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.
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.