The present invention relates to a prefabricated deck unit for a building system, the deck unit including: a reinforcing framework encased in a composite material, the reinforcing framework comprising: a first metal mesh unit, a plurality of interspaced spheroidal void formers attached to or in intimate contact with the first mesh unit, and a second mesh unit resting upon, or attached to, the spheroidal void formers, wherein the framework defines at least one terminal channel on at least one end of the deck unit, the terminal channel being shaped and dimensioned to snugly receive an elongate connecting rod for connecting the deck unit to a further structural element of the building system.