The easy answer to preventing ice dams is to keep the roof free of anything that will cause one, but do so in a safety conscious way. Do not risk serious injury, or worse, by getting on the roof. There are snow rakes available for consumer purchase. Most ice dams will form along the lower edge of the roof, so keeping as much of this clear as possible will help prevent one from forming.
In addition, there are possibly, or even likely air leaks which need to be sealed to prevent from reaching the roof. Keeping the roof cold, not warming it, will help, with the underside not approaching the freezing point, under 30 degrees being preferable. Keep the attic ventilated.
If you have a furnace in the attic, it may not be possible to prevent ice dams. Increasing the insulation in the attic, however should help, with the first area to insulate being the areas between the roof rafters. You can find out how much insulation your attic needs from your local building department, or perhaps a contractor. While adding insulation, be sure to leave an airspace between the roof deck and the insulation. This helps ensure condensation buildup from happening, which can possibly wear away at the roof deck. Prior to insulating add a layer of polystyrene rafter air channels, then add the insulation over the heating ducts. This will help minimize the amount of warm air allowed in to the attic. Also, insulate any hatchway in the attic.
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