Ice damming is something most of us don’t think of very often. This is when snow gets packed down & forms ice at the gutter. Then snow from the roof melts & runs into the ice. Since the water from melted snow has nowhere to go, it will have no choice but to seep back up thru the shingles, into the roof sheathing & eventually all the way into the drywall inside. If the water gets under the dam itself, large sheets of dangerous ice can slide off the roof & injure a person.
It is important to make sure your attic is ventilated very well. Details that show an insulation baffle between each truss at the top of the wall are critical to ensuring this happens. These “baffles”, which are sometimes Styrofoam, maintain a 1” free air space under the roof sheathing (usually plywood or osb) so that in the winter, the air temp under the roof sheathing is about the same as air outside to help keep snow from melting too fast.
In a roof that is properly ventilated, air should travel freely from the soffit vents up thru to & out of the ridge and gable vents. In the summer, this also helps the cooling bills because the hot air evacuates out the vents up high automatically drawing cooler air into the soffit vents below where it’s shaded. Some other systems can work, but a well-ventilated attic is a good way to keep your roof assembly dry & your utility bills in a more manageable range.
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