How to prevent ice dams!
To prevent an ice dam, don't heat the roof, keep it cold. That way, the snow on the roof eventually dissipates without making large amounts of meltwater. The underside of the roof deck should not exceed 30 F. The best way to maintain low temperatures is by ensuring that there is adequate insulation and sealing gaps that let warm air pass into the attic from the house. The attic must also be ventilated, so that cold air is introduced into it and heated air escapes rapidly. Some remodeling contractors
are under the impression that heat passing through the attic helps prevent ice dams, when just the opposite is true. Although excess heat moving from the attic through the roof rapidly melts snow, once the meltwater touches the cold eaves, it quickly freezes and forms an ice dam.
If you have a furnace in the attic, it may not be possible to prevent ice dams. Increased insulation, however, should help. First, insulate the areas between the roof rafters. It is important to keep an air space between the roof deck and the insulation in order to prevent a condensation buildup that can delaminate the roof deck. Prior to insulating, install polystyrene rafter air channels, which are available at home centers. Next, lay unfaced insulation batts or blankets over the furnace's heating ducts to help reduce the heat buildup in the attic. If there's a hatchway into the attic, build a cover for it out of rigid poly-styrene insulation.
If the gable and ridge vents do not generate sufficient air movement to dissipate the heat, you will need a motorized vent at one end of the attic to exhaust the heat, and an adequately sized vent on the opposite end of the attic to draw in cold air from the outside.
What to do if you get an ice dam
Homes with ice dams on the roof also often get parallel lines of moisture on the ceiling. The dark lines in the ceiling are called shadow lines. Although there may be insulation on the attic floor, the bottom chords of the trusses are usually not covered. They are exposed to the low winter temperatures and act like a thermal bridge to the ceiling below. This creates a cold strip on the ceiling on which condensation forms. Over time, this moisture traps dust and results in mildew growth, which shows up as shadow lines. First, clean off the mildew with a solution of 1 quart of bleach and 3 quarts of warm water. Rinse the surface with clear water, then let it dry.
If the surface has been stained from the mildew or is otherwise discolored, it will need to be painted. Apply a stain-blocking primer before applying a topcoat.
The cracks in the ceiling/wall joints are the result of truss uplift during the winter. The cracks could have been prevented by using brackets that allow the truss to lift without disturbing the drywall. If your home wasn't built with those brackets, the best way to deal with the problem is with a molding that is wide enough to cover the cracks. Nail it to the truss so it is free to move up and down.