Ice dams
March is typically the snowiest month on the Front Range, and snow on a roof is harmless until heat from the attic starts melting it from below. The meltwater runs down to the cold overhang at the eaves, refreezes, and builds a ridge of ice that backs the next round of meltwater up under the shingles, into the one place water should never reach.
- 1 Ridge caps. Worked loose by a temperature swing that happens every single day.
- 2 The field. High-altitude sun dries the covering out from above, faster than the calendar says.
- 3 Valleys. Where two slopes drain as one, and where meltwater runs longest.
- 4 Flashing. Chimney joints, and the line where the garage roof meets the house.
- 5 The vent boot. Rubber cracks early at this altitude, well before the shingles do.
- 6 Eaves. Ice builds at the edge wherever the attic is leaking heat into the roof.
A heat problem, not a snow problem
The dam forms because the roof over the living space is warm while the roof over the eaves is not, which is a statement about insulation and attic ventilation, not about winter. Homes that dam every year are losing heat into the roof deck; homes next door with the same snowfall and a colder, better-vented attic stay clean.
The lasting fix is air sealing, insulation depth and balanced ventilation. The code-required ice barrier membrane at the eaves is the backstop: it keeps the backed-up water out of the house while the real problem gets solved.
The telltale indoors is a ceiling stain near an exterior wall appearing in late winter: after weeks of freeze and thaw, not on any particular day. By the time the stain shows, the water has been finding its way in for a while, which is why eaves and attic belong on any inspection of an older Front Range roof.
Reading the ice, and what not to do about it
Where the ice forms says where the heat is escaping. A line of icicles running the full width of the house points at general loss: thin insulation, an attic nobody ever air-sealed. Ice over one bay and nowhere else points at something specific underneath it, and the usual suspects are a short list. A bathroom fan ducted into the attic instead of out through the roof. A run of recessed lights. A pull-down attic stair with nothing insulating its hatch, or a heating duct crossing cold space with an open joint. The distinction is worth making before anyone spends money, because the second kind is an afternoon and the first is a project.
What does not help is going after the ice itself. Chipping it takes granules and sometimes the shingle edge along with them. Rock salt in a stocking corrodes the metal at the very edge it is sitting on. Heated cable melts a channel through the dam without touching the reason the dam formed, which on a stubborn north-facing eave that genuinely cannot be fixed from the attic is a reasonable place to end up, and as a first answer means paying for electricity all winter to manage a heat leak that is already costing you the heat.