For Roof slabs, 8 mm, Iomm, 12mm bars are used according to the Load, span of the roof slab and also it's design factors. 6mm size is used as distribution bars. For Concreting 1:2:4 (150 grade) mix. is suitable, ie. 1 Part of cement:2 Parts of Sands:4 Parts of aggregates (20mm metal)based on water Cement ratio https://bit.ly/3KV51a1