For a one-story house, the roof slab thickness might typically range from 100mm to 125mm (4 to 5 inches), depending on factors like the span and load requirements. For two and three-storey houses, the thickness may increase to 125mm to 150mm (5 to 6 inches) or more, considering the additional loads and structural considerations associated with multiple floors.

**RCC roof slab** is horizontal structure is to provide flat surface on floor/ceiling in building. on the basis of reinforcement provided beam and column support and ratio of a span slab generally classified into one way slab and two way slab. one way slab is supported on two side by beam and ratio of long span to short span is greater than 2 whereas the two way slab supported on all four sides by beam and ratio of long span to short span is smaller than 2.

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**Flat slab** of reinforced slab supported directly by column without the use of beams, this type of slab generally easy to construct and require little Framework the loaded directly transferred to the column. Flate slab more suitable for span of 6 to 9m and for live load for 4 to 8KN/m2.

**Minimum thickness** of a rcc roof slab is completely dependent upon the types of slab, may be One way or Two way slab or flat slab, Span of slab, Design load upon the slab. The minimum thickness of a slab is not less than 4 inch according to the indian standard. But, the thickness is mostly depend upon the design load and the span. Other than that, the concrete mix grade and grade of Steel is also an important factor to the thickness of slab.

**General, 6 inch** (150mm) slab thickness is considered for residential and commercial building with reinforcement detail as per design. We recommended to use 5″ rcc roof slab thickness 1, 2 and 3 storey building.

**According to Indian standard** rcc concrete roof slab thickness in residential building construction is 4 inches (100mm). Use of 5″ to 6″ (125mm to 150mm) is recommended if the concrete will receive occasional heavy loads.

The **thickness of RCC roof slab** is vital factor in building construction designing and it is directly correlated with cost of a structural system building. for example in high rise building if increase the thickness of RCC slab by 5mm result significantly increases column axial load. then we have to increase the column size reinforcement, Foundation size and beam size and beam reinforcement. finally it affect the cost of construction.

Therefore we should limit the thickness in any building construction to Limit required by design the key factor affecting the minimum thickness of slab can be applied load,span of slab, durability of concrete, length of span fire safety requirement etc

Different standard would specify different slab thickness requirement however we can calculate the basic requirement and minimum thickness considering the above factor.

**How to calculate minimum thickness of RCC roof slab?**

**The calculation is only based on construction and detailing requirement according to BS8110 part 01**

1) RCC roof slab concrete cover is equal to 20 mm which is minimum specified in code for mild exposure condition with one hour fire resistance.

2) reinforcement bar diameter equal to 10 mm and we have 2 bar in top concrete fibre and 2 bar in bottom bottom concrete fibre in beam.

3) minimum clear cover spacing between bar is equal to to aggregate size + 5, generally we use 20mm aggregate for concreting work.

**Slab thickness formula:**– slab thickness formula calculation is only based on construction and detailing requirement according to BS8110 part 01 says that

**Slab thickness formula = (Cover ×4)+ (Dia of bar ×4)+ (aggregate size +5)**

Therefore minimum concrete thickness can be calculated as follow =20×2+10×4+20+5=105mm.

This is theoretical requirement for thickness of RCC roof slab as per is calculated reinforcement can be placed as precisely as calculate to maintain the gap between bar as aggregate size + 5, Further slab reinforcement will be disturbed by the beam reinforcement therefore, it is very difficult to achieve this requirement as limiting thickness to 105mm is practically difficult task. So we widely-used concrete slab thickness 125 mm.

**According to standard ACI318** have specified the minimum thickness of slab based on span of slab. Slab thickness for simply supported slab = span/20, one end continuous slab = span/24, both end continuous slab = span/28, cantilever slab = span/10 we used. Where span is clear distance between two support.

Assuming span between two support is 12 feet long, using Thumb Rule for calculation of slab thickness span/30, we get slab thickness = 12×12 inch/30=4.8″, that is taken as equal to 5″(125mm) thick.

**How to calculate RCC roof slab thickness?**:- actual thickness of RCC roof slab is calculated based on reinforcement design considering applied load,span of slab, durability of concrete, length of span, fire safety requirement. Considering clear cover of RCC roof slab is 20 mm, providing 4 nos of 12mm bar in beam and 20mm aggregate size for concreting work then therefore minimum concrete thickness can be calculated as follow =20×2+12×4+20+5=113mm, which is taken as 125mm (5″).

**RCC roof slab thickness for 1, 2, 3 or multi storey house**

Whether it is 1 storey, 2 storey,3 storey or 10 storey or high rise building or apartment, one way slab or two way slab,flat slab, the thickness of the slab is based on the span and type of slab and not on the number of stories.

**RCC roof slab thickness for 1, 2, 3 storey house**:- General, 6 inch (150mm) slab thickness is considered for residential 1 (G+0), 2 (G+1), 3 (G+2) storey and commercial building with reinforcement detail as per design. We recommended to use 5″ (125mm) rcc roof slab thickness for 1 (G+0), 2 (G+1), 3 (G+2) storey building.

Usually one way slabs and cantilever slabs will have more depth than two way slabs. The value of thickness in two way slabs(slabs supported on beams in all the edges and L/B less than 2.0) will be L/20, where L is the span. Thus a 4m slab May be assumed to have 200 mm depth.

However, it also depends on loading on the slabs. In design the assumed approximate depth has to be checked for stresses, deflection, punching shear(if it is a flat slab) etc. Of course flat slabs, which do not have any supporting beams will have more depth, and its design is much different than one way or two way slabs.

**Slab thickness for one way slab**

Thickness of one-way slabs must be at least L/20 for simply supported slabs; L/24 for slabs with one end continuous; L/28 for slabs with both ends continuous; and L/10 for cantilevers; where L is the span. We recommended to use 5″ to 6″(125mm to 150mm) rcc roof slab thickness for one way slab.

**Slab thickness for two way slab**

The minimum thickness of any two way slab without interior beams should not be less than the following: For slabs without drop panel 125 mm. For slabs with drop panel 100 mm. We recommended to use 4″ to 5″(100mm to 125mm) rcc roof slab thickness for two way slab.

**Slab thickness for flat slab**

Thickness of flat slabs must be at least L/20 for simply supported slabs; L/24 for slabs with one end continuous; L/28 for slabs with both ends continuous; and L/10 for cantilevers; where L is the span. We recommended to use 5″ to 6″(125mm to 150mm) rcc roof slab thickness for flat slab.

**Slab thickness for high rise building**

Thickness of slab for high rise building must be at least L/20 for simply supported slabs; L/24 for slabs with one end continuous; L/28 for slabs with both ends continuous; and L/10 for cantilevers; where L is the span. We recommended to use 4″ to 8″(100mm to 200mm) rcc roof slab thickness for high rise building.

**Slab thickness for apartment**

Thickness of slab for apartment must be at least L/20 for simply supported slabs; L/24 for slabs with one end continuous; L/28 for slabs with both ends continuous; and L/10 for cantilevers; where L is the span. We recommended to use 4″ to 8″(100mm to 200mm) rcc roof slab thickness for apartment.

**Slab thickness in inches:-** General, 4″ to 6″ slab thickness is considered for residential and commercial building with reinforcement detail as per design. We recommended to use thickness of slab is 5″ used for 1, 2 and 3 storey building.

**Slab thickness in mm:-** General, 100mm to 150mm slab thickness is considered for residential and commercial building with reinforcement detail as per design. We recommended to use thickness of slab is 125mm used for 1, 2 and 3 storey building.

**Slab thickness in m:-** General, 0.10m to 0.15m slab thickness is considered for residential and commercial building with reinforcement detail as per design. We recommended to use thickness of slab is 0.125m used for 1, 2 and 3 storey building.