Ice at the eaves
Every February, the same houses on a street grow a ridge of ice along the eaves while their neighbours stay clean. The difference is never the weather — both houses got the same winter. The difference is what is happening under the roof deck.
How the ridge builds
Snow on a roof is harmless until the surface beneath it warms above freezing. Heat leaking from the living space into the attic does exactly that: the snowpack melts from below, the meltwater runs down the slope, and when it reaches the overhang — which is over open air, not over a heated room, and is therefore genuinely cold — it refreezes. Repeat that for a few weeks and the ridge grows tall enough to hold standing water behind it, which then has nowhere to go but sideways and up, under the shingles.
That last part is what does the damage. Shingles shed water travelling downhill; they were never designed to hold back water sitting still. Once it gets past them it reaches the deck, the top plate and eventually the ceiling below — which is why the stain shows up on an interior wall in late winter, weeks after the ice first formed.
Why the fix is indoors
Chipping ice off an eave treats the symptom and damages the covering doing it. The lasting corrections are all on the warm side: sealing the air leaks that carry heat into the attic — around light fixtures, bath fans, the attic hatch, plumbing chases — then insulation depth, then balanced ventilation so the attic runs at outdoor temperature instead of somewhere in between. Houses that get all three stop damming.
The ice barrier membrane at the eaves is the backstop, not the solution: code requires it here precisely because this happens, and it keeps the backed-up water out of the structure while the real problem is dealt with. On a replacement, it is worth asking how far up the slope that membrane runs — the code minimum is a floor, and on a low-pitched roof with a deep overhang, more is cheap insurance.