metal roof installed over shingles to minimize ice damming

I have struggled with ice dams since moving into our bungalow 5 years ago. The roof is 3/12. I have increased the attic insulation (fiberglass batts) to R40+. I have installed air channels adjacent to the roof deck in the attic and installed additional vents to increase ventilation. Although there might be incremental improvements, the roof still suffers ice damming. So I am leaning toward installing a metal roof over top of my shingle roof with vent channels between.

The roof is in good shape since I cleaned off the old shingles and re-shingled 3 years ago. I am a n intermediate handy man and have no concerns I can handle this job. But are there any good websites or youtube videos showing the progression of one of these type jobs? I have hips on both sides of my roof and that might be the only real tricky part I think. I would also like to see how the vented channels between the old and new roof remain vented when a new fascia board will need to be installed and how that generally looks.

Does it really matter what brand of metal roof is used? I have a Menards in town and so would be inclined to order the metal roof through them.

I am also inclined to install a base layer of poly-iso sandwiched between 2x4 installed on their side on the old roof to provide a thermal break. I assume an ice-guard should be laid directly on top of the shingles underneath this poly-iso layer, correct.

How large is the roof? There are a whole host of ways that you can do this to make the roof a "cold roof".

Keep in mind, ice damns can form on cold roofs, but you can certainly lessen their likelihood with proper insulation and materials selections.

The entire roof, including attached single car garage is 1000'.

I totally realize ice dams are still possible, but at least water won't be working its way under the singles, like what we started experiencing last year and recently again this year. Although I am hoping that by adding a thermal break on top of the old roof and a vent barrier between the two decks that whatever ice damming does still occur will be considerably less than whats happening now.

I should add that the house is probably ~65 years old and the roof is stick framed and not trusses. I am not sure whether or not that might be a consideration for overall weight purposes of adding a metal roof over top of a shingled roof deck.

If you are going to do this...I would plan on removing the shingles. There is no point in having that additional thermal mass up there based on what you are trying to do. Can you remove the shingles as a part of this project?

I don't see how the shingles help in this situation.

Consider adding a recording humidity/temperature recorder in the attic. Consider a blower door test, and see if you're loosing humid air to the attic.

What's your climate zone?

Consider "Green Building Adviser" as a forum to ask this question which is really more about energy than a specific roofing choice. The cold roof with a ventilation channel may be a good solution, or not, depending on factors that internet advice can't really provide. Get local.