I added an addition several years age and now have eaves that form ice dams I understand that ventilating the attic may help but that is not an option. I have tried heated wires on the lower edge of the shingles but they are not durable and very short life.
I am looking for ways to do something from the exterior side of the roof and considering galvanized or aluminum flashing at the lower 18" of the shingles. Any thoughts or ideas?
The way to prevent ice damning is to prevent the snow on the roof from melting in the first place and that means adequate insulation and venting in the attic. If the insulation of your home is not adequate then warm air from the living space fills your attic and melts the snow on the roof, the melted water then run down the roof where it freezes over the eves because the eves are not warmed from below.
Ice-and-water barrier or heat cables do not address the reason for the ice damning in the first place. Ice-and-water barrier will minimize damage that can be caused by the ice and the heat cable can melt the ice ( or prevent it from becoming so thick it does cause damage.
You have to prevent the melting, anything else does not fix the problem.
This is not the best answer, nor is it an answer for everybody. However, depending on the situation, it can eliminate the problem until you are able to properly solve it.
If you remove the snow from the roof after every snowfall, then you'll never have problems with ice dams. I know this is not practical for most folks, but it may be a temporary workaround.
You should be able to find long handled roof snow rakes, or roof snow removal systems, at your local hardware store.
Most houses in the far north have steeper roofs than the standard 4/12 pitch. They also install sheet metal on the last 2 feet or so of roof edge. This prevents the snow from building up on the eave. It slides off before it can melt and refreeze. This can be a problem if you have a porch or sidwalk there.
In the future when you re-roof the house you might consider metal roofing to eliminate this problem.
Good luck with your project!
You dismissed attic ventilation as not being an option, but I think you need to know it may not be as difficult as you think.
Basic roof venting is comprised of both intake and exhaust vents. The ratio is 1:1, 50% intake and 50% exhaust. Intake happens down at your eves, and through convection the heat rises and goes out your exhaust vents near the ridge (top) of your roof. Most of the time, the problem with ice dams comes from the lack of intake vents. The most common intake is vented aluminum/steel/vinyl soffit, followed by wood soffit with holes drilled in it and covered with vent grills. Adding these intake vents at ice dam areas is the typical solution, but there are more solutions that you probably don't know about.
If the exhaust net free area is too small, then you can add a shingle over ridge vent. If you have a hip roof and don't have much horizontal ridge lines, you can also use hip vent. Alternative solutions for adding exhaust vents is with Omni-Wall vent for where the roof meets the wall and you can't put a regular vent. Adding regular roof louvers is the obvious option, but like I said earlier, exhaust is rarely the problem with ice dams.
Alternative solutions for adding intake vents in places other than at the soffit are vented drip edge and shingle-over edge vent. Either of these options should work.
