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Most builders in the snowy North take pains to ventilate their roof systems, even when it's not easy (see "Venting Details for Cathedral Ceilings," page 36). One important reason is to prevent the formation of damaging and dangerous ice at the eaves. Deep snow insulates a roof and traps heat inside the attic, but a wellvented attic will allow the heat out before it can melt the snow. So how can a person determine how much venting it takes to keep a given roof icefree? Code requirements like the 1:300 rule (one square foot of free vent area per 300 square feet of attic floor area) and the 1:150 rule are a good start, but in some