How thick should the slab be for a 2 storey building?

For an ordinary Nigerian residential floor the answer is almost always 150 mm, and it is set by the span rather than by the number of storeys. A 150 mm solid slab reaches about 3.25 m spanning one way between beams, and about 4.15 m where it is continuous over them. Past that the thickness goes up: 175 mm to about 4.90 m continuous, 200 mm to about 5.65 m. Every storey of a building has the same slab if it has the same beam layout, because each floor carries only itself.

Is a 125 mm slab thick enough?

Only over short spans. At residential loading a 125 mm slab reaches about 2.65 m simply supported and 3.35 m continuous, which suits a corridor, a toilet or a small store and not a sitting room. It also has very little to give: 125 mm satisfies two hours of fire resistance with nothing spare, and the reduction in effective depth from a cover error is proportionally much larger than it would be in a 150.

Why is slab thickness decided by deflection and not by strength?

Because a slab is thin relative to its span, so its stiffness runs out long before its bending capacity does. In the worked example on this site the 150 mm slab has three times the moment capacity it needs — K is 0.042 against a limit of 0.156 — and it still fails the span over depth check by a hair. That is the normal picture. It is also why adding steel to a slab that fails deflection helps only slightly, and adding 10 mm of thickness helps twice: it raises the effective depth and it lowers the steel stress that sets the modification factor.

How thick should a ground floor slab be?

A ground floor slab cast on compacted fill is not a suspended slab and none of the span tables apply to it. It is a slab on grade: it is carried by the ground under it, so its thickness is a serviceability and construction matter rather than a bending calculation. Common Nigerian practice is 100 to 150 mm on well compacted, well graded fill in layers, over a damp proof membrane, with light mesh or nominal bars to control shrinkage cracking. The thing that decides whether it cracks is the compaction and the curing, not the last 25 mm of thickness.

How thick should a concrete slab be?

Two quite different things get called a slab, and only one of them is what this page is about.

A suspended slab — the deck, spanning between beams, with air underneath. Designed by bending and deflection. Everything below applies to it.
A ground floor slab — cast on compacted fill, carried by the ground it sits on. It is not designed by span at all, and the tables here would be nonsense applied to it. There is a section at the foot of this page about what does decide it.

The span each thickness reaches

Maximum span of a solid one-way slab at ordinary residential loading, governed by the BS 8110 deflection check:

Thickness	Simply supported	Continuous
125 mm	2.65 m	3.35 m
150 mm	3.25 m	4.15 m
160 mm	3.50 m	4.45 m
175 mm	3.85 m	4.90 m
200 mm	4.45 m	5.65 m
225 mm	5.05 m	6.40 m
250 mm	5.65 m	7.10 m

Assumes grade 25 concrete at 24 kN/m³, finishes 1.2 and partitions 1.0 kN/m², imposed 1.5 kN/m², 20 mm cover, T12 bars, f_y 460 — the loading of the worked takedown on this site. Continuous spans use BS 8110 Table 3.12's 0.086nL², which carries conditions of its own.

The table assumes the steel provided equals the steel required. Rounding bars up lowers the service stress and raises the modification factor, which buys back roughly another 5 to 8% of span — that is exactly how the 150 mm slab in the worked example gets to within a hair of 3.5 m before failing. Use the table to choose a thickness, then let the design decide.

Two-way panels

Where the panel's long side is no more than twice the short side it spans both ways, and the load splits between them. Enter the table with the short span:

l_y / l_x <= 2 two-way — use l_x, the short side
l_y / l_x > 2 one-way — use the short side too; the long way is nominal steel

Doing that is conservative, and usefully so. A two-way panel carries less moment in its short span than a one-way strip of the same span, so its steel stress is lower, its modification factor is higher and it will reach a little further than the table says. What it must not do is span its long side across the table — the deflection check for a two-way panel is made on the short span.

Why deflection is what decides it

Take the site's worked slab: a 3.5 m span, first tried at 150 mm.

Bending K = 0.042 against K' = 0.156 — three times the capacity needed
Deflection span/d = 28.2 against allowable 28.0 FAIL

At 160 mm span/d = 26.1 against allowable 30.3 PASS

Ten millimetres moved the check by 4.2, because depth helps twice: it raises d directly, and it lowers the steel stress f_s that sets the modification factor. Steel alone cannot do that — doubling the reinforcement in a slab that fails deflection moves the answer by a few per cent and costs a great deal.

This is the opposite of a beam, which is deep relative to its span and is nearly always governed by strength. If you take one thing from both pages: slabs are sized by stiffness, beams by strength.

The four other things that set a thickness

Fire. A slab that is thin enough will not achieve a rating whatever the cover: 95 mm for one hour, 110 for an hour and a half, 125 for two. Nigerian low-rise residential practice is commonly one hour, which 150 mm clears easily — but the rating is an occupancy question, not a structural one.
Cover, top and bottom. A two-way slab has four layers of bar in it. At 20 mm cover each face with T12s, the mat alone occupies 64 mm, which is why slabs much under 100 mm stop being buildable before they stop being strong.
Concentrated loads. A blockwork wall standing on a slab, a water tank, a stair landing bearing on it — none of these are in the table, all of them are real, and the first one is common in Nigerian plans where a partition does not line up with a beam.
Openings. A stair void or a duct interrupts the span and needs trimming steel; a large one changes what the panel is. Structura does not design an opening, which is why the plan editor offers a void region that warns rather than a hole it pretends to have checked.

What an extra 25 mm costs

Worth knowing before you round up out of caution, because the slab is the biggest single item of dead load in the building and every member under it inherits the decision.

Dead load 0.025 x 24 = 0.6 kN/m² added, everywhere
Concrete 0.025 m³ per m²

On a 10 x 8 m floor
Concrete 80 x 0.025 = 2.0 m³
Cement 2.0 x 6.3 = 13 bags (1:2:4)
Extra load 80 x 0.6 x 1.4 = 67 kN at ultimate, into the columns and footings

Which is the argument for getting the thickness right rather than generous. Twenty-five millimetres over four floors is 270 kN arriving at the foundations — roughly one extra column's worth of load, paid for in footing area as well as in concrete.

The ground floor slab, which is a different question

It is carried by the ground, not by a span, so bending hardly enters into it. What decides whether it performs is underneath it:

Compaction. Fill placed in layers of 150 to 225 mm and compacted, not tipped to depth and levelled. A slab cracks because the fill under it settles, and no thickness fixes badly placed fill.
What the fill is. Laterite or granular material, free of topsoil, organic matter and building rubbish. Excavated topsoil put back is the commonest cause of a cracked ground floor in Nigerian residential work.
A damp proof membrane under the slab, lapped and turned up at the edges.
Nominal reinforcement — light mesh or bars at wide centres, to control shrinkage cracking rather than to carry load — and curing, which is what stops the cracks that appear in the first week.

Typical thickness is 100 to 150 mm. Where the ground is soft, made up or filled to any depth, the honest answer is that the slab should be suspended and spanning between beams — at which point the table at the top of this page applies again, and the soil is the thing to go and find out about.

Let the check decide the thickness

Structura designs one-way and two-way solid slabs and shows the deflection check with both numbers on the sheet — the actual span/depth and the allowable, with the modification factor that produced it — so a FAIL tells you which way to move. Change the thickness and run it again; it takes seconds.

Slabs are free to run, as many as you like.

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How thick should a slab be?