Waffle slab is a reinforced concrete roof or floor containing square grids with deep sides. This kind of slab is majorly used at entrance of hotels, Malls, Restaurants for good pictorial view and to install artificial lighting. This a  type of slab where we find hollow hole in the slab when the formwork is removed.  Firstly PVC trays (pods) are placed on shuttering then reinforcement is provided between the pods and steel mesh is provided at top of the pods and then concrete is filled. After concrete sets the formwork is removed and PVC pods are not removed. This forms hollow hole in it in which hole is closed at one end. The concrete waffle slab is often used for industrial and commercial buildings while wood and metal waffle slabs are used in many other construction sites. This is a one of the types of concrete slabs.

Where to use?

A waffle slab is a type of slab with holes underneath, giving an appearance of waffles. It is usually used where large spans are required (e.g auditorium, cinema halls) to avoid many columns interfering the space. Hence thick slabs spanning between wide beams (to avoid the beams protruding below for aesthetic reasons) are required.The main purpose of employing this technology is for its strong foundation characteristics of crack and sagging resistance. Waffle slab also holds a greater amount of load compared with conventional concrete slabs.

Types of Waffle slabs:

Based on the shape of Pods (PVC Trays) They are classified into different types some of them are:

Triangular pod system
Square

Advantages:

Waffle slabs are able to carry heavier loads and span longer distances than flat slabs as these systems are light in weight
Waffle slab can be used as both ceiling and floor slab.
Suitable for spans of 7m – 16m ; longer spans may be possible with posttensioning.
These systems are light in weight and hence considerable saving is ensured in the framework as light framework is required
Disadvantages:

Waffle slab is not used in typical construction projects.
The casting forms or moulds required for pre- cast units are very costly and hence only economical when large scale production of similar units are desired.
Construction requires strict supervision and skilled labour.