Abstract: The present invention relates to a lightweight composite architectural panel assembly, comprising: a) at least a first and a second end joist spaced parallelly along a perimeter of the assembly, each having a length; b) at least one web plate spaced parallelly between the end joists; an upper surface arranged in parallel to a lower surface; and attached to the at least first and a second end joist and the web plate, forming one or more channels disposed vertically between the upper and lower surfaces and longitudinally extending along the upper and lower surfaces, thereby forming an open-ended boxlike body; and c) a structural vertically load-bearing sheathing layer enveloping and adherent to the outer surface of the open-ended boxlike body, wherein the sheathing layer comprises a composite skin comprising a fibre mesh and a crosslinked polymeric resin system.