Ice Dams are a sure sign of a building not functioning properly. They are sort of like a big leg cast on someone hobbling around on crutches: You know happy, healthy people aren't supposed to walk like that you're just not sure how they got that way. Ice Dams are caused by warm air coming in contact with the underside (sheathing) of the roof and melting the snow that sits on top of the roof.
The warm air can get to the underside of the roof deck through inadequate insulation (conduction) or through heated air leaking up from below (convection). Actually it's not the hot air rising so much as the denser/colder air sinking that drives this but that's just splitting building science hairs.
In my experience looking at numerous ice dams in lots of different types of buildings it's almost always caused by air leakage. Occasionally there's just not enough room between living surface and roof sheathing and the heat just moves directly through the ceiling material.
Living in New England I often see people putting electrically heated wires onto the roof to melt the snow before it can freeze which is not fixing the problem it's just an expensive band aid.
Often people will recommend adding venting to cool the surface of the roof sheathing to prevent ice dams (I'm looking at YOU roofers). While this can help if there is not much heat being added from the living area and can be very important in moving moisture out of an attic, I believe it is better to keep the heat and moisture out of the attic to begin with! This is your heat and your comfort and your money flowing through those holes. Sometimes the problem is easy to spot as when there are un-insulated heating ducts running through the attic:
Sometimes it's a bath fan not being vented directly to the outside or leaky recessed lights. Sometimes in older homes, the interior partitions will not have any type of top plate on them as they were built differently back in the day so every interior wall below the attic is dumping warm air into the attic in the winter. This is why air sealing is so important. This is also something that you can do yourself if you are inclined before adding more insulation.
Air Sealing an attic space
Article on drafty attics
Fine Homebuilding article on Ice Dams