Below is what our roofing expert John Stout from Go Roof Tune Up
had to say in response to your question:
Thank you for an interesting question.
If it is necessary to install Solar Panels on your roof, the roof should be new.
If you now have a “30 Year” Shingle roof that is 15 years old, it may not have 15 years of life left. Or it could have more than 15 Years left. If you install the solar over your shingles with the thought of removing and replacing the solar panels after your metal roof is on may not be a savings of money. The cost to install solar panels are part of the sale of your system. To remove and reinstall your solar panels may be very expensive as a stand-alone project.
The answer to your question is have the metal roof installed now. Install solar system on the metal roof. Aside from normal maintenance, don’t think about it for another 30 years.
You may have the best looking roof on the block. Enjoy your new roof and your Solar System!