There's a material the roofer can put down that prevents ice dams. It's standard practice where this occurs.

The old timers up here in Vermont have roof rakes. They go out and scrape the first couple feet of snow off the roof edges. That prevents the ice dams.

Ice dams can also occur if the gutters are not cleaned before winter sets in. We had one around the chimney and it damaged the plaster. Turned out that when the roof was replaced, they didn't bother to replace the old flashing.

If you can afford it, have a structural engineer come out and inspect your corners and tell you what caused the ice dams and how to prevent them. Before hiring one, ask if this is the sort of thing they do. Otherwise, a home inspector might be able to do it.

Ice dams occur because of insufficient insulation and ventilation in the attic.

Think of the attic as a hat. It needs to be below freezing in the attic.

What happens is that the sun + heat from inside your home heats the attic, and the snow (at the peak of your roof) melts, and runs down the roof. When the melted snow gets to the eave, it freezes (cause the temp of the ever is below 32 deg) and creates an ice dam.

You need more vents in the eaves and more insulation in the attic.

The above posts sound right on to me. Condition of shingles and flashing, gutter cleaning, venting and insulation.

I would have an expert in roofing and insulation look at it, no need for a structural engineer.

The winter of '11 was the worst in our 25 yrs in MA. We were closing on the house on2/28/11 and we had 5 weeks in a row with major storms. I had 4' of snow on the roof and in 25 yrs had never had a problem with ice dams or interior damage. Yes, clean the gutter and downspouts. Yes, check to make sure the insulation is laid correctly in the attic. Neighbors all over the area were loosing downspouts and gutters and suffering internal damage. You couldn't find peopel to beat the ice off the roof and if you could it frequently resulted in damage to the shingles/down sports and gutters. I had to have a contractor in to repair a wall and ceiling in the M bath. With the closing just a couple weeks away I had to do something.

I took 100' of garden hose, connected it to my tankless water heater. Installed a spray nozzle with a shower like pattern. Oh, first I shoveled the roof off. No small job on a 3000sq ft house. I began spraying hot water at the low end ( downspout end) of the roof. I ran hot water for 3 1/2 hours. At the end the roof was as clean as a summer day. There was no shingle or gutter damage. The neighbors, who thought and were possible correct, that I was nuts were asking for my help. Two things. Spray the water from the outside to the inside. Otherwise you will build up water above the ice dam creating more problems than you solve. Cut bread loaves, again to allow water to run off. One more thing. The back of my house was three stories to the ground. The front only two. Don't fall off the roof.

A side benefit was that the water run off melted the ice in the driveway. This just demonstrates that well and truly, there is more than one way to skin a cat!

Up on a 3 story ice covered roof spraying water for 3+ hours, what could possibly go wrong ;-)

Two words: roof rake.

They still make 'em. There is only one plane on my house where the ice tends to build up. The south-ish side has flashing the last 12 inches and the sun on it during the day. The north-ish side is raked every time it snows.

I use mine.