Heat tape used on roofs and gutters to prevent ice buildup is called "heat cable." The term "heat tape" is another product—an insulated electrical wire that is flat versus rounded and usually applied to water pipes to keep them from freezing and bursting. Heat cable is a similar product, but it is rounded, sturdier, and designed for installation on the edge of your home's roof near the eaves. Sometimes the term is used interchangeably, but there is a difference.
In cold climates that experience snow in winter, heat cables can prevent ice buildup along the eaves and inside the gutters and downspouts for roofs of all types—metal, shingle, rubber, tar, wood, and plastic. Heat cables can also get very hot, from 150 F to 500 F.
Ice buildup can cause significant damage—both to the roofing system and through water that backs up under the shingles and flows down into the home. Heat cable is easy to apply and install, but preparation and prior understanding are essential.
The Physics of Ice Dams
Ice dams occur because snow and ice on the roof melt due to the home's interior heat. When this mass travels down the roof, it freezes again as it reaches the colder exposed overhangs or eaves.
Over time, the frozen melt-off can build into a dam barrier that causes ice and water to back up under the shingles. Inside the home, the dripping water can damage the interior ceiling and wall surfaces. Additionally, the sheer weight of ice dams may damage the roof overhangs and gutters.
Before You Begin
Heat cable is applied by looping it back and forth in a zig-zag pattern along the edge of the roofline and gutters. When plugged into an electrical supply, the cable warms up enough to prevent melting snow from freezing when it reaches the overhang area of the roofline. Rather than freezing and causing an ice dam and the ensuing water back up under the shingles, the water drips to the ground.
Heat cable is sold in various lengths, ranging from 30 to 200 feet. Heat cables have 3-prong grounded plugs for connecting directly to outdoor electrical outlets. Do not use extension cords with heat cables. For safety reasons, heat cables must be plugged directly into a GFCI (ground-fault circuit interrupter) receptacle. If your outdoor outlet is not GFCI-protected, replacing it with a new GFCI outlet is easy. This should be done before you begin the project.
Heat cable comes in two types: standard and self-regulating. Standard heat cable is also called "constant wattage" cable. It plugs in and generates the same amount of heat—at total capacity—regardless of outside temperatures. If you're using constant wattage cable, it works best to turn it on for about an hour or two before the snow starts falling. This helps warm the roof so that the snow will melt immediately as it falls.
Self-regulating heat cable is variable, taking into consideration the ambient temperatures and the surface temperature of the material the cable is touching. Self-regulating heat cable is the best cable for roofs since it draws more energy to heat the cable when it's colder and consumes less energy when it's warmer.
It is also essential to ensure the heat tape you use is "UL-listed." This listing indicates that it has been tested by Underwriter's Laboratories, an independent, non-profit testing laboratory that reviews a wide range of products for safety and quality. Manufacturers who avoid UL listing usually have substandard products that are more likely to fail.
If the heating cable is installed incorrectly or if the wrong type of product is used, it could create a fire or electrical shock hazard. This is also true for heat cable that is old and deteriorated. If your home has a heat cable installed before you move in or is more than five years old, it is a good idea to have it checked for proper function. You might want to replace the old cable with a new one to ensure safety.
How to Install Heat Cable on Your Roof
Equipment / Tools
- Extension ladder
- Tape measure
- UL-listed heat cable with roof clips
Measure for Cable
The first step is to determine how much heat cable you need. There are several steps and calculations in this process.
Begin by measuring the length of your roofline. Next, measure the depth of the eave overhang from the edge of the roof (not the gutter) straight back to the outside wall.
If the eave overhang is 12 inches deep, multiply the roof-line measurement by 4. If the eave is between 12 and 24 inches deep, multiply the roof-line measurement by 5.3. If the eave depth is between 24 and 36 inches, multiply the roof-line measurement by 6.8.
Now, measure the length of each downspout and add this measurement to the previous computation. If a downspout is not at the end of a cable run, double its measurement here—the cable will need to go all the way down and then back up inside the downspout.
Lastly, measure the distance between the edge of the roof, where you will start the heat cable, and the electrical outlet where you will plug in the cable.
Add up all of these dimensions. This is the length of the heat cable you will need.
Begin Routing Cable
Route the cable from the electrical outlet to the starting point on the roof. Clip the starting point of the cable to a shingle that is slightly further up the roof than the outside wall. This will ensure that the cable will completely cover the roof overhang.
Secure the cable to the shingle with one of the provided cable clips.
Create the First Loop
Run the cable back down to the gutter at an angle, and form it into a loop. Using one of the provided eave clips, attach it to the bottom edge of the last shingle. The cable should form a loop that extends partway into the gutter.
Continue the Zig-Zag
Run the cable back up the roof in a zig-zag pattern, creating a triangular shape that is about 15 inches wide. At the top of each triangle, bend the cable and secure it to a shingle with the shingle clip.
Repeat the same zig-zag pattern as many times as needed to cover the entire perimeter of the roof.
Run Cable Through Gutters and Downspouts
Once you have reached the end of the roofline, you can begin laying heat cable into the gutter itself. Here, the cable can be applied to the bottom of the gutter or suspended with hangers or S-hooks attached to the looping portions of the heating cable.
When you reach downspouts, feed the cable in a loop down into the downspout, extending the loop as far down as possible.
Continue installing the gutter cable back to the cable's starting point, then thread the end of the cable down through the last downspout to the outlet.
Plug the cable into the GFCI outlet and make sure the heat cable is working correctly. Once the installation is correct, leave the cable unplugged until it snows. To save energy, keep the cable unplugged when there is no snow or ice on the roof.
When to Call a Professional
The prospect of working many feet above the ground on an extension ladder in tall homes with high roofs may be unnerving. If you feel uneasy about heights, leave the work to a professional. A nervous DIYer is an accident-prone DIYer. A roofing specialist or experienced handyperson can install heat cables at an affordable price. Professional heat cable installation is also a good thing to consider if you already have new gutters or new roofing installed.
How much does it cost to run roof heat cable?
Typical heat cable uses about six to nine watts per foot per hour, so a constant wattage cable that's about 100 feet long running for 24 hours a day for one month would cost about $40 to $60 in additional energy costs.
Should you leave heat cable plugged in?
Roof heating cables should only be left on overnight when the temperatures are lower than 40 F or in icy conditions. Avoid leaving heat cables plugged in 24 hours a day, although you can if you're hit with a big winter storm.
How long does heat cable usually last?
Heat cable last about three to five years; the more you use it, the shorter its lifespan.