3 days vs an afternoon sounds like a framing difference as much as a material one. Stick framing a roof takes 3 days no matter who does it, so of course the purlin setup went faster. The real question is whether 18 months is long enough to call it. Phoenix gets what, 8 inches of rain a year? A wood roof that leaked every monsoon for years is one thing, but a dry stretch in the desert is not a hard test for gaskets and seams.