Three things are required for an ice dam to form: snow, heat to melt the snow, and freezing temperatures to refreeze the ponding water behind and underneath the frozen mass. Ice dams can form when snow or ice accumulates on a roof at the eaves. Heat escaping from the house at the wall-roof interface and heat from warming daytime temperatures and sun melts some of the dammed frozen water which backs up under roof shingles. As a result, the backed up water finds a path through or around the underlayment and sheathing into attic size of ice dams.
More snow and freezing temperatures followed by thaws increase the likelihood and size of ice dams.
Water-stained ceilings, dislodged roof shingles, sagging gutters and icicles, peeling paint, and damaged wall finishes are all evidence of ice dams where water has penetrated the attic and walls.
Air leaks through wire and plumbing penetrations in the ceiling beneath the attic and attic ductwork that isn’t insulated properly will also exacerbate the freeze/thaw cycle effecting buildup of frozen roof masses.
Most of the measures to curtail ice dams are “Do-It-Yourself” projects using materials that are either on hand or very affordable. Projects like reroofing and adding additional attic insulation which may be left to the professionals merely require the correct specifications to be included in the contracts (i.e., self-adhering bituminous flashing at eaves and install/repair baffles).
In cold climate states many roofing contractors will inspect roofs and perform the tasks necessary to eliminate ice dams including removal of ice and snow masses from the eaves. Inspections run $250-$500 and repair costs are based on the scope of the project.