Have seen a number of details for rain screens on exterior walls and cold roofs.  They are all over the board and was wondering if there was a sweet spot for the air gap for different situations.  Is it a function of the location of the build (temp., humidity, rain etc.) or more a materials/ease of construction issue?

I see that in Canada a wall needs a minimum 10mm (3/8") screen.  Most US details I have seen use a 1x (3/4") as the furring.  I'm sure that is adequate but is it needed?  If a person could rip 3/8" or 1/2" plywood for use as the furring cheaper than purchasing 1x would that be just as effective?

Also, most cold roof air gap details I have seen are accomplished with a 2x (1 1/2").  That seems overkill to me especially since I am dealing with a pretty wide eave to begin with.  Is there a thinner yet equally effective air gap in this situation?  Thanks.  (Zone 5 & 4 Marine).