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Venting a "hot roof"

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I have a 1 1/2 style cape cod house. I have a problem with Ice dams. The primary culprit is that the top section of the roof is considered "hot" (no venting is going on. Only 4 inches of insulation (4" trusses) between the outside and inside of the house. I'm going to reroof the house with asphalt shingles. Someone recommended to remove the turtle vents and use a ridge vent. In order to get complete circulation of the roof (especially over the "hot" section, add 1" x 4"'s from top to bottom, and add on another layer of decking, thus allowing air to move all the way to the top of the ridge. My question is whether or not a 1" space will be sufficient to get the air to circulate?
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