I have a metal edging on my roof in an area about 4 wide. Can I run heating coils over this metal section of the roof,because it seems to be an area that has ice dams. Presently I have a coil above the metal area, but thought that was the only possible venue.
Heating Coils on a metal roof section
roof
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I'm a contractor in Maine and we certainly have a lot of experience with ice dams. An ice dam can be the quickest way to force water under the roof shingles even on a roof in good shape.
I have had mixed results with heat tapes. When they are used properly, they can be somewhat effective. Most folks expect to see a nice bare zig zag pattern along the edge of the roof with the ice gone around the tape. Not how it works!
If you carefully read all the directions on many of the competing brand heat tapes out there, you will see that they do not claim to remove ice dams. They are designed only to create a small open tunnel under the ice dam to allow water that is trapped above the dam to drain off the roof. This is great in theory but here is where the reality sets in. If the tape does not extend slightly beyond or below the drip edge of the roof, it won't work. If the top edge of the tape does not extend above the ice dam, they will not work. If the gutters fill with ice and bridge the ice to the roof, they won't work.
Many of the tapes instruct you to only operate them when outdoor temps are 30 degrees and above. I have found that they are ineffective in temps much below 20 degrees. They also caution not to let them run continuously.
Running them in the bottom of a gutter is almost a complete waste of time and money. Since they only are capable of melting a small area around the tape, a gutter full of ice will stay full of ice! If the downspout is blocked with ice, forget it!
What I have also found is that folks get a false sense of security when they have them installed and neglect to remove the snow from the bottom 3 to 6 feet of their roofs, figuring the tape is a substitute for roof shoveling. Obviously, when they are installed, shoveling or using a roof rake gets a little tricky, so as not to rip the tape right off the roof.
I guess you can tell I am not a fan of these heat tapes. One method I have found to be extremely effective with dealing with an ice damn are nylon stockings or pantyhose (cut into two sections) filled with salt, calcium chloride or potassium chloride placed over the dam. They last several days and melt slowly and make a nice wide valley in the ice. I have seen hundreds of gallons of water pour off from behind an ice dam using them. Chopping the ice dam can damage the shingles and can be dangerous, but the filled stockings are fairly quick and easy.
If you have plants below or are concerned about salt staining your roof or siding etc, I highly recommend Potassium Chloride which will not harm plants, grass etc. You can usually find potassium chloride at your local hardware store.
I can't take credit for discovering this method, but thankfully I learned it from a tip on This Old House many years ago. I usually make up dozens of them in advance, so I am ready for all the calls from folks who wait too long and call to see if I can come fix the water leaking through their ceilings!
Ice melting cables are just a stopgap. You should fix the real problem. The easiest way to get rid of ice damming is to improve the attic ventilation.
Assuming you have existing vents, the biggest culprit is usually the eave venting. Check your eave vents to make sure they are clear. If so, you need more of them. If not, clear them and then add more.
If you have modern soffit with perforated aluminum panels, pull some of them out and make sure that they are not screwed on to solid wood of plywood soffits (we see this all the time). If so, cut holes in the ply or wood and then reinstall aluminum.
If they are installed directly to rafter tails or the like, look up inside and make sure that there is a free passage way from the eave up over the wall and into the attic space. It's common for overenthusiastic insulators to wedge insulation into that joint, thus effectively preventing airflow from the eaves into the attic space. This is vital for proper ventilation, which eliminates eave icing entirely.
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Best Answer
You mean you have heating coils further up the roof - toward the peak - from the metal? That's not a good idea, since it melts the snow from the bottom up, and the water from that melting runs down onto the metal & freezes there (that's the coldest part of your roof, because the overhang isn't heated by the house's interior). The result is ice damming.
It's also not a very good idea to add heating coils on top of the metal. If for some reason they happen to not function (say a power outage or a switch failure), they'll get torn off by the sliding snow/ice. A better idea is to heat from inside the soffit, allowing that heat to rise through the metal roof.
If that's aluminum, you really should coat it with something; aluminum is hydrophilic, meaning water likes to cling to it either as water or as ice. I'd love to see you replace it with copper, which isn't nearly so hydrophilic.