An ice dam forms when snow melts high on a warm roof, runs down, and refreezes at the cold eave. The water then backs up under the shingles.
Why the roof is warm
Air leaking out of the house — around lights, hatches and top plates — plus thin insulation. Both are attic problems, not roof problems.
Where heat cable belongs
As a stopgap on a roof you cannot fix yet, not as the fix.