A quote from a technical paper (called "The Seven Deadly Sins of Trussed Rafter Construction") on Fink truss roofs: "Typically the bottom chord of a truss is designed for a superimposed load of 0.25 kN/m2 which the author often refers to as empty cardboard boxes loading."
So whilst the top of a Fink truss frame is designed specifically to carry the load imposed by the roof cladding, snow and wind, the bottom chord is designed to carry only a thin skin of plasterboard and little more. Not surprising when you consider that the bottom chords on this roof are 75 x 38mm (3 x 1.5in) on about a 4.1m (13.4ft) span (based on those blocks being 340mm long blocks)
There are exceptions to this, where the builder has specified that the bottom chord be capable of supporting a load, but that does need to be specified at time of manufacture