When a large amount of snow accumulates on a roof, heat loss from insufficient insulation and poor attic ventilation can melt the snow from the underside. As the water reaches the roof edge, where it’s colder, it forms a dam. The dammed water backs up under the roofing and leaks into your home. The best way to prevent thawing from the underside is to maintain a cold roof.*
If you currently have ice dams which are causing leaks into the house your best bet is to either remove as much snow from the lower 2-3′ of your roof as you can (this is very labor intensive and in some cases dangerous!) or create a “salt sock” using salt and usually some pantyhose and lay it across the ice dam near a downspout to breack the dam and let the water flow through.
Unfortunately there is little we roofers can do with these leaks while the snow is on the roof. (other than remove some snow) We can provide you with an assessment of your current ventilation, insulation, and ice and water shield and make recommendations for improving these 3 things and also provide you with a cost estimate.