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Calculating Floor Slab Thickness

Calculating the floor slab thickness is necessary already in the preliminary design phase. Here are the thicknesses based on the main types

How thick is a floor slab? What is the minimum thickness of a concrete floor slab?

If you want to have a general idea about the preliminary design of a floor slab, read this article. This information can be helpful in the initial project phase to quickly evaluate the feasibility of the intervention, the approximate costs, the volume, and the overall height of the building, etc.

If, on the other hand, you are already one step ahead in the design and need to accurately calculate the floor slab thickness based on regulatory requirements and obtain all the necessary documentation to authorize the intervention, you must necessarily use a structural calculation software.

EdiLus 3D view

EdiLus – 3D View

Floor Slab Thickness

There are many types of floor slabs classified based on construction materials (steel, wood, concrete, etc.), function (roof, intermediate floor, etc.), or construction type (cast in place, prefabricated, etc.). Therefore, the thickness cannot be the same in all cases and also varies based on the span the slab must cover, the construction type, material strength, projected loads (accidental, permanent, etc.), regulatory requirements, etc.

Before getting to the thickness, let’s take a brief look at the most common types of floor slabs:

  • lightweight concrete;
    • cast in place;
    • with prefabricated joists and interposed brick blocks;
    • with reinforced concrete slabs and lightweight blocks;
    • prefabricated panels;
    • SAP type (with prefabricated joists of brick and reinforced concrete);
  • wood;
    • single framing;
    • double framing;
    • triple framing;
    • with cassettes;
    • glulam;
    • solid wood;
  • steel;
    • with vaults;
    • beams and blocks;
    • profiled sheeting;
  • reinforced concrete;
    • solid slab;
    • hollow core slabs;
  • mixed
    • concrete and prestressed concrete;
    • etc.

Calculating Floor Slab Thickness

Given that the floor slab thickness depends on a wide variety of factors, let’s analyze, for example, the most common ones and see how to evaluate the minimum thickness of a slab covering an average span of 5 m, in order to ensure stability and safety. We will consider the reference measurements of the structural frame only, leaving out all functional and finishing layers (screeds, insulations, plasters, floorings, etc.) which should be evaluated separately.

However, it is important to emphasize that the following is a preliminary assessment and for correct sizing of a floor slab, taking into account all the characteristics and variables at play, it is always advisable to use a structural calculation software. This way, you work safely and automatically generate all the necessary documentation to obtain permits and authorizations before starting the work.

In any case, the preliminary design phase is very important when working on a structural project because it constitutes the starting point of the design, a preliminary phase to the creation of the 3D model, created with a structural calculation software.

Obviously, the initial trial results obtained from the preliminary design should be properly verified by the structural calculation software. Evaluating thicknesses and section dimensions will greatly increase the likelihood of meeting all strength and deformability checks, reducing changes to the 3D model in the design phase.


Floor with vaults

Lightweight Concrete Floor Slab

The lightweight concrete floor slab is characterized by a system consisting of reinforced concrete joists alternating with rows of lightweight blocks for lightening purposes.

The overall thickness of the slab must take into account the following limits:

  • total thickness = slab thickness + block height > 16 cm. This relationship comes from adding the minimum height of the blocks on the market (usually 12 cm) to the minimum thickness of the slab equal to 4 cm;
  • total thickness > span/25. Where L indicates the span of the single bay.


In this case, by calculating the slab thickness equal to or greater than 1/25 of the span and assuming an average span of 5 meters, we can say that the thickness of a lightweight concrete floor slab is approximately 20 cm.

It is also important to consider that:

  • the base of the joist section must not be less than 1/8 of their spacing (i) and in any case not less than 8 cm. In the case of series production in the factory of complete floor panels, the minimum base of the joists can also be reduced to 5 cm;
  • the spacing of the joists (i) must always be less than 15 times the thickness of the slab (hs);
  • the minimum thickness of the slab must not be less than 4 cm;
  • the maximum width of the block must not exceed 52 cm.

Steel Floor Slab

Steel structures usually have smaller sections compared to reinforced concrete. In fact, a floor slab with a span between 4 and 5 meters, with steel profiles placed every 80/100 cm and decking on top, can have profiles about 12 cm high, for a total thickness between 15/18 cm. The thickness increases based on the construction type, as in the case of floor slabs with profiled sheeting.


Steel floor with profiled sheeting

Wooden Floor Slab

The wooden floor slab can be of various types but usually consists of a main structure of beams, possibly with a secondary structure and closing elements such as:

  • floorboards;
  • decking;
  • brick vaults;
  • stone slabs;
  • bricks;
  • etc.

The chosen type will also affect the calculation because it will impact the structural self-weight of the slab.

Under the same conditions as steel, wooden slabs require beams with a height of 23 cm placed every 1.30/1.40 meters approximately, small beams, and overlapping decking, for a total thickness between 32/35 cm.

To reduce the resistant section of the main structural element, it is good practice to place the main beams along the shorter side of the area to cover and generate a lower bending moment.

Timber floor material layers

Wooeden floor material layers

The methodology of preliminary assessment can be extended to all structural elements (columns, projecting beams, etc.).

Obviously, the values calculated in the preliminary design phase can be optimized and reduced if all checks performed by the software are satisfactory.

To design a floor slab with a 3D structural calculation software, I recommend watching these video tutorials that show you the steps to follow and guide you through the design.