Rib and block slabs with polystyrene void formers are constructed using pre-cast concrete ribs spaced 600mm "center to center". polystyrene void formers are placed between these ribs. The width of the void formers dictates the beam spacing and provides a flush soffit. A 25 MPa in-situ structural concrete topping is placed on top of the ribs and void formers with welded wire mesh reinforcement placed inside the topping to reduce shrinkage cracks on the surface of the slab. Every fifth to seventh row of blocks is omitted and cross-rib steel (2Y10's, 2Y12's, 1Y16) are placed perpendicular to the rib direction above each line of temporary propping, these cross-ribs prevent possible stress fractures caused by the slab settling to its final position. PB5 280mm polystyrene void formers are used with a 340mm slab. Temporary propping is used with rib and block slabs and are placed on the cross-ribs at a maximum of 1000mm apart "center to center".