If you had ice dams in the past now is the time to act. Ice Dams happen when you have snow on the roof and the heat that escapes form inside your house / attic melts the snow. As snow melts, water runs down the roof, and when it gets to the roof edge (eaves) which are unheated, it once again freezes.
As more and more water freezes along roof eaves, the ice builds up, creating ice dams. As even more water runs down, it now has nowhere to go, so it backs up under the shingles, and leaks into your roof.
The main cause of ice dams is lack of proper attic insulation AND roof ventilation. Learn more about ice dams http://www.roofcalc.org/top-10-ice-dams-prevention-products/