When a Roof Rake Is Worth It for Ice Dams
What raking does
A dam is built from the snow on the lower few feet of roof, above the eave. Pull that snow off and there is nothing to melt and refreeze there. It does not fix the attic, but it is cheap, works the same night, and needs no contractor.
Where it is worth owning one
This site suggests a rake where a city averages 20 inches of snow or more a winter and is at least moderate on a typical house: 1,208 of the 3,206 cities. 624 of those are only moderate, where a rake after the biggest storms may be all that is needed; the snowiest of them are Boulder, CO (93 in of snow, 11.4 dam days), Longmont, CO (93 in of snow, 11.4 dam days), Louisville, CO (93 in of snow, 11.4 dam days). Below 20 inches, storms are rarely deep enough for the snow left on the roof to matter.
How to do it without making things worse
Work from the ground with a long-handled roof rake, clearing the bottom three or four feet after any storm that leaves more than about six inches. Leave an inch or so of snow on the shingles rather than scraping them bare, which tears granules off. Stay clear of power lines, and never climb onto a snowy or icy roof. If a dam has already formed, raking the snow above it still slows it down; chipping the ice itself damages the roof. See the safety notes.
When raking is not enough
If you rake and still get dams, the heat is coming from the attic. That is the case for air sealing, and the reason the snowiest towns are not always the worst, covered in why snowfall misleads.