Standing seam roofing

[Thomas-in-Kentucky]

Quinton,

Simplifying is always a good thing.  Lumber is not cheap - even (especially!) if you have to saw it yourself!  If you can get rid of a layer of wood, then I'd say do it.  I don't see any reason why you couldn't run the 2x3's sideways.  I'm sitting in a metal building that we built that way (sans poly-iso insulation - actually there's some really thin insulation screwed just under the metal to help with sweating).  Pitch is 4:12.  2x4 trusses are on 4 foot centers.  2x4s (laid flat) run horizontally on 2 foot centers.  Metal roofing (not standing seam) screwed to the horizontal 2x4s.

If your (real) rafters don't overhang the walls at the eaves (we bought oversized trusses so it wasn't a problem), you'll have to figure out a way to build that overhang in, and that might be difficult with your 2x3's running horizontal.  (on the other hand, this configuration makes the gable overhangs easier to build)

FWIW, if you're cutting your own 2x3's, consider cutting 3x3's.  If for some reason the lumber ends up thick-and-thin in one direction (2.75 to 3.25), you can just roll it over and use the other 3" side.  I did this for mini-floor joists in my timber frame, and it has helped me avoid several would-be-culls.

-Thomas