Vent under metal roofing

Hi all,

I'm going to give a friend a hand putting metal roofing over an existing roof (rolled asphalt - 3/12 pitch). There are no soffit vents (no soffits on the house), just two gable vents.

I was thinking about how to add some ventilation. My thought was to cut a strip vent (2" wide) in the existing roof deck (two feet up above the gutters) along the length if the house, then cover with screen, or cobra vent material. Then lay wood purlins and attach the metal roofing.

I was also going to seal the gable vents and add a ridge vent.

Does this sound like a decent plan?

Thanks for the advice