Ice dams in Manchester
一年 97 天結凍後回溫、年積雪 113 cm——屋頂融雪流到屋簷重新結冰，水往上倒灌到瓦片底下; 年積雪 113 cm——低坡度段與遮陽棚要承受的是雪的重量.
一年 97 天結凍後回溫、年積雪 113 cm——屋頂融雪流到屋簷重新結冰，水往上倒灌到瓦片底下
年積雪 113 cm——低坡度段與遮陽棚要承受的是雪的重量
屋齡中位 1966 年——多數屋頂已經換過至少一次，接縫與收邊的做法是上一代的
Manchester crosses freezing between night and day on about 97 days a year, and each of those cycles melts a little roof snow and refreezes it lower down.
How the damage actually happens
Heat leaking into the attic warms the middle of the roof deck while the eave, which overhangs unheated air, stays below freezing. Snow melts up high, runs down, and refreezes where the deck is cold. The ridge of ice that builds up holds the next lot of meltwater against the shingles, and shingles are designed to shed water downhill, not to hold it.
Manchester averages about 113 cm of snow a year, and it is the snow that stays on the roof rather than the snow that falls which decides what happens at the eave.
What actually fixes it
- Air-seal the attic floor so heat stops reaching the deck - this is the real fix
- Insulate to the depth the code calls for, after sealing, not instead of it
- Ventilate soffit to ridge so the deck stays close to outside temperature
- Only then consider membrane at the eave, which limits damage rather than preventing ice
Heated cables are the opposite of a fix: they melt a channel through the ice, use electricity every winter, and leave the cause untouched. They have a place on a roof that genuinely cannot be ventilated, and nowhere else.
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