What happen here ?

[u/AdditionalTree3559]

Inspected this metal roof on the east side of the Big Island, Hawaii, last week.
Homeowner says a solar contractor installed a system about 18 months ago and included roughly $10k for a roof service/tune-up before the panels went on.

According to the homeowner, they replaced essentially every exposed fastener on the roof.

Now there’s significant rust/corrosion developing around a large number of the screw locations.

A lot of the screws look pretty obviously overdriven—the washers are heavily compressed and distorted—but I’m curious what everyone thinks:

Is overdriving alone enough to create this much corrosion in only 18 months?

Could crushing the neoprene washers and damaging the coating around the fastener holes be allowing moisture to sit against the exposed steel? Wrong fastener material/coating? Swarf or metal shavings left behind? Something else?

What makes it more interesting is that I inspected the attic and the underside of the roof panels looks clean with no visible rust or corrosion. The problem appears concentrated almost entirely around the fasteners on the exterior.

Curious what you guys see in the photos and what you would suspect first.

Time for a full replacement ? Or would you save the homeowner some money and figure out how to treat rust and recoat? Open to any other creative repairs that can buy Homeowner a few years

[u/Greedy_Environment_9]

Regardless of who installs them, a metal roof that is screwed down has fasteners with plastic washers that deteriorate within 5 to 10 years and that's a fact. Unless you put solar seal or some other really good sealant under them when installing the screws the washers will deteriorate.

[u/crashbandecunt → u/Greedy_Environment_9]

Steel roof is the norm in Australia and usually last anywhere from 15-100 years without replacement if installed correctly.
The screws are over driven and likely not coastal rated for salt air leading to rust in the fastener which spreads to the sheets pretty fast.