I live in a 14 unit townhouse. And ice dams originate on the two units on the top level: the highest point of the roof. The balance of the pitched roof does not present a problem. There are chase ways in the attic housing exhaust ducts for the stove fan, bathroom fans, and dryer vent. There are eight skylights on the roof, 4 of them in the top units. The chase ways and skylight are insulated, attic insulation is R40 loose fill. The soffits are perforated metal, and there are inlet baffles between the rafters. there are ten 12"X12" vents in this section of roof. There are two other buildings the same size that are not affected by ice dams, and they do not have skylights.
Answer:
No matter how you approach it, the solution here is going to include better ventilation. I appreciate that there are soffit vents and that there is upper attic cavity ventilation but you're telling me that the soffits need more. I don't know how long this has been going on but if it started when the building was brand new, you might have a more serious problem with vapour barrier. Where uppermost walls meet uppermost ceilings, there may be a void in the VB and if that's the case, the void may have to be clad.
