simple straight eave that distinction is academic, because you can lay the zigzag without crossings. On a roof with a valley, a dormer, a complicated gutter return or a downspout that doubles back, crossings are hard to avoid — and that is where the extra money buys something real.

Read the output figure precisely. WarmlyYours publishes 5 watts per linear foot at 50 degrees F. That is a reference condition, not a maximum: a self-regulating cable draws more as it gets colder, which is the whole point. When you size the circuit, assume more than 500 watts for a 100-foot run rather than exactly 500.

Everything else on this page applies equally here: it is a plug-in kit needing an outdoor GFCI receptacle, and it manages a symptom rather than fixing the building.

This is the kit most people end up buying, and the specification explains why: 100 feet at a published 5 watts per foot on a standard 120-volt outdoor outlet, arriving pre-assembled with the spacers and shingle clips already in the box.

Those clips matter more than the listing makes them sound. The cable has to be held in a zigzag along the eave at a consistent spacing, and it has to be attached without putting a fastener through the roofing. The clips are the entire mechanism for doing that, and buying a bare cable without them means a second order before you can start.

The thermostat is the other feature worth paying for. A heat cable with no control is a 500-watt appliance that runs whenever it is plugged in, which is how people end up shocked by a January bill. Frost King states the kit switches on and off automatically with temperature.

The honest limitation is that it is constant wattage rather than self-regulating. Every foot draws the same power whether it is buried in an ice dam or sitting in dry air, and — more importantly — if the cable touches or crosses itself, the heat concentrates at that point rather than backing off. Self-regulating cable is designed to tolerate exactly that, and it is the upgrade worth understanding before you buy.

Before ordering any length, read the section on this page about what a heat cable does and does not fix. It manages a symptom. The cause is heat leaking into your attic, and that is an insulation job — the same argument we make about roof rakes.

Length is the specification people get wrong, because a heat cable is not laid in a straight line. It goes along the eave in a zigzag — up the roof, back down, along — and then usually into the gutter and down the downspout. That consumes cable far faster than the width of the house suggests. A 20-foot eave with a modest triangle pattern and one downspout can easily want 60 to 80 feet.

This is the kit for when the arithmetic comes out past 100 feet. Easy Heat publishes 160 feet at 5 watts per foot, so 800 watts total, on 120 volts, with UL and CSA listings and a grounded three-wire plug.

Do the amp arithmetic before ordering. 800 watts at 120 volts is roughly 6.7 amps — close to half of a standard 15-amp circuit, on a load that runs for hours at a time. If the outdoor receptacle you plan to use shares a circuit with anything else that matters, that is the conversation to have with an electrician before the cable arrives, not after.

The six-foot cord is the practical constraint nobody reads. The cable is pre-assembled with the cord at one end, so the run has to *finish* within six feet of a receptacle. On a long eave that dictates which end you start from, and occasionally dictates that you need an outlet added.

One honest note about why you might be reading this entry. If your roofline genuinely needs 160 feet of heat cable, that is worth pausing on. A large heated area is a large heat-loss area, and the money may do more work in the attic than on the roof — the argument is laid out in the body of this page.

Most ice dam problems are not evenly distributed around a house. There is one valley where two roof planes meet and dump meltwater into a cold corner, or one stretch above a doorway where the dam forms every single year while the rest of the eave is fine.

For that, a perimeter of cable is the wrong purchase. This is thirty feet at a published 5 watts per foot — 150 watts, about 1.25 amps — which is small enough to plug in almost anywhere and cheap enough to treat as an experiment.

Treating it as an experiment is genuinely the right frame. Heat cable does not work equally well on every roof, and finding out on one problem area costs a fraction of finding out around the whole house. If the spot stays clear all winter, you know the approach works on your building and can extend it. If it does not, you have learned that cheaply.

The limitation is arithmetic. Thirty feet sounds like a lot until you lay it in a zigzag: a triangle pattern two feet tall over an eight-foot span, plus a run down one downspout, and it is gone. Measure the pattern before you decide this length is enough.

There is also a failure mode specific to spot treatment worth knowing about. Clearing a channel through one section of dam sometimes just relocates where the water refreezes — it runs along behind the remaining ice and forms the next dam a few feet over. On a roof where the whole eave is marginal, spot heating can chase the problem around rather than solve it.

This is the largest kit in the range and it is on this page mostly as a boundary marker. Easy Heat publishes 240 feet and 1,200 watts at 120 volts, which works out to roughly ten amps.

Ten amps is the number that changes the nature of the purchase. A standard 15-amp household circuit should not be loaded above about 12 amps continuously, so this cable effectively wants a circuit of its own. That is a licensed electrician and a panel with a spare slot, and it means the true cost of the installation is not the price of the cable.

The energy side is proportional. Twelve hundred watts is 1.2 kilowatt-hours for every hour it runs. We are not going to invent an electricity rate for you — multiply it by your own and by a realistic number of winter hours, and the figure that comes out is the one that should inform the decision.

There is a real case for it. Some houses genuinely have a long, complex, north-facing roofline that dams every year and cannot be practically re-insulated — a finished attic, a cathedral ceiling, a historic building. For those, one continuous run with no junctions sitting in meltwater is the right engineering answer, and this is that run.

But for most people who find themselves looking at a 240-foot kit, the honest read is different: the reason so much roofline needs heating is that so much heat is reaching it. That is the argument the rest of this page makes, and it is worth taking seriously before spending this much on managing the symptom.