Abstract: Lightweight structural panel units for construction of space enclosing structures each of which is formed from a blank of flat, suitably rigid, yet foldable sheet of material, which are profiled and pre-creased with a unique configuration. Upon folding, the blanks form a substantially hollow three dimensional polygonal beam-like unit having a shape and configuration which positively aligns and interlocks when conjoined with a plurality of similar folded beam-like units to form a substantially hollow 3-dimensional “structural panel unit” having symmetrical cross bracing inside and sufficient rigidity to enable sufficiently large areas to be produced for the construction of space enclosing structures. Each structural panel unit has a configuration and profile that positively aligns and interlocks when engaged with similar structural panel units to complete a desired space-enclosing structure which has an exterior facing surface and an interior facing surface separated by a substantially hollow void.