Then you need to sit closer.
"Records show 'licensed contractors' who presumably 'know what they are doing' can build single-story houses that fail within a few years, but duplexes are basically stacking two of them over each other to test if they were really good at their job."
If the existence of bad contractor was an argument for anything, we should go back to living in caves.
I can read just fine. Maybe you should consider that your entire statement must be processed together...and you cannot just carve out bits.
Let me then explain that you seem to not understand what a house is...because a duplex isn't a stacked set of single houses. If that was the case you could theoretically explain any house with 2 stories as a duplex...which defeats the purpose of having two units together but separated. A duplex has the bottom block without a roof...because the roof is the floor to the upper unit. Cool....that shouldn't cause any issues, because it's basically the definition of the word. You have the same single roof...that any normal unit would have. Yes, you can in fact renovate old duplexes into a single much larger unit by simply adding an internal staircase and calling it a day. This...has literally no effect on solar.
If instead what you wanted to suggest was row housing, which is entirely different due to most of them lacking internal structures but requiring support from their neighbors, then we're off to the races. I am thinking for you...so if you mean something else do interject. The greatest concern in a row house is that the internal structure is only designed for supporting weight in roughly two directions. Those would be directly perpendicular to the base of the unit, and at whatever angle the roof is sloped. Their transverse structure is instead supported by the other row houses pushing in...with only the end of many row houses having real transverse load capability without all of the others present.
Now...in this case installing solar is a crap shoot. Where are the houses tied together? Whose wall is their neighbor's, and what happens if you install such a large transverse load that you press outwards enough to damage the housing row? The answer to all of these questions is "I'll see you in special litigation."
Wow...so if you install solar panels in row housing you're taking a huge gamble that nothing screws up, the contractors (if they have a brain) will either write clauses into their contracts to make this your responsibility, or worst case scenario your neighbor is getting a new roof leak because your "competent" contractors can't magic a world where the coefficient of thermal expansion for solar panels matches OSB, wood, steel, and shingles all at the same time.
Yeah....my two cents is that "good" contractors leave these nightmares behind...and from experience I know that you're going to sign with a company to install who contracts out to a reserve company that will be dissolved in 3-5 years...so there's no chance for the inevitable screw-ups of lowest bid labor to come through. This is the same way that the intelligent people out there see something like "Tony's Roofing," which is "Tony T's Roofing" five years later, and "Cousin Tony's Roofing" in another 5 years. Same owner, same labor, but because the old company dissolved and "sold" their stuff to a new shell company everything is going to look great on paper.
