George from Fort Frances, Ontario has a problem of snow melting off of his new addition. He is trying to pin down the problem because this causes ice formation on the edge of the roof.
He had a ridge vent installed, but never checked to see if his soffit vents worked. A ridge vent, without good soffit venting will not vent an attic. In fact it can increase problems of snow blowing in through the ridge vent. When the soffits are open all along the edge of the roof, the wind will force its way up through the soffit vents and out the ridge vent -- then you have one of the best roof ventilation systems.
The photo shows a cross section of an ice dam and how it is formed by heat from the house melting the bottom of the snow pack. This then freezes on the cold outer portion of the roof.
The photo is from one of my favorite Reader's Digest books -- How A House Works.