What is a slab floor?
A slab floor is a floor structure consisting of a flat, solid plate of reinforced concrete. The plate carries the loads to its supports by itself — there are no separate beams, ribs or hollow cores inside it.
That sets it apart from a hollow core slab floor (with voids running lengthwise) or a ribbed floor. A slab floor is heavier per square metre, but structurally simple and geometrically free: it will follow any plan shape.
Two forms of construction
- Cast in situ. The floor is poured onto formwork. That costs time (form, reinforce, pour, cure, strike), but produces a fully monolithic structure without joints.
- Precast. The plate arrives from the factory. An intermediate form is the lattice girder slab (composite floor slab): a thin precast bottom shell that acts as permanent formwork and receives a structural topping on site.
Spanning one way or two ways
The great strength of the slab floor is that it can span two ways when supported on all four sides. The forces then distribute along the shortest route to the supports, which reduces the required thickness. If the slab bears on only two opposite edges, it behaves as a wide beam and the span governs.
Rule of thumb for a first estimate in housing: the thickness of a two-way spanning slab is often around 1/30 to 1/35 of the shorter span, and around 1/25 to 1/30 for one-way spanning. The final thickness always comes out of the structural engineer’s calculation, which weighs deflection, vibration and fire resistance alongside strength.
What to watch
- Reinforcement in the right place. Bottom steel in the span, top steel over the supports. Displaced mesh is a classic cause of cracking.
- Concrete cover. Too little cover leads to corrosion of the reinforcement over time.
- Punching shear at point supports. If the slab rests on columns rather than walls (a flat slab), punching shear reinforcement is needed around each column.
- Openings. Service penetrations and stair openings must be known in advance; drilling afterwards quickly hits the main reinforcement.
Related terms
- Lattice girder slab
- Hollow core slab floor
- Reinforced concrete
- Concrete cover
- Concrete floor
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