For me I would want the old shingles torn off. No point in having the extra weight up there and the new steel will probably lay flatter and look less rippled when it heats up in the sun. I like solid sheeted then textured underlayment then on site formed standing seam metal. All fasteners are covered under next sheet and edges are all crimped onto bottom drip edge and end fascia trim so no wind gets underneath. That's just me others opinions may vary.
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New Standing seam roof on house--over shingles???
