A slab is a flat, horizontal reinforced-concrete plate that forms the floor or roof of a building. It is one of the most common RCC elements in a house, carrying the loads of people, furniture and finishes and transferring them to the supporting beams, walls or columns.
Slabs are classified as one-way or two-way depending on how they span. A one-way slab spans in a single direction between two supports (used when one pair of sides is much longer than the other); a two-way slab spans in both directions and suits roughly square rooms, distributing load more efficiently. Typical residential slab thickness is 100–150 mm.
Estimating a slab means calculating its concrete volume (length × width × thickness), the cement, sand and aggregate from the chosen grade, and the reinforcement steel — about 80 kg per cubic metre as a planning figure, though the exact amount comes from the structural design and bar bending schedule.