
The decking issues should be addressed. That may require removing all of the old shingles or just in select areas and then building things back up to the shingle height. Overall, it's probably safest and most economical to remove all of the old shingles.
Is is an acceptable practice to install metal roofing over rotted plywood sheeting? We have a home with lots of missing and or damaged asphalt shingles. This has been an ongoing problem for 6+ years and we want to replace our asphalt shingles with metal. Should we remove our existing asphalt shingles before installing metal shingles?