ConstructionStudent Guide

How to Recognize One Way Slab & Two Way Slab with Naked Eye

In the world of civil engineering, there are many types of concrete slabs but the most important are:

  • One-way slab and
  • Two-way slab
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Both types of concrete roofing are quite strong, and each has its own strengths and weaknesses. Both methods are used by roofers in Oxford, London and elsewhere in the country and there are other roofing options available if desired. However, the price of one-way and two-way roofing will differ quite a lot. So, it is essential to choose the one that you feel most comfortable with.

One way slab and two way slab

TIP! “Also remember that the thicker the concrete slab, the less the risk of deflection”

This article will explain how to recognize one-way and two-way slabs with the naked eye. First, learn about the basics of these slabs and we will explain some quick points by which you will identify these with the naked eye.

One-Way Solid Roofing Design Spreadsheet Download Here
Ribbed Roof Design Spreadsheet Download Here
Flat Roof Design Spreadsheet Download Here
2-Way Slab Design Spreadsheet Download Here
One-way roof design spreadsheet Download Here

What Is One Way Slab?

While designing or analyzing structure of concrete roofing, if bending is considered to be occurring in one direction only then it is called one-way slab.

On Which Condition It Is Used?

  • If roof is supported by beams on two opposite sides, no matter what the dimensions are, the it is designed as a one-way slab.
  • If it is supported by beams on all four sides, then we check the ratio of longer span(Lx) to shorter span(Ly). If this value is ≥2 then the roof is designed as a one-way slab.

Examples Of One Way Slab

The best examples are:

  • Veranda
  • Car parking or garage
  • Long span hall

Now, lets get into the basics of two way slab.

What Is Two-Way Slab?

A two-way slab is a concrete structure that spans in two directions and provides support for most upper floors in building structures. It is also referred to as a “flat plate” since it has no beams or columns protruding below its surface. The two-way slab is typically constructed with reinforced steel embedded in the sides, making it much stronger than conventional slabs.

This type of structure provides the strength needed to carry more weight and withstand external forces, such as earthquakes and other disturbances. Moreover, the use of a two-way slab results in greater floor heights and easier maintenance, which are positive aspects for building designers. Overall, the two-way slab offers an economical alternative for multi-storey buildings that need a durable and stable structural element to provide efficient load bearing capacity.

On Which Condition It Is Used?

If it is supported by beams on all four sides and the ratio of longer span(Lx) to shorter span(Ly) is less than 2, then the it is designed as a two-way slab.

Examples Of Two Way Roofing

The best example are:

  • Square shape are always two way.
  • Mostly used in residential

Difference Between One Way & Two Way Slab

Before concluding let’s summarize basic points which justify the difference Between these two types:

  • In this, a major portion of loads is carried in one direction. Whereas in a 2-way slab load is carried along both directions. See the figure below.
  • The (Lx/Ly) ratio is more than or equal to 2 in the one-way slab, whereas (Lx /Ly) is less than 2 in the two-way.

How To Recognize One-Way Slab With Naked Eye

  • It will be rectangular in shape.
  • Mostly, it has beams on two sides
  • The x-axis and y-axis length to width ration is greater than or equals 2.
  • Main reinforcement is provided in one direction only (short span)

Reinforcement of One Way Roofing

  • Main Reinforcement – It should be laid down in a short span with alternate bent-up bars.
  • Distribution Reinforcement – It should be laid down in a long span on top of the main reinforcement and no bent-up bars.
  • Extra Bars – They should be laid down on top of bent-up bars.


Now, you are well aware of two types of roofing. You’ll need to decide which type of roofing is best for your home.


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