9.1 Step 0: assess the roof's remaining life

• Asphalt shingle: 20–30 year lifespan. If yours is already over 20 years old, re-roof before installing solar — otherwise the next reshingle will require pulling the panels off and reinstalling them.
• Concrete tile: 40–50 year tile lifespan, but the underlayment only lasts 20–25 years. For roofs over 20 years old, a "Tile Lift & Re-felt" is the right intermediate step.
• Metal / Standing Seam: 40+ year lifespan, and solar attaches without roof penetrations. Easiest to install on, easiest to service. The best option.

The roofing industry is also highly financialized, and roofer-only quotes are often inflated. Get multiple quotes here too.