Ice at the eaves
Every February, ridges of ice build along the eaves of houses across this region and water finds its way into ceilings that were dry in January. It reads like a roofing failure. It is usually a heating and ventilation failure that the roof is reporting.
The mechanism, plainly
Warm air escapes from the living space into the attic. It heats the underside of the deck over the heated part of the house, and the snow on that part melts. The meltwater runs down the slope until it reaches the overhang — which hangs past the walls and stays at outdoor temperature — where it refreezes. That ridge grows, and the water backing up behind it eventually sits deep enough to run uphill under the covering.
Which is why the roof is not the culprit. Shingles are designed to shed running water, not to hold back standing water pushed backwards. No covering on the market survives that condition indefinitely, and replacing the covering without changing the attic reproduces the problem on a newer roof.
What actually changes it
Two things, in order. Air sealing between the living space and the attic — the gaps around light fixtures, the hatch, the top plates and the chimney chase that let heated air up in the first place. Then balanced ventilation: intake low at the eaves, exhaust high at the ridge, sized to each other so outdoor air moves through and keeps the deck cold. Exhaust alone makes it worse, because it pulls more heated air out of the house.
On the older stock in this region — three-deckers, mill-era two-families, anything built before ventilation was standard practice — the honest answer is often that intake and exhaust have to be created rather than corrected. That work costs a fraction of its later value on the day the roof is already open, and a substantial amount at any other time.
What belongs in the covering itself
An ice barrier membrane at the eaves is required by code here, and the minimum is measured inward from the edge. Treat that minimum as a floor rather than a target: on a shallow pitch with a deep overhang, ice can build past where the code run ends. Ask for the coverage in feet, not as a yes or no, and ask what it costs to extend it — the answer is usually small enough that the question settles itself.