Sunken Slab

The sunken slab is a kind of slab at a certain depth (200 or 300 mm or any other depending on the design), which is at the level below the floor.

This additional depth is used to hold the pipes and utility nozzles, then the floor is filled up with sand or other lightweight material up to the normal floor level.

The sunken slab is also called sunk slab, it is sunk slab which is at a lower level than the surrounding slabs.

The purpose of providing a sunk slab is to prevent water from spilling over from the sunk slab area towards the surrounding slabs.

Method of construction of a Sunken Slab:

Concrete R.C.C. (Floor and sunken slabs) must be mixed with a waterproof material to get a dense underwater concrete.


Then cement and waterproofing materials must be thinned in water and sprayed on RCC sunken slab.


On top of that, a layer of plaster should be provided using a mortar plasticizer with cement mortar.

The brickwork of the walls and floor must be achieved with cement mortar blended with a mortar plasticizer.

Tile fixing tile for flooring and walls must be non-shrink, waterproofing adhesive to make tiled area waterproof.

Sanitary pipe joints must be sealed with a specially constructed sealant to prevent any water leakage.

Material used to fill Sunken Slab:

Lightly broken CLC blocks are used to fill sunken slabs and these blocks are the best choice for light weight cinder, broken bricks, brick jelly, coal lamps, burn bitter lamps and AAC blocks.

It begins to gain strength over time until some moisture is available in the atmosphere also requires less maintenance and it is cement-based products in general.

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Advantages of Sunken Slab:

Sunk slabs have following advantages as follows:

  1. In such slabs, all plumbing pipes are concealed inside the slab.
  2. Special plumbing fixtures in sunken strips are not required to reduce noise in bathroom.
  3. These slabs provide a good plumbing system, also used where the safety system is to be installed.
  4. The sealing design is compromised and creates special ventilation.

Disadvantages of Sunken Slab:

Sunk slabs have following disadvantages as follows:

  1. The height decreases from slab to slab.
  2. Experience or skilled labour is required to fabricate this kind of slabs.
  3. In any such slab, waterproofing and brickbat construction take a longer time for construction.

Application of sunken slab:

  • Bathroom/ Toilet / Wash Area Floor: Floor lattices and drainage lines can be moved within the submerged area.
  • Porch Slab: In these places, the beams are inverted so that the beam does not face downward and a plain surface is available.
  • Middle landing on the ladder: The end beam is designed as an inverted beam so that sufficient headroom is available below the landing.

Also, read 1. Post Tension Slab

Frequently Asked Questions:

What is sunken slab?

The sunken slab is slabs which are cast a certain depth (200 or 300 mm) below normal floor level.


The concrete slab is a typical structural component of modern buildings, with a flat, horizontal floor made of cast concrete.

Steel-reinforced slabs, typically between 100 and 500 mm thick are often used for flooring and roof construction, whereas thin clay slabs could also be used for outdoor flooring.


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