Roof Deicing with Heat Cable part 1
To effectively prevent ice dams, roof heat cables should extend at least 6 inches beyond the plane of your exterior wall. This ensures the cable crosses the “origin zone” where ice dams typically form—right above the top plate where your heated attic space ends and the cold overhang begins. By creating a melted pathway through this critical zone, water can safely drain off your roof instead of refreezing at the eaves.
What Causes Ice Dams?
As a refresher, here’s how ice dams are made:
Warm air escapes a heated area of the home interior, warming the roof deck. This melts the snow above, which sends meltwater down the roof to the unheated eave, where it refreezes to form an ice dam.
The University of Minnesota Extension explains the cycle clearly: “The snow on a roof surface that is above freezing will melt. As water flows down the roof it reaches the portion of the roof that is below 32°F and freezes. This causes the ice dam.”
In short, warm attic air heats the roof and melts snow, while the roof edges remain cold. This temperature difference causes melted snow to refreeze at the eaves, forming an ice dam that can trap water behind it. Without intervention, this trapped water can work its way under shingles and cause significant damage to your roof and home interior.
The origin zone of ice dams is normally located directly above the top of the exterior wall—exactly where the heated portion of your attic ends and the cold soffit begins.
While proper insulation and ventilation can help reduce uneven roof temperatures, heat cables provide a reliable solution for keeping drainage paths clear in areas prone to ice dam formation.
Why Heat Cable Should Extend a Minimum of 6″ Beyond the Plane of the Exterior Wall
The “6-inch Rule” is simply this: The height of the heat cable tracing pattern—how far up the roof the cable is installed—is determined by the “Origin Zone” of the ice dam. The location where an ice dam originates is typically above the top plate of the exterior wall, where the heated portion of the attic ends and the unheated soffit begins.
This is exactly why most heat cable manufacturers mandate the tracing height goes at least 6 inches higher than the plane of the exterior wall below. The idea is to create a melted pathway through the origin zone, thus allowing meltwater from the roof above an escape path off the roof and into the gutter.
If your heat cable doesn’t extend beyond this origin point, it won’t create the melted pathways needed for meltwater from higher on the roof to escape. Instead, that water will simply refreeze right where the ice dam would naturally form—defeating the entire purpose of the heat cable system.
Another Perspective: The Magic Line
Here’s the anatomy of the standard stick-framed roof-to-wall intersection:
There is a critical reference point referred to as “The Magic Line,” which is the vertical plane extending upwards from the building’s top plate. The heat cable installation must extend above this line at a minimum.
The structure consists of a ceiling with insulation above it, between the rafters. The rafters support the shingles on the exterior of the roof. This entire roof structure rests on the top plate of the wall. The overhang of the roof creates a soffit. A gutter is attached at the edge of the roof to collect runoff. The heat cable is installed on top of the shingles in a zigzag pattern, extending from the gutter up the roof—and critically, past that Magic Line.
How Do You Identify the Correct Placement for Heat Cable?
Identifying proper placement involves locating the vertical plane above your home’s exterior wall. This imaginary line rises up from the top plate of the exterior wall and intersects the roof—essentially marking where the heated part of your attic ends.
Your heat cable pattern should cross above this line by at least 6 inches. In practice, this means:
- Locate your exterior wall: From inside your attic, identify where the top plate of your exterior wall sits
- Visualize the vertical plane: Imagine a line extending straight up from that top plate through your roof
- Extend your cable past it: Your heat cable installation should zigzag up the roof slope until it’s at least 6 inches past this plane
By using the top of the wall as your reference point, you ensure the cable covers the critical origin zone of ice dams. This guarantees that the section of roof where ice dams typically form is kept warm enough to maintain a melted path for runoff.
How Much Heat Cable Do I Need?
The length of heat cable you need depends on your roof’s specific dimensions and layout. Key factors include:
| Factor | What to Measure |
| Eave length | Total linear feet of eaves requiring protection |
| Eave depth | How far your overhang extends from the wall |
| Roof valleys | Any valleys where water concentrates |
| Gutters | Linear feet of gutters needing cable |
| Downspouts | Number and height of downspouts |
Rather than guessing, use our Heat Cable Calculator to determine exactly how much cable you need. Input your specific roof measurements, and the calculator will account for your roof’s particulars—including eave length, gutters, downspouts, and valleys—to output an accurate estimate of cable length needed.
The calculator also provides guidance on the number of clips and accessories required for installation, ensuring you get everything you need in one order.
Ready to Protect Your Roof?
Installing heat cable correctly means following the 6-inch rule to ensure your system actually prevents ice dams instead of just warming your eaves. If you’re ready to get started, use our Heat Cable Calculator to determine exactly what you need.
Have questions about installation? Our team has solved thousands of ice dam problems. Call us at 877-387-4218 or check out our installation guide for detailed instructions.
Looking for professional installation? We can connect you with qualified installers in your area who know how to get the job done right.
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Contact Information:
- Address: 1515 5th Street South, Hopkins Minnesota, 55343
- Phone: 1-877-242-4210
- Website: www.radiantsolutionscompany.com