Most northeast codes allow only 3 roof layers maximum...so adding a 4th metal roof layer is probably not an option.
That said, a metal roof is much much more expensive than composite shingles.
After removal, you could expect to pay perhaps $150 per square to install new composite shingles (depending upon slope and complexity) and at least $500 per square or more for a new metal roof....
K2's experience with snow avalance off metal roof is unfortunately common. Here too, an good roofer experienced in metal installation would have priced in, and installed the appropriate snow fall protection. However, too many salesmen focus on the sale of their product, and ignore the other aspects of the roof system that should really be looked at (insulation, ventilation, snow fall location, etc.).
As well, "expensive" is relative. A "cheap" installation can be much more expensive than an "expensive" installation if the workmanship is not there.
Oh, and when you find a good roofer - don't nickle and dime them, they are worth more to you than anything you can pay them.
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