Denver winters are famous for their mood swings: a 65-degree sunny afternoon followed by a snowstorm and single digits by morning. The metro sees more than 60 freeze-thaw cycles in a typical winter — among the highest of any major U.S. city — and every one of those cycles works on your roof.
The most expensive consequence is the ice dam. Here's the mechanism: heat escaping from your living space warms the upper roof deck, melting snow even when it's below freezing outside. That meltwater runs down to the eaves — which extend past the heated wall line and stay cold — and refreezes. Over days, a ridge of ice grows into a dam. Water pooling behind it has nowhere to go but backward, up under the shingles, and into your walls and ceilings.
The damage shows up as interior stains that mysteriously appear during warm winter afternoons, icicles thick as your arm along the gutters, and water streaks down exterior walls. By the time you see interior evidence, insulation and decking have often been wet for weeks.
Prevention beats removal every time. The root cause is almost always in the attic: air leaks around light fixtures, chases, and hatches; insulation below the R-49 recommended for Colorado; or blocked soffit vents that prevent cold air from flushing the roof deck. Sealing those leaks and balancing ventilation keeps the entire deck uniformly cold — snow melts from the sun, not from your furnace, and dams never form.
If you already have a dam, one rule matters most: never let anyone attack it with a hammer, hatchet, or pressure washer. All three destroy shingles. Professional steam removal is the only method shingle manufacturers approve — it melts channels through the ice without touching the roofing. When you next re-roof, make sure ice-and-water shield membrane extends at least 24 inches past the interior wall line at every eave; it's cheap insurance and part of Denver's adopted code for good reason.