Standing Seam & Pitch...and Galvalume or not?

We're building a 2-building 'house' in the Big Bend region of Texas, specifically right next to BB Nat'l Pk, which is Chihuahuan Desert. So ~10" rain/yr, with droughts lasting ~6+ months typically, and a year rarely.

Each building will have simple shed roof with 2/12 slope, with roof acting as catchment. Since hi end of shed in both cases faces S, solar panels will probably not be on roof unless bracketry & angling are simpler & less expensive than I imagine they will be -- so TBD on that aspect.

One is 27.5' (E-W) by 34' (N-S); the other is 29'10" x 28'. In both cases that does not include tbd-but-deep overhangs on all sides.

We want/need metal roof (standing seam assumed & desired), and I have a couple of questions after a multi-day google-fest, which has given me just enough knowledge to be dangerous...

Here are my questions:

1) Snap-lock or mechanically sealed (2/12)?

2) Is Galvalume necessary?

Obviously not a marine environment & the rains evaporate quickly. 60-90 mph winds are not uncommon, & 10-20 mph sandblasting is a near-constant. So massive abrasion over time. The airborne particulates can have high mineral content, but I don't think there are salts per se.

Hail is a huge issue -- just last week there were literally dozens of house & vehicle windows (maybe hundreds) broken in a vicious storm featuring hail ranging from golf ball to softball size.

House walls will be non load-bearing infill-under-metal-roof style papercrete, which consists partly of Portland cement; and I've read that Galvalume is a no-go when in proximity with masonry products. Is this relevant wrt papercrete?

I know I want either good galvanized 24-26g steel, or Galvalume. And I will have the Kynar 500 type-paint on whichever I choose.

I've read numerous "Galvalume is always better than galvanized in at least some respect" sentences in a multitude of on-line info...

...but I have no idea if Galvalume is better or not in my case.

Any insight would be greatly appreciated.

Thanks, John

Great advice from Todd there. Hit those benchmarks that he mentioned and the roof should live a long life.

Thank you Todd & Eric.

One follow-up question for now -- there is a scarcity of builders down there, so beggars can't be choosers.

If the builder is disinclined to do mech seaming on the roof, but in all other respects seems fine, would you consider snap-lock a deal-breaker? Or would it appear to be a situation where mech-seaming is definitely preferred, but not absolutely mandatory?

I know that may be a tough question to answer definitively.

Same for the Galvalume -- I'm not sure the local vendors stock that in the colors I'd prefer, but in any case, if there's a big price jump up to the Galvalume (think special order w/freight, etc)...once again...preferred, or mandatory (compared to 26/24g galvanized with PVDF)?

Aaand...given the rafter lengths -- is that an issue that strongly argues for 24g over 26g? Does that specific issue also impact the Galvalume-or-not decision, or is rafter length fairly irrelevant wrt Galvalume?

Thanks again.

Sorry -- forgot to add -- apparently the papercrete proximity to the theoretical Galvalume is not an issue?

Wind driven rains and a lower pitch are not my favorite descriptors for a snap lock roof to be perfectly honest.

Thanks again to both of you.

There will be appropriate barriers between the p-crete & roof, so that question appears to be answered.

Yep -- I hear you on the mech seaming wrt wind & rain & length...do it once; do it right seems to apply strongly here.

But just in case that can't happen (ugh), can you elaborate more on this? In my ignorance I confess I have no idea what you're saying here:

@[email protected] said:
and also to clip fasteners versus a nail hem panel. If you go the snap lock route, is a clip fastened panel an option?

I think I know what you mean by nail hem vs clip fastener -- is clip-fastened better?

And I didn't specifically state that all interior walls rise to ceiling which matches roof pitch; there's no dropped flat ceiling. Does that lessen the concern wrt rafter length, or is that irrelevant to what we're talking about?

Thanks again.