... building code max. (R-50)???
Anytime snow melts from a roof due to lost heat from the interior, or sun shining on the roof, but air is below freezing, icecycles will form. The real danger here is ice cycles that can fall when thawing and kill in extreme cases, AND ice dams can form forcing ice back up under shingles damaging roof. That is why it is often a good idea to install heat tape under the edge of roofs in areas that are prone to heavy freezes and snow.
The roof can also be heated enough to melt snow/ice by the sun. I wouldn't assume it's being caused by heat escaping from inside. That said, just because you have R50 doesn't mean it's perfect - it's not. Some heat will still escape. Prefect insulation is not feasible.
"R" I think stands for "Resistance" factor against heat loss. It resists the heat transfer but doesn't stop it. There is no such thing as perfect insulation. Snow itself is a rather good insulator. So even though you are only losing very little heat, the snow on top of the snow directly on the roof helps trap that heat at the bottom. So the first layer will melt.
All the answers are OK, BUT nobody has said anything about the over hang of the house. This is not insulated and when the water runs off it comes to the roof edge and then freezes. To get away from this do what I did and move to Florida, If that is not possible get the special heat tapes for roofs and install in a "V" configuration and that will help a hole lot. Hope that this helps you out.
you need ventilation. the insulation is to keep heat i the living area of your home, your attic should be as cold as outside. make sure you have good soffit vents(intake) and good ridge or roof vents(exhaust). if you already have proper ventilation holes then make sure nothing is blocking them. many times folks push the insulation too far to the ends, blocking the soffit vents. you're loosing heat from your home into the attic which melts the roof snow and the outside temp then quickly turns this into ice. proper insulation as well as ventilation work hand in hand to combat this.
